Programmable matter by folding

Programmable matter is a material whose properties can be programmed to achieve specific shapes or stiffnesses upon command. This concept requires constituent elements to interact and rearrange intelligently in order to meet the goal. This paper considers achieving programmable sheets that can form themselves in different shapes autonomously by folding. Past approaches to creating transforming machines have been limited by the small feature sizes, the large number of components, and the associated complexity of communication among the units. We seek to mitigate these difficulties through the unique concept of self-folding origami with universal crease patterns. This approach exploits a single sheet composed of interconnected triangular sections. The sheet is able to fold into a set of predetermined shapes using embedded actuation. To implement this self-folding origami concept, we have developed a scalable end-to-end planning and fabrication process. Given a set of desired objects, the system computes an optimized design for a single sheet and multiple controllers to achieve each of the desired objects. The material, called programmable matter by folding, is an example of a system capable of achieving multiple shapes for multiple functions.     

Mechanosensitive remodeling of the bacterial flagellar motor is independent of direction of rotation

Motility is important for the survival and dispersal of many bacteria, and it often plays a role during infections. Regulation of bacterial motility by chemical stimuli is well studied, but recent work has added a new dimension to the problem of motility control. The bidirectional flagellar motor of the bacterium Escherichia coli recruits or releases torque-generating units (stator units) in response to changes in load. Here, we show that this mechanosensitive remodeling of the flagellar motor is independent of direction of rotation. Remodeling rate constants in clockwise rotating motors and in counterclockwise rotating motors, measured previously, fall on the same curve if plotted against torque. Increased torque decreases the off rate of stator units from the motor, thereby increasing the number of active stator units at steady state. A simple mathematical model based on observed dynamics provides quantitative insight into the underlying molecular interactions. The torque-dependent remodeling mechanism represents a robust strategy to quickly regulate output (torque) in response to changes in demand (load).

Many bacteria swim through aqueous environments to acquire resources, to disperse progeny, and to infect hosts (1, 2). The rotation of flagella (3, 4), powered by the bidirectional flagellar motor (5–7), drives motility in many bacteria. In Escherichia coli, the flagellar motor consists of over 20 different proteins that self-assemble at the cell wall in varying copy numbers (8–10). Motor structure (Fig. 1A) includes a rotor embedded in the inner cell membrane, a drive shaft, and a flexible hook that transmits torque to the filament (10, 11). The cytoplasmic ring (C ring), which contains copies of the proteins FliG, FliM, and FliN, is mounted on the cytoplasmic face of the rotor and is responsible for directional switching of the motor (12). The rotor is driven by up to 11 ion-powered MotA5B2 stator units (13–16) that surround the rotor and generate torque. MotA engages FliG, whereas MotB is mounted on the rigid framework of the peptidoglycan cell wall (17–20). Motor-bound units exchange with a pool of unbound units in the inner membrane (10, 21).Open in a separate windowFig. 1.Bacterial flagellar motor’s structure and its T-S curve. (A) Schematic representation of the flagellar motor of Gram-negative bacteria. The rotor consists of the MS ring (M for membranous and S for supramembranous) embedded in the inner membrane (IM) and the C ring embedded in the cytoplasm. Stator units (MotA–MotB complexes) that span the inner membrane bind to the peptidoglycan (PG) layer and apply torque on the C ring. The torque is transmitted via a rod (driveshaft) and a flexible hook (universal joint) to the flagellar filament. L and P rings (L for lipopolysaccharide and P for peptidoglycan) are embedded in the outer membrane (OM) and the peptidoglycan, respectively, and act as bushings. Inset shows the outline of an E. coli cell with a square demarcating the region that is represented in detail. (B) T-S curve of the CCW (solid orange) and CW (dashed blue) rotating flagellar motors compared in this study. Data are from refs. 31 and 58. See Materials and Methods for details.Motor function is regulated by inputs from the environment. Detection of specific ligands by chemoreceptors drives a two-component signaling cascade that controls the direction of rotation of the motor (22–24). Upon binding the response regulator CheY-P, the C ring undergoes a concerted conformational change that reverses motor rotation from counterclockwise (CCW) to clockwise (CW), as viewed from outside the cell. This change in the direction of rotation is the basis of run-and-tumble motility in E. coli (CCW = run, CW = tumble). Changes in viscous load trigger remodeling of the stator (25–27), whereby, at high loads, the number of motor-bound stator units increases, and vice versa. Dynamics of stator remodeling have only been quantified in CCW rotating motors, using electrorotation (28) and magnetic tweezers (29, 30). The observed dynamics were rationalized using the CCW torque–speed (T-S) relationship (Fig. 1B) (28). CCW and CW rotating motors have different T-S relationships (Fig. 1B), likely due to differences in stator–rotor interactions (31–33). How the differences in T-S relationship affect stator remodeling in CW motors is unknown. Additionally, the molecular mechanisms underlying the load-dependent remodeling phenomenon remain poorly understood.Here, we use electrorotation to study the dynamics of load-dependent stator remodeling in CW rotating motors. We find that, just like CCW motors, CW rotating flagellar motors release their stator units when the motor torque is low, and recruit stator units when the torque increases again. The rates of stator unit release and recruitment in CW and CCW motors collapse onto a single curve when plotted against torque, despite their dissimilar T-S relationships. The collapse of remodeling data suggests a universal model for torque dependence in the mechanically regulated remodeling of the bacterial flagellar motor. Our in vivo measurements of stator assembly dynamics advance the understanding of a large protein complex with multiple parts.     

