Submersed micropatterned structures control active nematic flow,topology, and concentration

Coupling between flows and material properties imbues rheological matter with its wide-ranging applicability, hence the excitement for harnessing the rheology of active fluids for which internal structure and continuous energy injection lead to spontaneous flows and complex, out-of-equilibrium dynamics. We propose and demonstrate a convenient, highly tunable method for controlling flow, topology, and composition within active films. Our approach establishes rheological coupling via the indirect presence of fully submersed micropatterned structures within a thin, underlying oil layer. Simulations reveal that micropatterned structures produce effective virtual boundaries within the superjacent active nematic film due to differences in viscous dissipation as a function of depth. This accessible method of applying position-dependent, effective dissipation to the active films presents a nonintrusive pathway for engineering active microfluidic systems.

Active fluids are inherently out of equilibrium; they locally transform internal energy into material stresses that can result in spontaneous hydrodynamic motion. An increasing number of biophysical systems, including colonies of bacilliform microbes (1–4), cellular monolayers (5–9), and subcellular filaments (10–12), display such collective active motion, orientational order, and topological singularities. Controlling active dynamics is essential not only to fully understanding how such biological systems employ self-generated stresses but also, in order to develop active microfluidic devices.To this end, recent work considers how confining walls (13–15), arrangements of obstacles (16, 17), and the dynamics of topological defects (18) dictate active nematic flow. Control of active material concentration has been studied from the perspectives of coexistence of phases in self-propelled rods (19–21) and motility-induced phase separation (22–24). Controlled accumulation and depletion of active matter have been engineered in bacterial systems to concentrate cells (25, 26) and to drive bacterial-ratchet motors (27–29). Similarly, substrate gradients modify cellular motility, driving density variation (30) and directed migration (31, 32).In addition to varying concentration and flow, topology has been controlled by including externally driven flows (33–35) and curvature (36, 37). Recent work shows that locally altering activity modifies defect populations (38–41), and anisotropic smectic sublayers below active nematic sheets can constrain orientation (42). Such studies demonstrate how underlying sublayer properties have pronounced effects on active dynamics and suggest approaches for engineering control of active matter.We propose a micropattern-based method for controlling active nematic dynamics without contiguous contact with active films. By patterning oil-submersed solid substrates below two-dimensional (2D) active nematic films with geometrical structures of differing height, we achieve effective virtual boundaries within active films that control topological defect populations, collective flow, and concentration of active nematic material without penetrating the film. By implementing underlying submersed patterned microstructures, we tune the depth of the oil layer to adjust dissipation within the superjacent film and thereby, generate a highly tunable technique for controlling the active dynamics. Presently, we introduce four initial submersed structures: micropatterned trenches (Fig. 1 A–C), undulated substrates (SI Appendix, Fig. S1), stairways (Fig. 1 D–F), and pillars (Fig. 1 G–I).Open in a separate windowFig. 1.Submersed micropatterns control active nematic dynamics. (A–C) Trench setup. An active film resides at the oil–water interface above different substrate depths. The active flows drag the underlying oil layer, but viscous dissipation is depth dependent, affecting active nematic film dynamics. (B) Fluorescence microscope image of the active nematic bundled microtubule film above a submersed trench. (Scale bar: 250 μm.) (C) Simulation results for the vorticity field within the superjacent active nematic layer. The flow behaviors within the low-friction region (between the dashed lines) are distinct from the behavior in the high friction region (beyond the dashed lines). Plus-half (minus-half) defects denoted by dark green (magenta) symbols behave differently in the two regions. (D–F) Stairway setup. (E) Fluorescence microscope image of the micromilled stairway and the superjacent bundled microtubule film. Step location is indicated by dashed lines. The oil depth increases from left to right. The differences in oil depth alter the length scale of the active turbulence above each step. (Scale bar: 250 μm.) (F) Simulations results for discrete steps in the effective friction (dashed lines). The effective friction coefficient decreases from left to right. The color bar is shared with C. (G–I) Pillar setup. (H) Fluorescence microscope image of the bundled microtubule film above the SU-8 micropillar. (Scale bar: 100 μm.) (I) Simulation results show that the active nematic concentration ϕ is depleted within the high-friction region encircled by the pillar perimeter (dashed line).     

From the Cover: Fiber networks amplify active stress

Large-scale force generation is essential for biological functions such as cell motility, embryonic development, and muscle contraction. In these processes, forces generated at the molecular level by motor proteins are transmitted by disordered fiber networks, resulting in large-scale active stresses. Although these fiber networks are well characterized macroscopically, this stress generation by microscopic active units is not well understood. Here we theoretically study force transmission in these networks. We find that collective fiber buckling in the vicinity of a local active unit results in a rectification of stress towards strongly amplified isotropic contraction. This stress amplification is reinforced by the networks’ disordered nature, but saturates for high densities of active units. Our predictions are quantitatively consistent with experiments on reconstituted tissues and actomyosin networks and shed light on the role of the network microstructure in shaping active stresses in cells and tissue.Living systems constantly convert biochemical energy into forces and motion. In cells, forces are largely generated internally by molecular motors acting on the cytoskeleton, a scaffold of protein fibers (Fig. 1A). Forces from multiple motors are propagated along this fiber network, driving numerous processes such as mitosis and cell motility (1) and allowing the cell as a whole to exert stresses on its surroundings. At the larger scale of connective tissue, many such stress-exerting cells act on another type of fiber network known as the extracellular matrix (Fig. 1B). This network propagates cellular forces to the scale of the whole tissue, powering processes such as wound healing and morphogenesis. Despite important differences in molecular details and length scales, a common physical principle thus governs stress generation in biological matter: Internal forces from multiple localized “active units”—motors or cells—are propagated by a fiber network to generate large-scale stresses. However, a theoretical framework relating microscopic internal active forces to macroscopic stresses in these networks is lacking. Here we propose such a theory for elastic networks.Open in a separate windowFig. 1.Biological fiber networks (green) transmit forces generated by localized active units (red). (A) Myosin molecular motors exert forces on the actin cytoskeleton. (B) Contractile cells exert forces on the extracellular matrix. (C) The large nonlinear deformations induced by a model active unit in the surrounding fiber network result in stress amplification, as shown in this paper. Fiber color code is shown in D. (D) Each bond in the network comprises two rigid segments hinged together to allow buckling.This generic stress generation problem is confounded by the interplay of network disorder and nonlinear elasticity. Active units generate forces at the scale of the network mesh size, and force transmission to larger scales thus sensitively depends on local network heterogeneities. In the special case of linear elastic networks, the macroscopic active stress is simply given by the density of active force dipoles, irrespective of network characteristics (2). Importantly, however, this relationship is not applicable to most biological systems, because typical active forces are amply sufficient to probe the nonlinear properties of their constitutive fibers, which stiffen under tension and buckle under compression (3). Indeed, recent experiments on reconstituted biopolymer gels have shown that individual active units induce widespread buckling and stiffening (4, 5), and theory suggests that such fiber nonlinearities can enhance the range of force propagation (6, 7).Fiber networks also exhibit complex, nonlinear mechanical properties arising at larger scales, owing to collective deformations favored by the networks’ weak connectivity (3, 8). The role of connectivity in elasticity was famously investigated by Maxwell, who noticed that a spring network in dimension d becomes mechanically unstable for connectivities z < 2d. Interestingly, most biological fiber networks exhibit connectivities well below this threshold and therefore cannot be stabilized solely by the longitudinal stretching rigidity of their fibers. Instead, their macroscopic mechanical properties are typically controlled by the fiber bending rigidity (9). In contrast to stretching-dominated networks with connectivities above the Maxwell threshold, such weakly connected, bending-dominated networks are soft and extremely sensitive to mechanical perturbations (9–11). In these networks, stresses generated by active units propagate along intricate force chains (12, 13) whose effects on force transmission remain unexplored. Collections of such active units generate large stresses, with dramatic effects such as macroscopic network stiffening (14–16) and network remodeling (5, 17).Here we study the theoretical principles underlying stress generation by localized active units embedded in disordered fiber networks (Fig. 1C). We find that arbitrary local force distributions generically induce large isotropic, contractile stress fields at the network level, provided that the active forces are large enough to induce buckling in the network. In this case, the stress generated in a biopolymer network dramatically exceeds the stress level that would be produced in a linear elastic medium (2), implying a striking network-induced amplification of active stress. Our findings elucidate the origins and magnitude of stress amplification observed in experiments on reconstituted tissues (4, 18) and actomyosin networks (14, 17). We thus provide a conceptual framework for stress generation in biological fiber networks.     

