Magnetic field-induced assembly of oriented superlattices from maghemite nanocubes

Tailoring the structure of nanocrystal superlattices is an important step toward controlled design of novel nanostructured materials and devices. We demonstrate how the long-range order and macroscopic dimensions of magnetic nanoparticle arrays can be controlled by the use of a modulated magnetic field. Inducing a dipolar attraction during the initial stage of the drying-mediated self-assembly process was sufficient to assemble the superparamagnetic oleate-capped maghemite nanocubes into large and defect-free superstructures with both translational and orientational order. The characteristic dimensions of the superlattice are controlled by the particle concentration as well as the duration of the applied magnetic field. The superparamagnetic maghemite nanocubes assemble into large and highly oriented thin arrays by applying the magnetic field perpendicular to the substrate surface only during the initial phase of drying-mediated self-assembly. Micrometer-sized and thick three-dimensional mesocrystals are obtained when the drying dispersion is subjected to an external magnetic field of moderate strength for the entire duration of the assembly process. The discovery of how translational and orientation order of nanocrystal superlattices can be induced by a temporal modulation of an anisotropic interparticle force offers new insight on the importance of the initial nucleation stage in the self-assembly process and suggests new routes for controlled self-assembly of dipolar nanocrystals.     

Assembly of two distinct dimers and higher-order oligomers from full-length tau

Abnormal accumulation of tau as filamentous structures is a neuropathological hallmark of neurodegenerative diseases referred to as tauopathies. Little is known about the role of native cysteine residues in tau assembly because their substitution with other amino acids has no effect on tau filament morphology. To understand the process involved in tau oligomerization, we analysed both heparin-induced assembly of different forms of recombinant human tau and assembly of tau from COS-7 cells transiently expressing different human tau constructs. Here, we demonstrated that tau assembly involves two distinct dimers (cysteine-dependent and cysteine-independent) that differ in resistance to reduction. During assembly, an increase of cysteine-dependent tau oligomer was observed prior to detection of increased thioflavin T fluorescence signals. The latter event was accompanied by an increase of cysteine-independent dimer. Fewer higher-order oligomers and aggregates were assembled from four-repeat tau containing two amino-terminus inserts that have either the C291A/C322A mutation (cysless-4R2N) or a hexapeptide deletion at residues 306-311 (DeltaPHF6-4R2N) compared with those assembled from wild-type tau. Assembly of distinct types of dimers was also observed in lysates from COS-7 cells expressing wild-type 4R2N and brain extracts from mice expressing P301L mutant tau. In contrast, COS-7 cells expressing cysless- or DeltaPHF6-4R2N tau contained very little cysteine-dependent dimer. Together, the results indicate that intermolecular disulfide crosslinking along with PHF6 hexapeptide facilitates tau oligomerization and that this event is accompanied by cysteine-independent intermolecular bridging of microtubule-binding domain, leading to assembly of higher-order oligomers. The levels of these dimers may be used to gauge the potential for tau assembly.     

Dynamic hybrid materials for constitutional self-instructed membranes

Constitutional self-instructed membranes were developed and used for mimicking the adaptive structural functionality of natural ion-channel systems. These membranes are based on dynamic hybrid materials in which the functional self-organized macrocycles are reversibly connected with the inorganic silica through hydrophobic noncovalent interactions. Supramolecular columnar ion-channel architectures can be generated by reversible confinement within scaffolding hydrophobic silica mesopores. They can be structurally determined by using X-ray diffraction and morphologically tuned by alkali-salts templating. From the conceptual point of view, these membranes express a synergistic adaptive behavior: the simultaneous binding of the fittest cation and its anion would be a case of “homotropic allosteric interactions,” because in time it increases the transport efficiency of the pore-contained superstructures by a selective evolving process toward the fittest ion channel. The hybrid membranes presented here represent dynamic constitutional systems evolving over time to form the fittest ion channels from a library of molecular and supramolecular components, or selecting the fittest ion pairs from a mixture of salts demonstrating flexible adaptation.     