釉质仿生矿化模型的研究

目的：构建釉质生物矿化的模型,包括有机基质模板（类釉原蛋白寡肽序列）的建立和无机离子供体（包裹钙磷离子的温度敏感性脂质体）的合成,体外实现类釉质样结构的再矿化。方法：首先通过标准固相法合成所需“类釉原蛋白”寡肽[（Gln?Pro?Ala）4?Thr?Lys?Arg?Glu?Glu?Val?Asp],并用CaCl2溶液诱导其进行自组装;其次采用二棕榈酰磷脂酰胆碱和二肉豆蔻卵磷脂为原料,通过相交融合法分别合成包裹钙、磷离子的温度敏感性脂质体;最后在37℃时,将酸蚀后的牙片浸泡在包裹钙、磷离子的脂质体与寡肽的混悬液中,作为实验组。将酸蚀后的牙片浸泡在包裹钙磷离子的脂质体混悬液中,作为对照组,促进脱矿后的釉质再矿化。矿化后的牙片通过扫描电镜（ SEM）、傅立叶变换红外光谱仪（ FTIR）和X射线衍射仪（ XRD）进行表征。结果：实验组的脱矿釉质表面均匀有序的沉积了一层釉质样的羟基磷灰石晶体（ HA）结构,而对照组只沉积了较少量无序的HA晶体。结论：通过类釉原蛋白寡肽有机矿化模板的建立,以及仿生釉质矿化过程中钙、磷离子的输送,构建了釉质仿生矿化模型,并实现了脱矿釉质表面类釉质样微结构的再生。     

Structure,self-assembly,and properties of a truncated reflectin variant

Naturally occurring and recombinant protein-based materials are frequently employed for the study of fundamental biological processes and are often leveraged for applications in areas as diverse as electronics, optics, bioengineering, medicine, and even fashion. Within this context, unique structural proteins known as reflectins have recently attracted substantial attention due to their key roles in the fascinating color-changing capabilities of cephalopods and their technological potential as biophotonic and bioelectronic materials. However, progress toward understanding reflectins has been hindered by their atypical aromatic and charged residue-enriched sequences, extreme sensitivities to subtle changes in environmental conditions, and well-known propensities for aggregation. Herein, we elucidate the structure of a reflectin variant at the molecular level, demonstrate a straightforward mechanical agitation-based methodology for controlling this variant’s hierarchical assembly, and establish a direct correlation between the protein’s structural characteristics and intrinsic optical properties. Altogether, our findings address multiple challenges associated with the development of reflectins as materials, furnish molecular-level insight into the mechanistic underpinnings of cephalopod skin cells’ color-changing functionalities, and may inform new research directions across biochemistry, cellular biology, bioengineering, and optics.