Shape control and compartmentalization in active colloidal cells

Small autonomous machines like biological cells or soft robots can convert energy input into control of function and form. It is desired that this behavior emerges spontaneously and can be easily switched over time. For this purpose we introduce an active matter system that is loosely inspired by biology and which we term an active colloidal cell. The active colloidal cell consists of a boundary and a fluid interior, both of which are built from identical rotating spinners whose activity creates convective flows. Similarly to biological cell motility, which is driven by cytoskeletal components spread throughout the entire volume of the cell, active colloidal cells are characterized by highly distributed energy conversion. We demonstrate that we can control the shape of the active colloidal cell and drive compartmentalization by varying the details of the boundary (hard vs. flexible) and the character of the spinners (passive vs. active). We report buckling of the boundary controlled by the pattern of boundary activity, as well as formation of core–shell and inverted Janus phase-separated configurations within the active cell interior. As the cell size is increased, the inverted Janus configuration spontaneously breaks its mirror symmetry. The result is a bubble–crescent configuration, which alternates between two degenerate states over time and exhibits collective migration of the fluid along the boundary. Our results are obtained using microscopic, non–momentum-conserving Langevin dynamics simulations and verified via a phase-field continuum model coupled to a Navier–Stokes equation.Active matter describes particulate systems with the characteristic that each “particle” (agent) converts energy into motion (1, 2). Active matter covers a range of length scales that include molecular motors in the cytoskeleton (3–5), swimming bacteria (6–8), driven colloids (9, 10), flocks of birds and fish (11–14), and people and vehicles in motion (15). Over the last decade, studies of active matter have demonstrated behavior not seen in equilibrium systems, including giant number fluctuations (16, 17), emergent attraction and superdiffusion (18–20), clustering (21, 22), swarming (23–27), and self-assembled motifs (28, 29). These systems provide interesting theoretical and engineering challenges as well as opportunities to explore and target novel behaviors that proceed outside of thermodynamic equilibrium.Of particular interest are systems found in nature or inspired by natural phenomena. Biological systems usually operate in confined regions of space––think of intracellular space, interfaces and membranes, and the crowding of cells near surfaces. The role of hydrodynamics in confinement has been studied for biological swimmers, such as bacteria and sperm, showing accumulation at the walls (30–32) and upstream swimming along surfaces (33) or in a spiral vortex (34–36). Attraction to walls has also been reported in the absence of hydrodynamics for disks (37, 38), spheres (39), and dumbbell swimmers (40). But, whereas these examples study the behavior under the influence of hard boundaries, biological swimmers typically interact with soft boundaries, such as membranes and biofilms. Another design variable is the possibility that the boundary itself is active, as in the surface of a bacterium covered with flagellae or, as demonstrated recently, active nematic vesicles (41).In this work, we propose and investigate an active matter system under flexible, active confinement. We call this system an active colloidal cell. Our realization of an active colloidal cell consists of independent particles, called spinners (42), that translate and rotate in two dimensions and are constrained within a finite area by a flexible boundary that is also built from spinners. Each spinner has a gear-like geometry, which consists of a large central disk and four smaller satellite disks (Fig. 1A). Similar gear-shaped rigid aggregates of self-propelled particles have been formed experimentally (43). Spinners are freely mobile in the cell interior. On the cellular boundary, spinners are connected to one another by a flexible chain of beads attached by finitely extensible springs. Both the interior and the boundary spinners can be subject to a clockwise or counterclockwise driving torque, which makes them active.Open in a separate windowFig. 1.Schematic of the confined spinner models. (A) The active colloidal cell is made up of spinners driven counterclockwise (blue) or clockwise (yellow). Boundary spinners are connected by a flexible bead–spring chain (gray). We compare the behavior of a continuum model (B) to a microscopic model (C). The compartmentalization of interior spinners is visualized by coloring the Voronoi tessellation in the microscopic model.Rotationally driven particles can synchronize and self-organize (44, 45) in the absence (42) and in the presence (46–48) of hydrodynamic interactions. Crystallization has recently been observed in rotating magnetic Janus colloids (49) and fast-moving bacteria (50). Spinners in the interior of the cell resemble molecular motors that push themselves forward on their neighbors and, thus, sustain convective dynamics. The effect of the boundary spinners is similar to that found in the cilia of living tissues, which stir nearby fluid. Our results demonstrate that a natural consequence of the activity present in the colloidal cell is control over both its external shape and internal structure. We report compartmentalization into regions of clockwise and counterclockwise spinners––a behavior which is affected by, and can be controlled via, properties of the enclosing boundary configuration as previously suggested (51). Transitions in the internal structure of the colloidal cell occur as its radius increases, and as the composition of the interior spinners and the patterning of the boundary are varied.A previous study of spinners in bulk (42) showed phase separation into clockwise- and counterclockwise domains. Cates and collaborators (6, 52, 53) have suggested that phase separation is a generic consequence of local energy input in an otherwise equilibrium system. Here and in the study of bulk spinners we demonstrate phase separation due to local rotational, rather than translational, energy input. We obtain our results using a particulate, microscopic model (Fig. 1C) as well as a continuum model (Fig. 1B). This allows us to conclude that the phenomena we observe are robust with respect to details of the model.In this study we use two models to study the behavior of an active colloidal cell, illustrated in Fig. 1. The microscopic model describes spinners as individual particles and simulates their motion using Langevin dynamics. It resolves the behavior of individual spinners but does not include hydrodynamic effects. In contrast, the continuum model describes the spinner system as a viscous binary fluid, which is governed by an incompressible Navier–Stokes equation coupled to a Cahn–Hilliard equation. Both models are described in detail in Materials and Methods below. Note that the microscopic model was introduced in earlier work using Brownian dynamics (42) and is extended here to include boundaries.     

Electrochemical borylation of carboxylic acids

A simple electrochemically mediated method for the conversion of alkyl carboxylic acids to their borylated congeners is presented. This protocol features an undivided cell setup with inexpensive carbon-based electrodes and exhibits a broad substrate scope and scalability in both flow and batch reactors. The use of this method in challenging contexts is exemplified with a modular formal synthesis of jawsamycin, a natural product harboring five cyclopropane rings.

Boronic acids are among the most malleable functional groups in organic chemistry as they can be converted into almost any other functionality (1–3). Aside from these versatile interconversions, their use in the pharmaceutical industry is gaining traction, resulting in approved drugs such as Velcade, Ninlaro, and Vabomere (4). It has been shown that boronic acids can be rapidly installed from simple alkyl halides (5–19) or alkyl carboxylic acids through the intermediacy of redox-active esters (RAEs) (Fig. 1A) (20–24). Our laboratory has shown that both Ni (20) and Cu (21) can facilitate this reaction. Conversely, Aggarwal and coworkers (22) and Li and coworkers (23) demonstrated photochemical variations of the same transformation. While these state-of-the-art approaches provide complementary access to alkyl boronic acids, each one poses certain challenges. For example, the requirement of excess boron source and pyrophoric MeLi under Ni catalysis is not ideal. Although more cost-effective and operationally simple, Cu-catalyzed borylation conditions can be challenging on scale due to the heterogeneity resulting from the large excess of LiOH•H₂O required. In addition to its limited scope, Li and coworkers’ protocol requires 4 equivalence of B₂pin₂ and an expensive Ir photocatalyst. The simplicity of Aggarwal and coworkers’ approach is appealing in this regard and represents an important precedent for the current study.Open in a separate windowFig. 1.(A) Prior approaches to access alkyl boronic esters from activated acids. (B) Inspiration for initiating SET events electrochemically to achieve borylation. (C) Summary of this work.At the heart of each method described above, the underlying mechanism relies on a single electron transfer (SET) event to promote decarboxylation and form an alkyl radical species. In parallel, the related borylation of aryl halides via a highly reactive aryl radical can also be promoted by SET. While numerous methods have demonstrated that light can trigger this mechanism (Fig. 1B) (16, 25–31), simple electrochemical cathodic reduction can elicit the same outcome (32–35). It was postulated that similar electrochemically driven reactivity could be translated to alkyl RAEs. The development of such a transformation would be highly enabling, as synthetic organic electrochemistry allows the direct addition or removal of electrons to a reaction, representing an incredibly efficient way to forge new bonds (36–40). This disclosure reports a mild, scalable, and operationally simple electrochemical decarboxylative borylation (Fig. 1C) not reliant on transition metals or stoichiometric reductants. In addition to mechanistic studies of this interesting transformation, applications to a variety of alkyl RAEs, comparison to known decarboxylative borylation methods, and a formal synthesis of the polycyclopropane natural product jawsamycin [(–)-FR-900848] are presented.     

Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors

The noble gases are elements of broad importance across science and technology and are primary constituents of planetary and stellar atmospheres, where they segregate into droplets or layers that affect the thermal, chemical, and structural evolution of their host body. We have measured the optical properties of noble gases at relevant high pressures and temperatures in the laser-heated diamond anvil cell, observing insulator-to-conductor transformations in dense helium, neon, argon, and xenon at 4,000–15,000 K and pressures of 15–52 GPa. The thermal activation and frequency dependence of conduction reveal an optical character dominated by electrons of low mobility, as in an amorphous semiconductor or poor metal, rather than free electrons as is often assumed for such wide band gap insulators at high temperatures. White dwarf stars having helium outer atmospheres cool slower and may have different color than if atmospheric opacity were controlled by free electrons. Helium rain in Jupiter and Saturn becomes conducting at conditions well correlated with its increased solubility in metallic hydrogen, whereas a deep layer of insulating neon may inhibit core erosion in Saturn.Noble gases play important roles in the evolution and dynamics of planets and stars, especially where they appear in a condensed, purified state. In gas giant planets, helium and neon can precipitate as rain in metallic hydrogen envelopes, leading to planetary warming and specifically the anomalously slow cooling of Saturn (1–8). In white dwarf stars cooling can be especially fast due to the predicted low opacity of dense helium atmospheres, affecting the calibration of these objects as cosmological timekeepers (9–12). In these systems, the transformation of dense noble gases (particularly He) from optically transparent insulators to opaque electrical conductors is of special importance (2, 9, 11, 12).Dense noble gases are expected to show systematic similarities in their properties at extreme conditions (13–17); however, a general understanding of their insulator–conductor transformation remains to be established. Xe is observed to metallize near room temperature under pressures similar to those at Earth’s core–mantle boundary (18, 19). Ar and He are observed to conduct only at combined high pressure and temperature (12, 13, 17). Ne is predicted to have the highest metallization pressure of all known materials—10³ times that of Xe and 10 times that of He (14, 18, 20, 21)—and has never been documented outside of its insulating state. Experimental probes of extreme densities and temperatures in noble gases have previously relied on dynamic compression by shock waves (12, 13, 17, 22–24). However, in such adiabatic experiments, light and compressible noble gases heat up significantly and can ultimately reach density maxima (12, 13, 17, 21, 24, 25), so that conditions created often lie far from those deep within planets (7, 8) and stars (9).Here we report experiments in the laser-heated diamond anvil cell (15, 16, 26–29) on high-density and high-temperature states of the noble gases Xe, Ar, Ne, and He (Fig. 1). Rapid heating and cooling of compressed samples using pulsed laser heating (26, 27) is coupled with time domain spectroscopy of thermal emission (26) to determine sample temperature and transient absorption to establish corresponding sample optical properties (Figs. S1 and S2). A sequence of heat cycles to increasing temperature documents optical changes in these initially transparent insulators.Open in a separate windowFig. 1.Creating and probing extreme states of noble gases. (A) Configuration of laser heating and transient absorption probing of the diamond anvil cell, with probe beams transmitted through the cell into the detection system. (B) Microscopic view of the diamond cell cavity, which contains a noble gas sample and a metal foil (Ir) which converts laser radiation to heat and has small hole at the heated region through which probe beams are transmitted to test optical character of samples. (C) Finite element model (26) (Fig. S3) of the temperature distribution in heated Ar at 51 GPa (Fig. 2), with solid–melt (16) and insulator–conductor (α = 0.1 μm⁻¹) boundaries in the sample marked dashed and dotted, respectively. (D) Schematic of time domain probing during transient heating. Temperature is determined from thermal emission (red) and absorption from transmitted probe beams: a continuous laser (cw; green) and pulsed supercontinuum broadband (bb; blue).     