Pd0@Polyoxometalate Nanostructures as Green Electrocatalysts: Illustrative Example of Hydrogen Production

Green-chemistry type procedures were used to synthesize Pd⁰ nanostructures encapsulated by a vanadium-substituted Wells-Dawson-type polyoxometalate (Pd⁰@POM). The cyclic voltammogram run with the Pd⁰@POM-modified glassy carbon electrode shows well-defined waves, associated with Pd⁰ nanostructures and the V^V/V^IV redox couple. The Pd⁰@POM-modified electrode displayed remarkably reproducible cyclic voltammetry patterns. The hydrogen evolution reaction (HER) was selected as an illustrative example to test the electrocatalytic behavior of the electrode. The kinetic parameters of the HER show the high efficiency of the Pd⁰@POM-modified electrode. This is the first example of electrochemical characterization of a modified electrode based on a vanado-tungstic POM and Pd⁰ nanostructures.     

Hollow core of Alzheimer’s Aβ42 amyloid observed by cryoEM is relevant at physiological pH

Recent cryoEM density maps of Aβ₄₂ fibrils obtained at low pH revealed two protofilaments winding around a hollow core raising the question if such tubular structures also exist at physiological pH. Based on the cryoEM measurements and on NMR data, we probe amyloid fibril organizations corresponding to the observed cryoEM density map. Our study demonstrates that the tubular Aβ₄₂ fibril models exist at both acidic and physiological pH; however, the relative populations of the polymorphic models shift with pH. At acidic pH, the hollow core model exhibits higher population than the other models; at physiological pH, although it is less populated compared to the other models, structurally, it is stable and represents 8% of the population. We observe that only models with C termini facing the external surface of the fibril retain the hollow core under acidic and physiological conditions with dimensions similar to those observed by cryoEM; on the other hand, the hydrophobic effect shrinks the tubular cavity in the alternative organization. The existence of the hollow core fibril at physiological pH emphasizes the need to examine toxic effects of minor oligomeric species with unique organizations.     

Supramolecular Behavior of the Amphiphilic Drug (2<Emphasis Type="Italic">R</Emphasis>)-2-Ethylchromane-2-Carboxylic Acid Arginine Salt (a Novel PPARα/γ Dual Agonist)

Purpose This study was conducted to evaluate the aggregation properties of an amphiphilic drug.Methods Aggregation of the drug was studied by various methods including phase-contrast and polarized microscopy, spectrophotometry, surface tensiometry, atomic force microscopy, and dynamic light scattering. Lymph-cannulated rats were used to assess fractions of drug that were absorbed into lymphatics.Results During the pharmaceutical development of an α/γ dual PPAR agonist, a derivative of a chromane-2-carboxylic acid (compound 1), it was discovered that the compound was able to form various aggregates in aqueous media from pH 6.5 to 7.1, whereas aggregating predominantly into micelles at higher pH values. Critical micelle concentrations seemed to be quite low, about 0.25 mM (0.17 mg/mL) in deionized water as determined by spectrophotometric (dye) and surface tensiometry (du Nuoy) methods. Aggregation of compound 1 into large supramolecular aggregates was visualized via phase-contrast microscopy and atomic force microscopy. The observed aggregates ranged from 250 nm to greater than 10 μm in size. Formation of liquid crystalline phases was observed by polarized microscopy as the material was gradually hydrated with water. Lymph studies in rats indicated that up to 6.9% of the orally administered dose of compound 1 in pH 6.5 buffer appeared in lymph, suggesting that supramolecular aggregation may also occur in vivo leading to partitioning between the portal and the lymph routes.Conclusions The aforementioned supramolecular aggregation was found to have a profound effect on the pharmaceutical development of the drug and potentially on in vivo absorption of the drug.     