Materials from naturally occurring and recombinant proteins are frequently employed for the study of fundamental biological processes and leveraged for applications in fields as diverse as electronics, optics, bioengineering, medicine, and fashion (1–13). Such broad utility is enabled by the numerous advantageous characteristics of protein-based materials, which include sequence modularity, controllable self-assembly, stimuli-responsiveness, straightforward processability, inherent biological compatibility, and customizable functionality (1–13). Within this context, unique structural proteins known as reflectins have recently attracted substantial attention because of their key roles in the fascinating color-changing capabilities of cephalopods, such as the squid shown in Fig. 1A, and have furthermore demonstrated their utility for unconventional biophotonic and bioelectronic technologies (11–40). For example, in vivo, Bragg stack-like ultrastructures from reflectin-based high refractive index lamellae (membrane-enclosed platelets) are responsible for the angle-dependent narrowband reflectance (iridescence) of squid iridophores, as shown in Fig. 1B (15–20). Analogously, folded membranes containing distributed reflectin-based particle arrangements within sheath cells lead to the mechanically actuated iridescence of squid chromatophore organs, as shown in Fig. 1C (15, 16, 21, 22). Moreover, in vitro, films processed from squid reflectins not only exhibit proton conductivities on par with some state-of-the-art artificial materials (23–27) but also support the growth of murine and human neural stem cells (28, 29). Additionally, morphologically variable coatings assembled from different reflectin isoforms can enable the functionality of chemically and electrically actuated color-changing devices, dynamic near-infrared camouflage platforms, and stimuli-responsive photonic architectures (27, 30–34). When considered together, these discoveries and demonstrations constitute compelling motivation for the continued exploration of reflectins as model biomaterials.Open in a separate windowFig. 1.(A) A camera image of a D. pealeii squid for which the skin contains light-reflecting cells called iridophores (bright spots) and pigmented organs called chromatophores (colored spots). Image credit: Roger T. Hanlon (photographer). (B) An illustration of an iridophore (Left), which shows internal Bragg stack-like ultrastructures from reflectin-based lamellae (i.e., membrane-enclosed platelets) (Inset). (C) An illustration of a chromatophore organ (Left), which shows arrangements of reflectin-based particles within the sheath cells (Inset). (D) The logo of the 28-residue-long N-terminal motif (RMN), which depicts the constituent amino acids (Upper) and their predicted secondary structures (Lower). (E) The logo of the 28-residue-long internal motif (RMI), which depicts the constituent amino acids (Upper) and their predicted secondary structures (Lower). (F) The logo of the 21-residue-long C-terminal motif (RMC), which depicts the constituent amino acids (Upper) and their predicted secondary structures (Lower). (G) The amino acid sequence of full-length D. pealeii reflectin A1, which contains a single RMN motif (gray oval) and five RMI motifs (orange ovals). (H) An illustration of the selection of the prototypical truncated reflectin variant (denoted as RfA1TV) from full-length D. pealeii reflectin A1.Given reflectins’ demonstrated significance from both fundamental biology and applications perspectives, some research effort has been devoted to resolving their three-dimensional (3D) structures (30, 31, 35–39). For example, fibers drawn from full-length Euprymna scolopes reflectin 1a and films processed from truncated E. scolopes reflectin 1a were shown to possess secondary structural elements (i.e., α-helices or β-sheets) (30, 31). In addition, precipitated nanoparticles and drop-cast films from full-length Doryteuthis pealeii reflectin A1 have exhibited β-character, which was seemingly associated with their conserved motifs (35, 36). Moreover, nanoparticles assembled from both full-length and truncated Sepia officinalis reflectin 2 variants have demonstrated signatures consistent with β-sheet or α-helical secondary structure, albeit in the presence of surfactants (38). However, such studies were made exceedingly challenging by reflectins’ atypical primary sequences enriched in aromatic and charged residues, documented extreme sensitivities to subtle changes in environmental conditions, and well-known propensities for poorly controlled aggregation (12, 14, 15, 30–32, 34–39). Consequently, the reported efforts have all suffered from multiple drawbacks, including the need for organic solvents or denaturants, the evaluation of only polydisperse or aggregated (rather than monomeric) proteins, a lack of consensus among different experimental techniques, inadequate resolution that precluded molecular-level insight, imperfect agreement between computational predictions and experimental observations, and/or the absence of conclusive correlations between structure and optical functionality. As such, there has emerged an exciting opportunity for investigating reflectins’ molecular structures, which remain poorly understood and the subject of some debate.Herein, we elucidate the structure of a reflectin variant at the molecular level, demonstrate a robust methodology for controlling this variant’s hierarchical assembly, and establish a direct correlation between its structural characteristics and optical properties. We first rationally select a prototypical reflectin variant expected to recapitulate the behavior of its parent protein by using a bioinformatics-guided approach. We next map the conformational and energetic landscape accessible to our selected protein by means of all-atom molecular dynamics (MD) simulations. We in turn produce our truncated reflectin variant with and without isotopic labeling, develop solution conditions that maintain the protein in a monomeric state, and characterize the variant’s size and shape with small-angle X-ray scattering (SAXS). We subsequently resolve our protein’s dynamic secondary and tertiary structures and evaluate its backbone conformational fluctuations with NMR spectroscopy. Finally, we demonstrate a straightforward mechanical agitation-based approach to controlling our truncated reflectin variant’s secondary structure, hierarchical self-assembly, and bulk refractive index distribution. Overall, our findings address multiple challenges associated with the development of reflectins as materials, furnish molecular-level insight into the mechanistic underpinnings of cephalopod skin cells’ color-changing functionalities, and appear poised to inform new directions across biochemistry, cellular biology, bioengineering, and optics.     

Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies

Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal’s curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies—potentially as more efficient mimics of structural color as it occurs in nature.Hierarchical design principles, i.e., the structuration of material over multiple length scales, are ubiquitously used in nature to maximize functionality from a limited choice of available components. Hierarchically structured materials often provide better performance than their unstructured counterparts and novel properties can arise solely from the multiscale structural arrangement. Examples can be found in the extreme water repellency of the lotus leaf (1); the outstanding mechanical stability and toughness of sea creatures such as sea sponges (2) and abalone shells (3); and the bright coloration found in beetles, birds, and butterflies (4, 5).To achieve the strongest visual effects, many organisms combine optical effects arising from light interacting with structured matter at different length scales (6). Structural periodicity on the scale of visible light wavelengths can result in regular optical density variations that give rise to bright, iridescent colors due to pronounced interference effects (4). At the micron scale, regular structural features act as diffraction gratings that produce vivid, rainbow coloration (7) and are used to control scattering (8) and to direct the emission of light by mirror-like reflections (9). At the molecular level, broadband absorption, for example by melanin, can be used to reduce unwanted scattering due to structural imperfections, which, if not eliminated, can lead to a whitish appearance (10).Several advantages of structural coloration compared with pigmentation hues have led to a strong interest in mimicking nature’s photonic design principles in technology (11, 12). These advantages include increased color longevity by the absence of photobleaching; the ability to use benign and nontoxic materials; broad variation in colors arising from simple changes in geometry of the same material; and the presence of vivid optical effects such as sparkle, luster, and iridescence, not achievable by conventional pigments. However, structural color requires a high degree of control over the nano- and microscale geometry of the colored material. Self-assembly processes involving colloidal particles have been identified as an attractive route to create ordered structures at the nanoscale without the need for expensive and serial nanofabrication methods (13–15). Colloidal particles are attractive building blocks for mimicking nature’s structural coloration strategies because they can be conveniently synthesized in a range of sizes comparable to those of visible light wavelengths. Colloids self-assemble into highly ordered, close-packed crystals on flat surfaces, giving rise to structural color by constructive interference of light reflected at the individual lattice planes (13, 15).Structural hierarchy, i.e., the organization of colloidal crystals into ordered superstructures, can be achieved by imposing a confining element on the crystallization process. A simple yet interesting confinement for the crystallization process is an emulsion droplet that creates colloidal assemblies known as photonic balls (16–19). Curvature imposed by the spherical confinement is a unique structural element of photonic balls. It is known that curvature induces different types of defect structures in 2D (20, 21) and 3D assemblies (22), which can affect the resulting optical properties. Photonic balls have recently gained attention in various optical applications, including pigments (23–26), sensors (27), magnetically switchable colorants (25, 28), and color-coded substrates for biomaterials evaluation (29, 30).Here, we provide a detailed physical understanding of the different optical phenomena occurring in photonic balls and trace the origin of these properties to structural details, especially curvature. We use focused ion beam-assisted cross-sectioning to visualize the effect of confinement on the internal morphology and find ordered, layered crystal planes near the interface and a more disordered region toward the center. Tailoring the degree of confinement allows us to control the crystallinity and thus, the