Shape-programmable magnetic soft matter

Shape-programmable matter is a class of active materials whose geometry can be controlled to potentially achieve mechanical functionalities beyond those of traditional machines. Among these materials, magnetically actuated matter is particularly promising for achieving complex time-varying shapes at small scale (overall dimensions smaller than 1 cm). However, previous work can only program these materials for limited applications, as they rely solely on human intuition to approximate the required magnetization profile and actuating magnetic fields for their materials. Here, we propose a universal programming methodology that can automatically generate the required magnetization profile and actuating fields for soft matter to achieve new time-varying shapes. The universality of the proposed method can therefore inspire a vast number of miniature soft devices that are critical in robotics, smart engineering surfaces and materials, and biomedical devices. Our proposed method includes theoretical formulations, computational strategies, and fabrication procedures for programming magnetic soft matter. The presented theory and computational method are universal for programming 2D or 3D time-varying shapes, whereas the fabrication technique is generic only for creating planar beams. Based on the proposed programming method, we created a jellyfish-like robot, a spermatozoid-like undulating swimmer, and an artificial cilium that could mimic the complex beating patterns of its biological counterpart.Shape-programmable matter refers to active materials that can be controlled by heat (1–5), light (6, 7), chemicals (8–13), pressure (14, 15), electric fields (16, 17), or magnetic fields (18–33) to generate desired folding or bending. As these materials can reshape their geometries to achieve desired time-varying shapes, they have the potential to create mechanical functionalities beyond those of traditional machines (1, 15). The functionalities of shape-programmable materials are especially appealing for miniature devices whose overall dimensions are smaller than 1 cm as these materials could significantly augment their locomotion and manipulation capabilities. The development of highly functional miniature devices is enticing because, despite having only simple rigid-body motions (34–36) and gripping capabilities (37), existing miniature devices have already been used across a wide range of applications pertaining to microfluidics (38, 39), microfactories (40, 41), bioengineering (42, 43), and health care (35, 44).Among shape-programmable matter, the magnetically actuated materials are particularly promising for creating complex time-varying shapes at small scales because their control inputs, in the form of magnetic fields, can be specified not only in magnitude but also in their direction and spatial gradients. Furthermore, as they can be fabricated with a continuum magnetization profile, m, along their bodies, these magnetic materials can also generate deformations with very high spatial resolutions. A continuum m can be approximated as a collection of magnetic dipoles continuously distributed within a material, where each dipole can produce locally varying stress when it interacts with the actuating fields. The actuating fields can deform the materials by either exerting magnetic forces (27, 30) or torques (18, 19) on these dipoles.Although shape-programmable magnetic soft materials have great potential, previous work can only rely on human intuition to approximate the required m and actuating fields for these materials to achieve their desired time-varying shapes. As a result, such heuristic and unsystematic methods can only program these materials for a limited number of applications, demonstrating only either simple deformations (21, 24–28) or very specific functionalities (18–20, 22, 23, 29–33).Here, we present a universal programming methodology that can automatically generate the required m and actuating fields for small-scale soft matter to achieve desired time-varying shapes (Fig. 1A). This universal method therefore has the potential to inspire a wide variety of miniature devices that could transform robotics, material science, and biomedicine. The proposed method consists of theoretical formulations, computational strategies, and fabrication procedures. Although the theory and computational method are universal for programming 2D or 3D time-varying shapes (S2. Programming Materials with 3D Time-Varying Shapes), our fabrication technique is only generic for making planar beams. Despite the limitations of our current fabrication technique, it is still significantly better than existing techniques, which at most can only create direction-varying m that have a uniform magnitude (18, 19). As we can fabricate continua m that have desirable nonuniform magnitude and orientation profiles, the potential of shape-programmable magnetic beams can now be fully realized.Open in a separate windowFig. 1.The programming methodology and a simple proof of concept. (A) The programming method for magnetic soft elastomeric composite materials to achieve the desired time-varying shapes. We illustrate this concept with an arbitrary beam that can be programmed to achieve the desired shapes shown on the Left. By using our proposed programming method (shown in the Center), we can automatically generate the required magnetization profile, m(s), and magnetic field control inputs, B(t), for the material (shown on the Right). The given m(s) and B(t) are only used as an illustration. (B) A graphical illustration for the theoretical formulations. Based on the desired kinematics in i, a quasistatic analysis can be conducted on ii. (C) A simple proof of concept of the proposed method in which a beam is programmed to create a shape resembling a cosine function when it is subjected to a 5-mT uniform magnetic field input. (i) Desired shape, simulated first derivative of the bending moment, and necessary magnetization profile along the beam. The desired first derivative of the bending moment is represented by the blue curve, whereas the dotted red curve represents the first derivative of the bending moment generated by magnetic actuation. As the blue and dotted red curves will totally overlap one another, they have been separated into two plots for clarity. The plotted magnetization profile is along the predeformed beam (see Fig. S6 for a more quantitative representation for the magnetization). Additional parameters for this device can be found in S9. Parameters for Each Case and 相似文献   

Chlorination of arenes via the degradation of toxic chlorophenols

Aryl chlorides are among the most versatile synthetic precursors, and yet inexpensive and benign chlorination techniques to produce them are underdeveloped. We propose a process to generate aryl chlorides by chloro-group transfer from chlorophenol pollutants to arenes during their mineralization, catalyzed by Cu(NO₃)₂/NaNO₃ under aerobic conditions. A wide range of arene substrates have been chlorinated using this process. Mechanistic studies show that the Cu catalyst acts in cooperation with NO_x species generated from the decomposition of NaNO₃ to regulate the formation of chlorine radicals that mediate the chlorination of arenes together with the mineralization of chlorophenol. The selective formation of aryl chlorides with the concomitant degradation of toxic chlorophenol pollutants represents a new approach in environmental pollutant detoxication. A reduction in the use of traditional chlorination reagents provides another (indirect) benefit of this procedure.

Chlorophenols are widely encountered moieties present in herbicides, drugs, and pesticides (1). Owing to the high dissociation energies of carbon‒chloride bonds, chlorophenols biodegrade very slowly, and their prolonged exposure leads to severe ecological and environmental problems (Fig. 1A) (2–4). Several well-established technologies have been developed for the treating of chlorophenols, including catalytic oxidation (5–11), biodegradation (12–15), solvent extraction (16, 17), and adsorption (18–20) Among these methods, adsorption is the most versatile and widely used method due to its high removal efficiency and simple operation, but the resulting products are of no value, and consequently, these processes are not viable.Open in a separate windowFig. 1.Background and reaction design. (A) Examples of chlorophenol pollutants. (B) Examples of aryl chlorides. (C) The chlorination process reported herein was based on chloro-group transfer from chlorophenol pollutants.With the extensive application of substitution reactions (21, 22), transfunctionalizations (23, 24), and cross-coupling reactions (25, 26), aryl chlorides are regarded as one of the most important building blocks widely used in the manufacture of polymers, pharmaceuticals, and other types of chemicals and materials (Fig. 1B) (27–31). Chlorination of arenes is usually carried out with toxic and corrosive reagents (32–34). Less toxic and more selective chlorination reagents tend to be expensive [e.g., chloroamides (35, 36)] and are not atom economic (37–39). Consequently, from the perspective of sustainability, the ability to transfer a chloro group from unwanted chlorophenols to other substrates would be advantageous.Catalytic isofunctional reactions, including transfer hydrogenation and alkene metathesis, have been widely exploited in organic synthesis. We hypothesized that chlorination of arenes also could be achieved by chloro-group transfer, and since stockpiles of chlorophenols tend to be destroyed by mineralization and chlorophenol pollutants may be concentrated by adsorption (18–20), they could be valorized as chlorination reagents via transfer of the chloro group to arene substrates during their mineralization, thereby adding value to the destruction process (Fig. 1C). Although chlorophenol pollutants are not benign, their application as chlorination reagents, with their concomitant destruction to harmless compounds, may be considered as not only meeting the criteria of green chemistry but also potentially surpassing it. Herein, we describe a robust strategy to realize chloro-group transfer from chlorophenol pollutants to arenes and afford a wide range of value-added aryl chlorides.     