原子力显微镜对硫酸软骨素分形结构的研究

自组装纳米结构一直是化学科学中的前沿课题。本文运用原子力显微镜(AFM)对硫酸软骨素溶液在Ca2+试剂作用下的自组装纳米结构随时间的形态学变化进行了研究,实验结果显示当硫酸软骨素的浓度大于1.0mg/mL时,该聚合物在水溶液中可以形成纳米尺寸的类似于胶束结构的自组装聚集体,且这种类似胶束结构体现了分形学上所说的自相似性,聚合物凝集的过程实际上就是分形生长的过程。该分形结构会因为Ca2+的作用而随时间而发生形态学改变。本文探讨了这种分形生长的机理,并对这种分形结构随时间变化的现象给予了初步解释,为高分子自组装为纳米分散粒子,特别是功能化的聚合物在水中形成稳定的纳米粒子的研究提供了一定的实验依据。     

Kinetic Studies and Temperature-Induced Morphological Changes of Peptide Decorated Nano-Objects via Polymerization-Induced Self-Assembly

In the present work, combining polymerization-induced self-assembly (PISA) with self-assembling peptides (SAPs) peptide−polymer hybrid nanostructures are prepared, harnessing the advantages of PISA and the self-assembling driving force of SAPs. A tripeptide methacrylamide denoted as MAm-GFF, where MAm, G, and F stand for methacrylamide, glycine, and phenylalanine, is copolymerized with glycerol monomethacrylate (GMA) by reversible addition−fragmentation chain transfer polymerization (RAFT) in dimethylformamide to produce a P(GMA₆₂-stat-(MAm-GFF)₇) macromolecular chain transfer agent (macro-CTA). This peptide-containing macro-CTA is then successfully chain-extended with poly(2-hydroxypropyl methacrylate) (PHPMA) by aqueous dispersion PISA, forming P(GMA₆₂-stat-(MAm-GFF)₇)-b-PHPMA₂₇ self-assembled objects. The impacts of temperature and monomer conversion on the morphologies formed during the PISA process are investigated by analyzing samples withdrawn at different time during the polymerization of HPMA using transmission electron microscopy (TEM) and dynamic light scattering (DLS) at different temperatures (5–70 °C).     

PLLA-Based Block Copolymers via Raft Polymerization—Impact of the Synthetic Route and Activation Mechanism

Designing supramolecular structures with well-defined dimensions and diverse morphologies via the self-assembly of block copolymers is renowned. Specifically, the design of 1D fiber nanostructures is extensively emphasized, due to their unique properties in many areas, such as microelectronics, photonics, and particularly in the biomedical field. Herein, amphiphilic diblock copolymers of P(l -lactide)-b-P(N-t-butoxy-carbonyl-N´-acryloyl-1,2-diaminoethane)-co-P(N-isopropylacrylamide) PLLA_n-b-P(BocAEAm)_m-co- P(NiPAAm)_Ɩ are developed. Two synthetic strategies are investigated to equip PLLA with a chain transfer agent (CTA), either by Steglich esterification of PLLA-OH or via the ring-opening polymerization of l -lactide using a CTA containing a hydroxyl functional group. The second strategy proves to be superior in terms of degree of functionalization. The corona-forming blocks, with degrees of polymerization of 200 and above are achieved in good definition by photo-iniferter RAFT polymerization (Đ ≤ 1.25), while a Đ of 1.75 is obtained by conventional RAFT polymerization. The self-assembly of the developed system leads to the formation of nanofibers with a height of 11 nm and a length of ≈300 nm, which is determined by atomic force microscopy (AFM). These fibers are the basis for new antimicrobial nanomaterials after deprotection, as the subject of upcoming work.     

DNA-Templated Nanowire Fabrication

DNA-based nanotechnology is a vibrant and expanding field. The specific molecular recognition properties and large aspect ratio of DNA make the molecule a promising template for bottom-up fabrication of nanowires and nanodevices. Fabricating well-defined DNA-templated nanowires requires aligned surface deposition and specific metallization of DNA molecules. DNA localization on surfaces has been achieved by bulk fluid flow or a moving air-water interface, and localization efficiency has been improved by surface modifications that favor DNA-substrate interaction. DNA-templated nanowires have been constructed from gold, silver, copper, palladium, and platinum, and template modifications have allowed the bottom-up construction of a simple electronic nanodevice. These achievements demonstrate the promising feasibility of using bottom-up nanofabrication to create increasingly sophisticated nanodevices.