resulting color arising from Bragg diffraction. Disorder and defects, caused by frustrated crystallization in the curved confinement, contributes to unwanted broadband scattering of light, which compromises the macroscopic color. We introduce gold nanoparticles as subwavelength, spectrally selective absorbers into the photonic balls to suppress light scattering in spectrally unwanted regions. This synergistic combination of plasmonic absorption with Bragg diffraction leads to a macroscopically purer coloration, if the photonic crystal stop band and gold nanoparticle absorption are appropriately adjusted. Finally, we observe more complex optical signatures in the reflection of light from photonic balls assembled from larger colloidal particles (d > 400 nm) that result from grating diffraction caused by a regular arrangement of colloids at the surface of the photonic balls.Our theoretical experimental examination of the influence of curvature and confinement on the different optical phenomena enables us to create a complete picture of the rich optical properties of these simple, hierarchical self-assembled structures.     

Self-reproducing catalyst drives repeated phospholipid synthesis and membrane growth

Cell membranes are dynamic structures found in all living organisms. There have been numerous constructs that model phospholipid membranes. However, unlike natural membranes, these biomimetic systems cannot sustain growth owing to an inability to replenish phospholipid-synthesizing catalysts. Here we report on the design and synthesis of artificial membranes embedded with synthetic, self-reproducing catalysts capable of perpetuating phospholipid bilayer formation. Replacing the complex biochemical pathways used in nature with an autocatalyst that also drives lipid synthesis leads to the continual formation of triazole phospholipids and membrane-bound oligotriazole catalysts from simpler starting materials. In addition to continual phospholipid synthesis and vesicle growth, the synthetic membranes are capable of remodeling their physical composition in response to changes in the environment by preferentially incorporating specific precursors. These results demonstrate that complex membranes capable of indefinite self-synthesis can emerge when supplied with simpler chemical building blocks.Lipid membranes are ubiquitous in all domains of life. Membranes are organizing structures needed to define physical boundaries, compartmentalize molecules within the cell, and provide sites for proteins that control transport and signaling. Natural membranes are also capable of growth through in situ synthesis of glycerophospholipids catalyzed by embedded integral membrane proteins that are continually synthesized by entrapped cellular machinery (1–3). Numerous studies of artificial membranes have demonstrated the ability of various amphiphiles to self-assemble into bilayer vesicles with properties reminiscent of cellular membranes (4–8). A limited number of these studies have demonstrated that seeding artificial membranes with catalysts that are capable of either generating additional amphiphiles, or of driving the recruitment of lipids from the environment, triggers an increase in membrane surface area, and, in some cases, causes vesicle budding and division (9–14). However, membrane growth in these systems inevitably leads to dilution of the catalyst and cessation of membrane formation (15, 16). Thus, a significant roadblock to synthetic membranes capable of continual phospholipid synthesis has been the lack of a mechanism by which the embedded molecular catalysts that are responsible for membrane expansion are able to repopulate indefinitely (15, 17). Here we report on the design of a simplified lipid synthesizing membrane that uses a synthetic, membrane-embedded catalyst that is capable of self-reproduction. To achieve simultaneous lipid and catalyst synthesis we use a shared catalytic triazole coupling reaction to generate both triazole phospholipids and additional membrane-embedded copper-chelating oligotriazole catalysts (18, 19). By substituting the complex network of biochemical pathways used in nature with a single autocatalyst that simultaneously drives membrane growth, our system continually transforms simpler, high-energy building blocks into new artificial membranes.     