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet–visible (UV–Vis) absorption bands. Bulky ligands on the metal suppress π–π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV–Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Conjugated polymers are macromolecules usually featuring a backbone chain with alternating double and single bonds (1–3). These characteristics allow the overlapping p-orbitals to form a system with highly delocalized π-electrons, thereby giving rise to intriguing chemical and physical properties (4–6). They have exhibited many applications in organic light-emitting diodes, organic thin film transistors, organic photovoltaic cells, chemical sensors, bioimaging and therapies, photocatalysis, and other technologies (7–10). To facilitate the use of solar energy, tremendous efforts have been devoted in recent decades to developing previously unidentified conjugated polymers exhibiting broad and strong absorption bands (11–13). The common strategies for increasing absorption involve extending π-conjugation by incorporating conjugated cyclic moieties, especially fused rings; modulating the strength of intramolecular charge transfer between donor and acceptor units (D–A effect); increasing the coplanarity of π conjugation through weak intramolecular interactions (e.g., hydrogen bonds); and introducing heteroatoms or heavy atoms into the repeat units of conjugated polymers (11–16). Additionally, appropriate solubility is a prerequisite for processing and using polymers and is usually achieved with the aid of long alkyl or alkoxy side chains (12, 17).Aromatic rings are among the most important building blocks for conjugated polymers. In addition to aromatic hydrocarbons, a variety of aromatic heterocycles composed of main-group elements have been used as fundamental components. These heteroatom-containing conjugated polymers show unique optical and electronic properties (4–10). However, while metalla-aromatic systems bearing a transition metal have been known since 1979 due to the pioneering work by Thorn and Hoffmann (18), none of them have been used as building blocks for conjugated polymers. The HOMO–LUMO gaps (E_g) of metalla-aromatics are generally narrower (Fig. 1) than those of their organic counterparts (19–22). We reasoned that this feature should broaden the absorption window if polymers stemming from metalla-aromatics are achievable.Open in a separate windowFig. 1.Comparison of traditional organic skeletons with metalla-aromatic building blocks (the computed energies are in eV). (A) HOMO–LUMO gaps of classic aromatic skeletons. (B) Carbolong frameworks as potential building blocks for novel conjugated polymers with broad absorption bands and improved solubility.In recent years, we have reported a series of readily accessible metal-bridged bicyclic/polycyclic aromatics, namely carbolong complexes, which are stable in air and moisture (23–25). The addition of osmium carbynes (in carbolong complexes) and alkynes gave rise to an intriguing family of d_π–p_π conjugated systems, which function as excellent electron transport layer materials in organic solar cells (26, 27). These observations raised the following question: Can this efficient addition reaction be used to access metalla-aromatic conjugated polymers? It is noteworthy that incorporation of metalla-aromatic units into conjugated polymers is hitherto unknown. In this contribution, we disclose a polymerization reaction involving M≡C analogs of C≡C bonds, which involves a unique carbyne shuttling strategy (Fig. 2A). This led to examples of metalla-aromatic conjugated polymers (polycarbolongs) featuring metal carbyne units in the main chain. On the other hand, the development of polymerization reactions plays a crucial role in involving certain building blocks in conjugated polymers (28–32). These efficient, specific, and feasible polymerizations could open an avenue for the synthesis of conjugated polymers.Open in a separate windowFig. 2.Design of polymers and synthesis of monomers. (A) Schematic illustration of the polymerization strategy. (B) Preparation of carbolong monomers. Insert: X-ray molecular structure for the cations of complex 3. Ellipsoids are shown at the 50% probability level; phenyl groups in PPh₃ are omitted for clarity.     

Spontaneous helielectric nematic liquid crystals: Electric analog to helimagnets

Recently, a type of ferroelectric nematic fluid has been discovered in liquid crystals in which the molecular polar nature at molecule level is amplified to macroscopic scales through a ferroelectric packing of rod-shaped molecules. Here, we report on the experimental proof of a polar chiral liquid matter state, dubbed helielectric nematic, stabilized by the local polar ordering coupled to the chiral helicity. This helielectric structure carries the polar vector rotating helically, analogous to the magnetic counterpart of helimagnet. The helielectric state can be retained down to room temperature and demonstrates gigantic dielectric and nonlinear optical responses. This matter state opens a new chapter for developing the diverse polar liquid crystal devices.

In nature , a new matter state usually arises as a result of unexpected combinations of hierarchical orderings. Helicity is one of the most essential nature of matter states for organizing superstructures in soft matters, spanning many length scales from the atomic to the macroscopic biological levels. When constructed from building blocks with inherent polarity, three hierarchical orderings could coexist in a helical structure: 1) the head-to-tail or polar symmetry of each building block (e.g., Fig. 1C), 2) the orientational order of a swarm of building blocks (Fig. 1A), and 3) the emergent helicity (Fig. 1B). While a simultaneous realization of these three orderings could lead to extraordinary material properties, such highly hierarchical structures are often challenging to achieve in man-made systems. Probably the most familiar example is the chiral magnet or helimagnet (Fig. 1B) in quantum systems, where the magnetic spins form two- or three-dimensional spiral structures (1, 2). The polar magnetic helical structures are considered mainly to originate from either the breaking of the space-inversion symmetry in crystal structures (3) or the magnetic frustration (1, 4, 5). Their strong magnetism-chirality coupling triggers enormous interests in condensed matter physics, leading to many unique quantum and information functionalities (6–9). From the mirror relationship between the magnetism and electricity, we anticipate the incidence of a possible electric version of the helimagnets, namely helielectrics. However, the diverse magnetic topological states rarely show up in electric systems, except a few recent breakthroughs (e.g., the observation of the electric skyrmions, polar vortices, and merons in metal-organic crystals) (10–12). The special electric states at nanoscale exhibit extraordinary properties such as local negative dielectric permittivity (13) and strain-polarization coupling (14, 15). Nevertheless, nearly all the aforementioned chiral magnet or electric-analog systems are based on elaborately fabricated inorganics. It is expected that the revolutionary realization of these topologies in a soft matter system would bring the advantages of flexibility, simple preparation, large-area film formation, and ease of integration into electric devices.Open in a separate windowFig. 1.Topological analogy: electric versus magnetic states. (A) Uniform magnetization or polarization. (B) Helimagnet or helielectric states. Possible helicoidal (top) and heliconical (bottom) textures are shown. (C) Molecular structure of the polar anisotropic entity, RM734. The molecular polar dipole is nearly parallel to the long molecular axis. (D) The ferroelectric nematic state with spontaneous polarization. (E) HN* state with heli realized by adding chiral generators into the polar chiral nematic state. One-dimensional polarization fields are also depicted in D and E for clarity. (F) The molecular structures of the chiral generators S1 and S2. (G) The state diagram of the two HN* materials by mixing RM734 with S1 or S2.Among the soft matter systems, liquid analogs of ferromagnet and helimagnet have been reported in liquid crystal (LC) colloids recently (16–20). For the electric versions, there already exist a category of materials possessing all the aforementioned three hierarchical orderings (i.e., the ferroelectric smectic LCs) (21–26). The smectic C* (SmC*) has layered heliconical structure with its local polarity aligning perpendicular to the long molecular axis. Confinement to thin LC cells leads to the unwound ferroelectric state of SmC* with microsecond switching time, thereby being a promising candidate for LC display applications. However, the unavoidable defect generation in the devices originated from the crystal-like structure has been one of the main technical difficulties. Moreover, the SmC* has intrinsically low fluidity and polarity (spontaneous polarization P_s < 1 μC). Here, we report a discovery of a helimagnetic analog state in polar LC materials, dubbed helielectric nematic (HN*). The spontaneous polar nematic ordering is coupled to the chiral orientational helicity (Fig. 1B), taking the form with a nearly helicoidal orientational field. Thanks to its much higher fluidity than the traditional SmC* ferroelectrics, uniform structures can be easily obtained by the typical thermal annealing process. The simultaneous observation of the traditional nonlinear second-harmonic generation (SHG) and SHG interferometry microscopies, as well as the optical observations of the selective reflection from HN* state, allow us to directly visualize the helical polar field. In contrast to the traditional nanoscopic helimagnetic or helielectric inorganics, a wide tunability of the periodic distance ranging from micrometers to near ultraviolet wavelength is achieved in the fluidic structure. Besides, the ability of switching between the polar and nonpolar helical LC states enables complementary physics study for the topology features in HN*. As gifts of the chirality–polarity interaction, the matter state uniquely expresses giant dielectric and SHG optical response, especially interesting SHG amplification when the SHG wavelength coincides with the reflection band of the HN* state.     

Electrophilic aromatic substitution reactions of compounds with Craig-Möbius aromaticity

Electrophilic aromatic substitution (EAS) reactions are widely regarded as characteristic reactions of aromatic species, but no comparable reaction has been reported for molecules with Craig-Möbius aromaticity. Here, we demonstrate successful EAS reactions of Craig-Möbius aromatics, osmapentalenes, and fused osmapentalenes. The highly reactive nature of osmapentalene makes it susceptible to electrophilic attack by halogens, thus osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene can undergo typical EAS reactions. In addition, the selective formation of a series of halogen substituted metalla-aromatics via EAS reactions has revealed an unprecedented approach to otherwise elusive compounds such as the unsaturated cyclic chlorirenium ions. Density functional theory calculations were conducted to study the electronic effect on the regioselectivity of the EAS reactions.

Aromaticity, a core concept in chemistry, was initially introduced to account for the bonding, stability, reactivity, and other properties of many unsaturated organic compounds. There have been many elaborations and extensions of the concept of aromaticity (1, 2). The concepts of Hückel aromaticity and Möbius aromaticity are widely accepted (Fig. 1A). A π-aromatic molecule of the Hückel type is planar and has 4n + 2 conjugated π-electrons (n = 0 or an integer), whereas a Möbius aromatic molecule has one twist of the π-system, similar to that in a Möbius strip, and 4n π-electrons (3, 4). Since the discovery of naphthalene in 1821, aromatic chemistry has developed into a rich field and with a variety of subdisciplines over the course of its 200-y history, and the concept of aromaticity has been extended to other nontraditional structures with “cyclic delocalization of mobile electrons” (5). For example, benzene-like metallacycles—predicted by Hoffmann et al. as metallabenzenes—in which a metal replaces a C–H group in the benzene ring (6), have garnered extensive research interest from both experimentalists and theoreticians (7–12). As paradigms of the metalla-aromatic family, most complexes involving metallabenzene exhibit thermodynamic stability, kinetic persistence, and chemical reactivity associated with the classical aromaticity concept (13–15). Typically, like benzene, metallabenzene can undergo characteristic reactions of aromatics such as electrophilic aromatic substitution (EAS) reactions (16–18) (Fig. 1B, I) and nucleophilic aromatic substitution reactions (19–21).Open in a separate windowFig. 1.Schematic representations of aromaticity classification (A) and EAS reactions (B) of benzene, metallabenzene, and polycyclic metallacycles with Craig-Möbius aromaticity.The incorporation of transition metals has also led to an increase in the variety of the aromatic families (22–25). We have reported that stable and highly unusual bicyclic systems, metallapentalenes (osmapentalenes), benefit from Craig-Möbius aromaticity (26–30). In contrast to other reported Möbius aromatic compounds with twisted topologies, which are known as Heilbronner-Möbius aromatics (31–34), the involvement of transition metal d orbitals in π-conjugation switches the Hückel anti-aromaticity of pentalene into the planar Craig-Möbius aromaticity of metallapentalene (35–38) (Fig. 1A, III). Both the twisted topology and the planar Craig-Möbius aromaticity are well established and have been accepted as reasonable extensions of aromaticity (39–43). There has been no experimental evidence, however, as to whether these Möbius aromatic molecules can undergo classical aromatic substitution reactions, such as EAS reactions, instead of addition reactions. Given the key role of EAS in aromatic chemistry to obtain various derivatives, we sought to extend the understanding of the reactivity paradigm in the metalla-aromatic family.Our recent synthetic efforts associated with the metallapentalene system prompted us to investigate whether typical EAS reactions could proceed in these Craig-Möbius aromatics. If so, how could substitution be achieved in the same way that it is with traditional Hückel aromatics such as benzenes? In this paper, we present EAS reactions, mainly the halogenation of osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene, which follow the classic EAS mechanistic scheme (Fig. 1B). With the aid of density functional theory (DFT) calculations, we characterized the effects on EAS reactivity and regioselectivity.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Chirality-matched catalyst-controlled macrocyclization reactions

Macrocycles, formally defined as compounds that contain a ring with 12 or more atoms, continue to attract great interest due to their important applications in physical, pharmacological, and environmental sciences. In syntheses of macrocyclic compounds, promoting intramolecular over intermolecular reactions in the ring-closing step is often a key challenge. Furthermore, syntheses of macrocycles with stereogenic elements confer an additional challenge, while access to such macrocycles are of great interest. Herein, we report the remarkable effect peptide-based catalysts can have in promoting efficient macrocyclization reactions. We show that the chirality of the catalyst is essential for promoting favorable, matched transition-state relationships that favor macrocyclization of substrates with preexisting stereogenic elements; curiously, the chirality of the catalyst is essential for successful reactions, even though no new static (i.e., not “dynamic”) stereogenic elements are created. Control experiments involving either achiral variants of the catalyst or the enantiomeric form of the catalyst fail to deliver the macrocycles in significant quantity in head-to-head comparisons. The generality of the phenomenon, demonstrated here with a number of substrates, stimulates analogies to enzymatic catalysts that produce naturally occurring macrocycles, presumably through related, catalyst-defined peripheral interactions with their acyclic substrates.

Macrocyclic compounds are known to perform a myriad of functions in the physical and biological sciences. From cyclodextrins that mediate analyte separations (1) to porphyrin cofactors that sit in enzyme active sites (2, 3) and to potent biologically active, macrocyclic natural products (4) and synthetic variants (5–7), these structures underpin a wide variety of molecular functions (Fig. 1A). In drug development, such compounds are highly coveted, as their conformationally restricted structures can lead to higher affinity for the desired target and often confer additional metabolic stability (8–13). Accordingly, there exists an entire synthetic chemistry enterprise focused on efficient formation and functionalization of macrocycles (14–18).Open in a separate windowFig. 1.(A) Examples of macrocyclic compounds with important applications. HCV, hepatitis C virus. (B) Use of chiral ligands in metal-catalyzed or mediated stereoselective macrocyclization reactions. (C) Remote desymmetrization using guanidinylated ligands via Ullmann coupling. (D) This work: use of copper/peptidyl complexes for macrocyclization and the exploration of matched and mismatched effect.In syntheses of macrocyclic compounds, the ring-closing step is often considered the most challenging step, as competing di- and oligomerization pathways must be overcome to favor the intramolecular reaction (14). High-dilution conditions are commonly employed to favor macrocyclization of linear precursors (19). Substrate preorganization can also play a key role in overcoming otherwise high entropic barriers associated with multiple conformational states that are not suited for ring formation. Such preorganization is most often achieved in synthetic chemistry through substrate design (14, 20–22). Catalyst or reagent controls that impose conformational benefits that favor ring formation are less well known. Yet, critical precedents include templating through metal-substrate complexation (23, 24), catalysis by foldamers (25) or enzymes (26–29), or, in rare instances, by small molecules (discussed below). Characterization of biosynthetic macrocyclization also points to related mechanistic issues and attributes for efficient macrocyclizations (30–34). Coupling macrocyclization reactions to the creation of stereogenic elements is also rare (35). Metal-mediated reactions have been applied toward stereoselective macrocyclizations wherein chiral ligands transmit stereochemical information to the products (Fig. 1B). For example, atroposelective ring closure via Heck coupling has been applied in the asymmetric total synthesis of isoplagiochin D by Speicher and coworkers (36–40). Similarly, atroposelective syntheses of (+)-galeon and other diarylether heptanoid natural products were achieved via Ullman coupling using N-methyl proline by Salih and Beaudry (41). Finally, Reddy and Corey reported the enantioselective syntheses of cyclic terpenes by In-catalyzed allylation utilizing a chiral prolinol-based ligand (42). While these examples collectively illustrate the utility of chiral ligands in stereoselective macrocyclizations, such examples remain limited.We envisioned a different role for chiral catalysts when addressing intrinsically disfavored macrocyclization reactions. When unfavorable macrocyclization reactions are confronted, we hypothesized that a catalyst–substrate interaction might provide transient conformational restriction that could promote macrocyclization. To address this question, we chose to explore whether or not a chiral catalyst-controlled macrocyclization might be possible with peptidyl copper complexes. In the context of the medicinally ubiquitous diarylmethane scaffold, we had previously demonstrated the capacity for remote asymmetric induction in a series of bimolecular desymmetrizations using bifunctional, tetramethylguanidinylated peptide ligands. For example, we showed that peptidyl copper complexes were able to differentiate between the two aryl bromides during C–C, C–O, and C–N cross-coupling reactions (Fig. 1C) (43–45). Moreover, in these intermolecular desymmetrizations, a correlation between enantioselectivity and conversion was observed, revealing the catalyst’s ability to perform not only enantiotopic group discrimination but also kinetic resolution on the monocoupled product as the reaction proceeds (44). This latter observation stimulated our speculation that if an internal nucleophile were present to undergo intramolecular cross-coupling to form a macrocycle, stereochemically sensitive interactions (so-called matched and mismatched effects) (46) could be observed (Fig. 1D). Ideally, we anticipated that transition state–stabilizing interactions might even prove decisive in matched cases, and the absence of catalyst–substrate stabilizing interactions might account for the absence of macrocyclization for these otherwise intrinsically unfavorable reactions. Herein, we disclose the explicit observation of these effects in chiral catalyst-controlled macrocyclization reactions.     

Topological gelation of reconnecting polymers

DNA recombination is a ubiquitous process that ensures genetic diversity. Contrary to textbook pictures, DNA recombination, as well as generic DNA translocations, occurs in a confined and highly entangled environment. Inspired by this observation, here, we investigate a solution of semiflexible polymer rings undergoing generic cutting and reconnection operations under spherical confinement. Our setup may be realized using engineered DNA in the presence of recombinase proteins or by considering micelle-like components able to form living (or reversibly breakable) polymer rings. We find that in such systems, there is a topological gelation transition, which can be triggered by increasing either the stiffness or the concentration of the rings. Flexible or dilute polymers break into an ensemble of short, unlinked, and segregated rings, whereas sufficiently stiff or dense polymers self-assemble into a network of long, linked, and mixed loops, many of which are knotted. We predict that the two phases should behave qualitatively differently in elution experiments monitoring the escape dynamics from a permeabilized container. Besides shedding some light on the biophysics and topology of genomes undergoing DNA reconnection in vivo, our findings could be leveraged in vitro to design polymeric complex fluids—e.g., DNA-based complex fluids or living polymer networks—with desired topologies.