Immunopotentiating and Delivery Systems for HCV Vaccines

Development of preventive vaccines against hepatitis C virus (HCV) remains one of the main strategies in achieving global elimination of the disease. The effort is focused on the quest for vaccines capable of inducing protective cross-neutralizing humoral and cellular immune responses, which in turn dictate the need for rationally designed cross-genotype vaccine antigens and potent immunoadjuvants systems. This review provides an assessment of the current state of knowledge on immunopotentiating compounds and vaccine delivery systems capable of enhancing HCV antigen-specific immune responses, while focusing on the synergy and interplay of two modalities. Structural, physico-chemical, and biophysical features of these systems are discussed in conjunction with the analysis of their in vivo performance. Extreme genetic diversity of HCV-a well-known hurdle in the development of an HCV vaccine, may also present a challenge in a search for an effective immunoadjuvant, as the effort necessitates systematic and comparative screening of rationally designed antigenic constructs. The progress may be accelerated if the preference is given to well-defined molecular immunoadjuvants with greater formulation flexibility and adaptability, including those capable of spontaneous self-assembly behavior, while maintaining their robust immunopotentiating and delivery capabilities.     

SNARE tagging allows stepwise assembly of a multimodular medicinal toxin

Generation of supramolecular architectures through controlled linking of suitable building blocks can offer new perspectives to medicine and applied technologies. Current linking strategies often rely on chemical methods that have limitations and cannot take full advantage of the recombinant technologies. Here we used SNARE proteins, namely, syntaxin, SNAP25, and synaptobrevin, which form stable tetrahelical complexes that drive fusion of intracellular membranes, as versatile tags for irreversible linking of recombinant and synthetic functional units. We show that SNARE tagging allows stepwise production of a functional modular medicinal toxin, namely, botulinum neurotoxin type A, commonly known as BOTOX. This toxin consists of three structurally independent units: Receptor-binding domain (Rbd), Translocation domain (Td), and the Light chain (Lc), the last being a proteolytic enzyme. Fusing the receptor-binding domain with synaptobrevin SNARE motif allowed delivery of the active part of botulinum neurotoxin (Lc-Td), tagged with SNAP25, into neurons. Our data show that SNARE-tagged toxin was able to cleave its intraneuronal molecular target and to inhibit release of neurotransmitters. The reassembled toxin provides a safer alternative to existing botulinum neurotoxin and may offer wider use of this popular research and medical tool. Finally, SNARE tagging allowed the Rbd portion of the toxin to be used to deliver quantum dots and other fluorescent markers into neurons, showing versatility of this unique tagging and self-assembly technique. Together, these results demonstrate that the SNARE tetrahelical coiled-coil allows controlled linking of various building blocks into multifunctional assemblies.