Recombination of genetic material involves the transient cleavage of two DNA segments that are spatially proximate in three dimensions (3D)—although not necessarily adjacent in one dimension—followed by alternative rejoining of DNA ends. Beyond its role in meiosis (1), similar topological processes involving the reconnection of DNA segments are also seen in the proliferation of transposable elements (2, 3) and the integration of viral DNA in the host genome (4). More recently, artificially driven DNA translocation- and recombination-like events have been used to map highly accessible genomic sites (5) and scramble synthetic yeast chromosomes (6). Recombination operations on a plasmid in vitro are known to yield linked or knotted DNA products (7–9). This observation suggests that unrestricted DNA recombination in vivo may pose a pressing topological problem to the cell (10, 11), but also that recombination may be employed to design topologically nontrivial DNA molecules.Enzyme-mediated recombination has been well studied on short plasmids in dilute conditions (8, 12–15). On the other hand, the topological consequences of recombination operations on long and entangled DNA are far less investigated or understood. Inspired by this problem, here, we study a system of ring polymers continuously undergoing cutting and reconnection operations—hereafter called “reconnecting” or “recombinant” rings—inside a sphere (Fig. 1A). We note that, at variance with meiotic recombination, where two finite chromosome sections are exchanged, our reconnection operations are performed by introducing an exchange event on a single site, followed by alternative rejoining of the polymer segments (Fig. 1A). Therefore, our model entails a highly simplified view of recombination, and its aim is consequently limited to exploring the generic and qualitative topological feature of recombination in confinement, rather than making quantitative predictions. At the same time, our system can be viewed more generally as a confined melt of living polymer loops. Living polymers are reversibly breakable: Like the polymers in Fig. 1A, they can break and rejoin locally (i.e., reconnect), while remaining in thermal equilibrium (16). A melt of living polymer loops can form spontaneously following polymerization of monomers, given an appropriate choice of the reaction kinetics (17). As we discuss below, such melts can, in principle, be realized experimentally and, hence, are potentially relevant to materials science.Open in a separate windowFig. 1.Phases of reconnecting rings. (A) We study ring polymers allowed to recombine/reconnect within a sphere of radius R. (B and C) The two panels show two possible states of the system at equilibrium and after relaxing the confinement. B is a sketch of an ensemble of many, small, and mostly unlinked rings, whereas C shows an ensemble of few, long, and linked rings. The snapshots are taken after releasing confinement for ease of visualization.We discover that, depending on polymer stiffness and the radius of the confining sphere, these systems display a topological transition between a regime with many short, unlinked, and segregated rings and another one with few long, mixed, and linked rings. Geometrically, this transition is naturally explained as the result of a competition between bending energy of the loops and entropy of the system. Topologically, it can be seen as a gelation transition and understood in terms of the critical overlap concentration c*, above which linking is expected to be entropically favored. Our gel of recombinant polymers is fundamentally different from other types of topological gels formed by nearly monodisperse loops in the presence of topoisomerase-like enzymes, such as the kinetoplast DNA network (18) or Olympic gels (19, 20), because in our case, multicomponent links are generically polydisperse and typically contain one or very few loops that are much longer than the rest and are often knotted. Similarly, our setup is distinct from that of previous works investigating the segregation of fixed-size polymer rings in a melt or under confinement (21–28) because the latter did not consider reconnection operations, which can change loop sizes and global topology.The topological gelation we find in this work suggests that unregulated single-site reconnection of DNA in vivo should be highly detrimental. On the other hand, being able to construct a phase diagram for gelation in vitro can be useful from a materials-science perspective, as it may provide an avenue to design experiments with DNA rings, which undergo recombination under confinement to yield linked and knotted products with desired topologies or materials with topologically controlled mesoscopic properties—e.g., Olympic ring-like gels (19, 20).     

Mechanical force effect on the two-state equilibrium of the hyaluronan-binding domain of CD44 in cell rolling

CD44 is the receptor for hyaluronan (HA) and mediates cell rolling under fluid shear stress. The HA-binding domain (HABD) of CD44 interconverts between a low-affinity, ordered (O) state and a high-affinity, partially disordered (PD) state, by the conformational change of the C-terminal region, which is connected to the plasma membrane. To examine the role of tensile force on CD44-mediated rolling, we used a cell-free rolling system, in which recombinant HABDs were attached to beads through a C-terminal or N-terminal tag. We found that the rolling behavior was stabilized only at high shear stress, when the HABD was attached through the C-terminal tag. In contrast, no difference was observed for the beads coated with HABD mutants that constitutively adopt either the O state or the PD state. Steered molecular dynamics simulations suggested that the force from the C terminus disrupts the interaction between the C-terminal region and the core of the domain, thus providing structural insights into how the mechanical force triggers the allosteric O-to-PD transition. Based on these results, we propose that the force applied from the C terminus enhances the HABD–HA interactions by inducing the conformational change to the high-affinity PD transition more rapidly, thereby enabling CD44 to mediate lymphocyte trafficking and hematopoietic progenitor cell homing under high-shear conditions.Leukocyte extravasation from blood to sites of infection and inflammation or to specific organs is achieved by a sequential adhesion cascade: (i) rolling, (ii) chemokine-induced activation, (iii) firm adhesion, and (iv) transcellular migration. Rolling is mediated by specialized cell surface adhesion molecules, such as selectins, CD44, and specific types of integrins (1, 2).Under conditions of hydrodynamic flow, receptor–ligand bonds are subjected to tensile mechanical force, which disrupts the receptor–ligand bond (Fig. 1A). In general, the lifetime of the receptor–ligand bond exponentially decreases with an increase of the mechanical force (3). However, there is growing evidence demonstrating that the lifetimes of some receptor–ligand bonds increase when moderate levels of force are applied (4–9). However, the underlying mechanism of this phenomenon is still elusive and in some cases controversial. For example, integrin and bacterial adhesin FimH-mediated adhesion have been explained by an “allosteric model,” in which mechanical force induces allosteric changes of the receptor, resulting in the stabilization of the high-affinity state (10, 11). Although selectin-mediated adhesion has been explained by the allosteric model (12), a different “sliding-rebinding model” was also reported (13). This model proposes that force tilts the binding interface to make it parallel to the direction of force, allowing the selectin ligand to slide on the selectin and to form new contacts. The sliding-rebinding model has also been used to explain the force-induced activation of von Willebrand factor-mediated adhesion and actin depolymerization (6, 8).Fig. 1.The effect of the tensile force on the two-state conformations of CD44 HABD. (A) Illustration of the tensile force applied between the receptor on the cells and the immobilized ligand under the fluid shear force. (B) The crystal structure of CD44 HABD ...CD44 is a transmembrane receptor for hyaluronan (HA) (14). CD44–HA interactions are involved in various physiological and pathological processes mediated over a wide range of hydrodynamic forces, including T-lymphocyte trafficking on the endothelium (15, 16), hematopoietic progenitor cell homing into bone marrow niches (17), and the progression of atherosclerosis (18). The HA-binding domain (HABD) of CD44 adopts two distinctive conformations representing the low- and high-affinity states for HA (19–21). HABD is composed of a conserved Link module and the N- and C-terminal extension segment (22). In the ordered (O) state, the C-terminal segment is well folded (Fig. 1B) (19), whereas it becomes disordered in the partially disordered (PD) state upon ligand binding (Fig. 1C) (20). In addition, solution NMR analyses demonstrated that HABD exists in an equilibrium between the O and PD states in both the HA-unbound and HA-bound states, with a transition rate of ∼500 ms, and that HA binding induces an equilibrium shift toward the PD state (21) (Fig. 1D). The Y161A mutant, which constitutively adopts the PD state, exhibits a higher affinity than wild-type HABD, indicating that the O and PD states represent the low- and high-affinity states for HA, respectively (21) (Fig. 1E). Cells expressing the Y161A mutant exhibited firm adhesion and impaired rolling on an HA substrate, suggesting that the two-state conformations are essential for the CD44-mediated rolling under flow conditions (21).Despite the importance of the mechanical force in rolling, the means by which it affects the CD44-mediated rolling remain poorly characterized. Recently, it was reported that the rolling of CD44-expressing cells is enhanced at the higher shear stress (23), raising the possibility that CD44 possesses some mechanochemical specializations to resist higher tensile force. Considering the fact that the C terminus of CD44 HABD is connected to the plasma membrane, the force applied from the C terminus of HABD would induce the allosteric transition from the O to the PD state, thereby providing the resistance to the applied force. On the other hand, our previous NMR studies demonstrated that more than 90% of HABD adopts the PD state in the HA-bound state (21), indicating that the free energy of the PD state can be lowered upon HA binding, regardless of the presence or absence of the tensile force. Therefore, it is worthwhile to investigate whether the CD44–HA interaction is strengthened by the tensile force.To assess the effect of the tensile force on the CD44-mediated rolling, we established a cell-free rolling system using cell-sized beads, which are coated with recombinant HABDs. The effect of the tensile force can be investigated by comparing the rolling activity of the beads coated with the ligand-binding domain via the N-terminal or the C-terminal tag (Fig. 1G) (10). We compared the rolling behavior of the beads with N- or C-terminally attached HABD and found that the rolling behavior was stabilized only at higher shear stress, when HABD was attached to the beads via the C-terminal tag. Steered molecular dynamics (SMD) simulations suggested that the force from the C terminus induces the dissociation of the “mechanosensitive latch” in the C-terminal region, which triggers the conversion from the O to the PD state. Based on these results, we propose that the tensile force from the C terminus stabilizes the CD44–HA bond by inducing a rapid transition from the O to the PD state, thereby sustaining the CD44-mediated cell rolling under higher shear stress conditions.     

Scaling and spontaneous symmetry restoring of topological defect dynamics in liquid crystal

Topological defects—locations of local mismatch of order—are a universal concept playing important roles in diverse systems studied in physics and beyond, including the universe, various condensed matter systems, and recently, even life phenomena. Among these, liquid crystal has been a platform for studying topological defects via visualization, yet it has been a challenge to resolve three-dimensional structures of dynamically evolving singular topological defects. Here, we report a direct confocal observation of nematic liquid crystalline defect lines, called disclinations, relaxing from an electrically driven turbulent state. We focus in particular on reconnections, characteristic of such line defects. We find a scaling law for in-plane reconnection events, by which the distance between reconnecting disclinations decreases by the square root of time to the reconnection. Moreover, we show that apparently asymmetric dynamics of reconnecting disclinations is actually symmetric in a comoving frame, in marked contrast to the two-dimensional counterpart whose asymmetry is established. We argue, with experimental supports, that this is because of energetically favorable symmetric twist configurations that disclinations take spontaneously, thanks to the topology that allows for rotation of the winding axis. Our work illustrates a general mechanism of such spontaneous symmetry restoring that may apply beyond liquid crystal, which can take place if topologically distinct asymmetric defects in lower dimensions become homeomorphic in higher dimensions and if the symmetric intermediate is energetically favorable.

Topologically nontrivial configurations of order, called topological defects, may appear generically and spontaneously when order is formed. As such, topological defects have been studied in diverse disciplines (1, 2), including cosmology (3), crystals and liquid crystals (2), superconductivity and superfluid (4–9), and biology (10–19) to name but a few. While there exist various kinds of defects characterized by different symmetries and properties, defects may also enjoy common properties across different disciplines. In this context, liquid crystal has the advantage that it is amenable to direct optical observations; various compounds and techniques exist; and as a soft matter system, it shows large response to external fields, being suitable for studying nonequilibrium and nonlinear effects (2, 20). This advantage has been recognized and used for decades, with a notable example of observing liquid crystal defects to test predictions for cosmic strings (21). Moreover, the scope of studies of liquid crystalline defects has been recently extended remarkably, including the use of defects as templates for molecular self-assembly (22) and the recent surge of investigations of active nematic systems bearing relevance to life phenomena (10–19).Despite this history, resolving fully three-dimensional (3D) structures of liquid crystal defects has not been straightforward, even for the simplest kind of defects, namely nematic disclination lines. Well-known techniques for 3D observation of defects and other orientational structures are fluorescence confocal polarizing microscopy (23, 24) and two- or three-photon excitation fluorescence polarizing microscopy (25–27). Both techniques allow one to reconstruct the 3D structure of the director field, by which one can determine the position and structure of defects in principle. To do so, however, one needs to reduce the effect of defocusing and polarization changes due to the birefringence of liquid crystal. For singular defects, such as nematic disclinations, scattering at the core gives another difficulty. The effect of birefringence can be significantly reduced by partial polymerization of the medium (28), but this cannot be used to study dynamics of defects.Here, we propose a method to capture dynamically evolving 3D structures of nematic disclination lines by using confocal microscopy and a recently reported accumulation of fluorescent dyes around the singular core of defects (29). This method allows us to visualize the disclinations directly (Fig. 1), without reconstructing and analyzing the director field. Using this technique, we observe reconnections of disclinations—a hallmark of such topological defect lines—and characterize the reconnection dynamics in terms of scaling and symmetry.Open in a separate windowFig. 1.Reconnections and loop shrinkage. (A–C) Sketches of an in-plane reconnection (A), an intersecting reconnection (B), and a loop shrinkage (C). (D–F) Confocal observations of an in-plane reconnection (D), an intersecting reconnection (E), and a loop shrinkage (F) (Movies S1–S4). E, Insets display side views of the event shown in E. (Scale bars: D–F, 50 μm; E, Insets, 20 μm.).     

In situ structure of intestinal apical surface reveals nanobristles on microvilli

Microvilli are actin-bundle-supported membrane protrusions essential for absorption, secretion, and sensation. Microvilli defects cause gastrointestinal disorders; however, mechanisms controlling microvilli formation and organization remain unresolved. Here, we study microvilli by vitrifying the Caenorhabditis elegans larvae and mouse intestinal tissues with high-pressure freezing, thinning them with cryo-focused ion-beam milling, followed by cryo-electron tomography and subtomogram averaging. We find that many radial nanometer bristles referred to as nanobristles project from the lateral surface of nematode and mouse microvilli. The C. elegans nanobristles are 37.5 nm long and 4.5 nm wide. Nanobristle formation requires a protocadherin family protein, CDH-8, in C. elegans. The loss of nanobristles in cdh-8 mutants slows down animal growth and ectopically increases the number of Y-shaped microvilli, the putative intermediate structures if microvilli split from tips. Our results reveal a potential role of nanobristles in separating microvilli and suggest that microvilli division may help generate nascent microvilli with uniformity.

Microvilli are membrane-bound cell-surface protrusions that contain a core bundle of actin filaments enveloped in the plasma membrane (1–3). Many epithelial cells develop microvilli above their apical surface to enhance functional capacity for a range of physiological tasks, including nutrient absorption in the intestine (4), solute uptake in the renal tubules (5), mechanosensation in sensory stereocilia of the inner ear (6), and chemosensation in the gut, lung, and urogenital tracts (7–9). Abnormal microvillar structure and function lead to human disorders, such as life-threatening nutrient malabsorption, osmotic imbalances, and inherited deafness in Usher syndrome (1, 3, 4, 10).An intestinal absorptive cell enterocyte develops up to 1,000 densely packed microvilli in an array known as the brush border. These fingerlike outward projections enhance the functional surface area for nutrient absorption and provide the barrier for host defense against pathogens and toxins (1, 3, 4). Because the gut epithelium undergoes constant regenerative renewal, microvillus assembly is a process that continues throughout our lifetime (1, 3, 4, 11). The long-standing questions regarding microvillus formation are how microvilli are formed with striking uniformity in sizes and how these protrusions are maximally packed in a hexagonal pattern.The tip of microvilli is known to be decorated by additional filamentous structures. The glycoprotein-rich glycocalyx localizes between the apical tip of microvilli and the luminal space (Fig. 1A), provides a barrier for pathogens, and serves as the interface for nutrient digestion (3, 4). The protocadherin-based adhesion tip links localize between adjacent microvilli (Fig. 1A) (12). The mammalian cadherin superfamily members, including CDHR2 and CDHR5, play essential roles in packing microvilli, increasing surface density, and controlling microvilli length (13). Other cadherins, specifically CDH23 and PCDH15, have been implicated in organizing the exaggerated microvilli found on inner-ear hair cells (14, 15).Open in a separate windowFig. 1.In situ cryo-ET of the C. elegans intestinal brush border reveals nanobristles on the lateral surface of microvilli. (A) A schematic diagram of an intestinal epithelial cell (Left) and two microvilli (Right) from the dotted box in Left. The glycocalyx and the protocadherin tip link are the characterized cell-surface structure at microvillar tips. This work shows that numerous nanobristles (magenta) decorate the lateral surface of microvilli. (B, Left and Center) Representative cryo-SEM images of the C. elegans L1 larvae before and after FIB milling. (Scale bars, 10 μm.) (B, Right) Representative FIB image of the ∼200-nm-thick cryo-lamella. (Scale bar, 5 μm.) (C) A 3D rendering of the C. elegans intestinal brush border showing various macromolecules and structures. Magenta, nanobristles; cyan, membrane; yellow, actin; beige, ribosome; green, mitochondria; orange, ER) Nanobristles and ribosomes were mapped back in the tomogram with the computed location and orientation. (D) A selected microvillus from E magnified for visualization. (E and F) Cryo-ET tomogram slices of microvilli (E, top view; F, side view). (Scale bars in C–F, 50 nm.)An individual microvillus is only 0.1 μm in diameter and 1 to ∼2 μm in height (Fig. 1A), the tiny dimension of which, along with their high density and lumen localization, becomes a technical hurdle for in situ structural investigations at high resolution (1–3). Despite our understanding of microvillar tip decorations, it is unclear whether any structure projects from the lateral surface of microvilli. Recent methodology advance of cryo-electron tomography (cryo-ET) makes the platform well suited to address the challenges of studying microvillus structure in animals (16, 17). Here, we used cryo-ET to reveal a previously unrecognized nanobristle structure on the lateral surface of microvilli. We provide evidence that nanobristle formation depends on a protocadherin family protein, CDH-8, and that nanobristles regulate microvilli separation.     

Gyrification from constrained cortical expansion

The exterior of the mammalian brain—the cerebral cortex—has a conserved layered structure whose thickness varies little across species. However, selection pressures over evolutionary time scales have led to cortices that have a large surface area to volume ratio in some organisms, with the result that the brain is strongly convoluted into sulci and gyri. Here we show that the gyrification can arise as a nonlinear consequence of a simple mechanical instability driven by tangential expansion of the gray matter constrained by the white matter. A physical mimic of the process using a layered swelling gel captures the essence of the mechanism, and numerical simulations of the brain treated as a soft solid lead to the formation of cusped sulci and smooth gyri similar to those in the brain. The resulting gyrification patterns are a function of relative cortical expansion and relative thickness (compared with brain size), and are consistent with observations of a wide range of brains, ranging from smooth to highly convoluted. Furthermore, this dependence on two simple geometric parameters that characterize the brain also allows us to qualitatively explain how variations in these parameters lead to anatomical anomalies in such situations as polymicrogyria, pachygyria, and lissencephalia.The mammalian brain is functionally and anatomically complex. Over the years, accumulating evidence (1, 2) shows that there are strong anatomical correlates of its information-processing ability; indeed the iconic convoluted shape of the human brain is itself used as a symbol of its functional complexity. This convoluted (gyrified) shape is associated with the rapid expansion of the cerebral cortex. Understanding the evolutionary and developmental origins of the cortical expansion (1–6) and their mechanistic role in gyrification is thus an important question that needs to be answered to decipher the functional complexity of the brain.Historically there have been three broad hypotheses about the origin of sulci and gyri. The first is that gyri rise above sulci by growing more (7), requiring the pattern of sulci and gyri to be laid down before the cortex folds, presumably by a chemical morphogen. There is no evidence for this mechanism. The second hypothesis considers that the outer gray matter consists of neurons, and the inner white matter is largely long thin axons that connect the neurons to each other and to other parts of the nervous system and proposes that these axons pull mechanically, drawing together highly interconnected regions of gray matter to form gyri (8–10). However, recent experimental evidence (11) shows that axonal tension when present is weak and arises deep in the white matter and is thus insufficient to explain the strongly deformed gyri and sulci. The third hypothesis is that the gray matter simply grows more than the white matter, an experimentally confirmed fact, leading to a mechanical buckling that shapes the cortex (11–14). Evidence for this hypothesis has recently been provided by observations of mechanical stresses in developing ferret brains (11), which were found to be in patterns irreconcilable with the axonal tension hypothesis. In addition, experiments show that sulci and gyri can be induced in usually smooth-brained mice by genetic manipulations that promote cortical expansion (15, 16), suggesting that gyrification results from an unregulated and unpatterned growth of the cortex relative to sublayers.Nevertheless, there is as yet no explicit biologically and physically plausible model that can convincingly reproduce individual sulci and gyri, let alone the complex patterns of sulci and gyri found in the brain. Early attempts to mechanically model brain folding (13) were rooted in the physics of wrinkling and assumed a thin stiff layer of gray matter that grows relative to a thick soft substrate of white matter. This model falls short in two ways. First, the gray matter is neither thin nor stiff relative to the white matter (17, 18). Second, this model predicts smooth sinusoidal wrinkling patterns, sketched in Fig. 1A, whereas even lightly folded brains have smooth gyri but cusped sulci. More complicated mechanical models including, e.g., elasto-plasticity and stress-related growth (14, 19, 20), lead to varying morphologies, but all produced simple smooth convolutions rather than cusped sulci.Open in a separate windowFig. 1.Wrinkling and sulcification in a layered material subject to differential growth. (A) If the growing gray matter is much stiffer than the white matter it will wrinkle in a smooth sinusoidal way. (B) If the gray matter is much softer than the white matter its surface will invaginate to form cusped folds. (C) If the two layers have similar moduli the gray matter will both wrinkle and cusp giving gyri and sulci. Physical realizations of A, B, and C, based on differential swelling of a bilayer gel (Materials and Methods), confirm this picture and are shown in D, E, and F, respectively.A fundamentally different mechanical instability that occurs on the surface of a uniformly compressed soft solid (21, 22) has recently been exposed and clarified, theoretically, computationally, and experimentally (23–26). This sulcification instability arises under sufficient compression leading to the folding of the soft surface to form cusped sulci via a strongly subcritical transition. In Fig. 1B, we show a geometry dual to that associated with wrinkling: A soft layer of gray matter grows on a stiff white-matter substrate. Unlike wrinkling, this instability can produce the cusped centers of sulci, but the flat bottom of the gray matter is not seen in the brain. This is a consequence of the assumption that the gray matter is much softer than the white matter—in reality the two have very similar stiffnesses (17, 18). We are thus led to the final simple alternate, sketched in Fig. 1C, where the stiffnesses of the gray and white matter are assumed to be identical. Such a system is subject to a cusp-forming sulcification instability discussed earlier, and can lead to an emergent pattern very reminiscent of sulci and gyri in the brain.     

Inside-out regulation of E-cadherin conformation and adhesion

Cadherin cell–cell adhesion proteins play key roles in tissue morphogenesis and wound healing. Cadherin ectodomains bind in two conformations, X-dimers and strand-swap dimers, with different adhesive properties. However, the mechanisms by which cells regulate ectodomain conformation are unknown. Cadherin intracellular regions associate with several actin-binding proteins including vinculin, which are believed to tune cell–cell adhesion by remodeling the actin cytoskeleton. Here, we show at the single-molecule level, that vinculin association with the cadherin cytoplasmic region allosterically converts weak X-dimers into strong strand-swap dimers and that this process is mediated by myosin II–dependent changes in cytoskeletal tension. We also show that in epithelial cells, ∼70% of apical cadherins exist as strand-swap dimers while the remaining form X-dimers, providing two cadherin pools with different adhesive properties. Our results demonstrate the inside-out regulation of cadherin conformation and establish a mechanistic role for vinculin in this process.

E-cadherins (Ecads) are essential, calcium-dependent cell–cell adhesion proteins that play key roles in the formation of epithelial tissue and in the maintenance of tissue integrity. Ecad adhesion is highly plastic and carefully regulated to orchestrate complex movement of epithelial cells, and dysregulation of adhesion is a hallmark of numerous cancers (1). However, little is known about how cells dynamically regulate the biophysical properties of individual Ecads.The extracellular region of Ecads from opposing cells bind in two distinct trans orientations: strand-swap dimers and X-dimers (Fig. 1 A and B). Strand-swap dimers are the stronger cadherin adhesive conformation and are formed by the exchange of conserved tryptophan (Trp) residues between the outermost domains of opposing Ecads (2–4). In contrast, X-dimers, which are formed by extensive surface interactions between opposing Ecads, are a weaker adhesive structure and serve as an intermediate during the formation and rupture of strand-swap dimers (5–7). Using cell-free, single-molecule experiments we previously showed that X-dimers and strand-swap dimers can be distinguished based on their distinctly different response to mechanical force. When a strand-swap dimer is pulled, its lifetime decreases with increasing force, resulting in the formation of a slip bond (8, 9) (Fig. 1B). In contrast, an X-dimer responds to pulling force by forming a catch bond, where bond lifetime initially increases up to a threshold force and then subsequently decreases (8, 10) (Fig. 1B). It has also been shown that wild-type Ecad ectodomains in solution can interconvert between X-dimer and strand-swap dimer conformations (9, 11). However, the biophysical mechanisms by which Ecad conformations (and adhesion) are regulated on the cell surface are unknown.Open in a separate windowFig. 1.Overview of experiment. (A) The extracellular region of Ecad from opposing cells mediates adhesion. The cytoplasmic region of Ecad associates either directly or indirectly with p120 catenin, β-catenin, α-catenin, vinculin, and F-actin. (B) Strand-swap dimers form slip bonds (blue) and X-dimers form catch bonds (red). Ecads interconvert between these two dimer conformations. Structures were generated from the crystal structure of mouse Ecad (PDB ID code 3Q2V); the X-dimer was formed by alignment to an X-dimer crystal structure (PDB ID code 3LNH). (C) Graphics showing the cell lines used in experiments and Western blot analysis of corresponding cell lysates.The cytoplasmic region of Ecad associates with the catenin family of proteins, namely, p120-catenin, β-catenin, and α-catenin. The Ecad–catenin complex, in turn, links to filamentous actin (F-actin) either by the direct binding of α-catenin and F-actin or by the indirect association of α-catenin and F-actin via vinculin (12) (Fig. 1A). Adhesive forces transmitted across intercellular junctions by Ecad induce conformational changes in α-catenin (13, 14), strengthen F-actin binding (15), and recruit vinculin to the sites of force application (16, 17). However, vinculin and α-catenin do not merely serve as passive cytoskeletal linkers; they also dynamically modulate cytoskeletal rearrangement and recruit myosin to cell–cell junctions (13, 18–20). Studies show that α-catenin and vinculin play important roles in strengthening and stabilizing Ecad adhesion: bead-twisting experiments show force-induced stiffening of Ecad-based junctions and cell doublet stretching experiments demonstrate reinforcement of cell–cell adhesion in vinculin- and α-catenin–dependent manners (18, 19, 21).Currently, actin anchorage and cytoskeletal remodeling are assumed to be the exclusive mechanisms by which α-catenin and vinculin strengthen Ecad adhesion (22–24). Here, we directly map the allosteric effects of cytoplasmic proteins on Ecad ectodomain conformation and demonstrate, at the single-molecule level, that vinculin association with the Ecad cytoplasmic region switches X-dimers to strand-swap dimers. We show that cytoskeletal tension, due to vinculin-mediated recruitment of myosin II, regulates Ecad ectodomain structure and adhesion. Finally, we demonstrate that only ∼50% of Ecads are linked to the underlying cytoskeleton and that while about 70% of Ecads form strand-swap dimers the remaining form X-dimers, which provides cells with two Ecad pools with different adhesive properties.