Alexander N. Semenov

Engineering of peptide β-sheet nanotapes

A set of principles are outlined for the design of short oligopeptides nwhich will self-assemble in appropriate solvents into long, semi-flexible, npolymericβ-sheet nanotapes. Their validity is demonstrated nby experimental studies of an 11-residue peptide (DN1) which forms nnanotapes in water, and a 24-residue peptide (K24) which forms nanotapes nin non-aqueous solvents such as methanol. Circular dichroism (CD) nspectroscopy studies of the self-assembly behaviour in very dilute nsolutions (µm) reveal a simple transition from a random ncoil-to-β-sheet conformation in the case of DN1, but a more ncomplex situation for K24. Association of DN1 is very weak up to a nconcentration of 40 µm at which there is a sudden increase in the nfraction of peptide in the β-sheet structure, indicative of nan apparent ‘critical tape concentration’. This is shown to narise from a two-step self-assembly process: the first step being a ntransition from a random coil to an extended β-strand nconformation, and the second the addition of this β-strand nto a growing β-sheet. Both peptides are shown to gel their nsolvents at concentrations above 2×10 n -3 n volume nfraction: these gels are stable up to the boiling point of the solvents. nRheology measurements on gels of the 24-residue peptide in 2-chloroethanol nreveal that the tapes form an entangled network with a mesh size of n10–100 nm for peptide volume fractions 0.03–0.003; the npersistence length of the tape is 13 nm or greater, indicative of a nmoderately rigid polymer; the tapes are about a single molecule in nthickness. The mechanical properties of the gels in many respects are ncomparable to those of natural biopolymers such as gelatin, actin, amylose nand agarose.

Supramolecular Self-Assembly and Radical Kinetics in Conducting Self-Replicating Nanowires

By using a combination of experimental and theoretical tools, we elucidate unique physical characteristics of supramolecular triarylamine nanowires (STANWs), their packed structure, as well as the entire kinetics of the associated radical-controlled supramolecular polymerization process. AFM, small-angle X-ray scattering, and all-atomic computer modeling reveal the two-columnar snowflake internal structure of the fibers involving the π-stacking of triarylamines with alternating handedness. The polymerization process and the kinetics of triarylammonium radicals formation and decay are studied by UV-vis spectroscopy, nuclear magnetic resonance and electronic paramagnetic resonance. We fully describe these experimental data with theoretical models demonstrating that the supramolecular self-assembly starts by the production of radicals that are required for nucleation of double-columnar fibrils followed by their growth in double-strand filaments. We also elucidate nontrivial kinetics of this self-assembly process revealing sigmoid time dependency and complex self-replicating behavior. The hierarchical approach and other ideas proposed here provide a general tool to study kinetics in a large number of self-assembling fibrillar systems.

Anisotropic Self-Assembly of Supramolecular Polymers and Plasmonic Nanoparticles at the Liquid–Liquid Interface

The study of supramolecular polymers in the bulk, in diluted solution, and at the solid–liquid interface has recently become a major topic of interest, going from fundamental aspects to applications in materials science. However, examples of supramolecular polymers at the liquid–liquid interface are mostly unexplored. Here, we describe the supramolecular polymerization of triarylamine molecules and their light-triggered organization at a chloroform–water interface. The resulting interfacial nematic layer of these 1D supramolecular polymers is further used as a template for the precise alignment of spherical gold nanoparticles coming from the water phase. These hybrid thin films are spontaneously formed in a single process, without chemical prefunctionalization of the metallic nanoparticles, and their ordering is improved by centrifugation. The resulting polymer chains and strings of nanoparticles can be co-aligned with high anisotropy over very large distances. By using a combination of experimental and theoretical investigations, we decipher the full sequence of this oriented self-assembly process. In such a highly anisotropic configuration, electron energy loss spectroscopy reveals that the self-assembled nanoparticles behave as plasmonic waveguides.

On connection between constructive involutive divisions and monomial orderings

This work considers the basic issues of the theory of involutive divisions, namely, the property of constructivity which assures the existence of minimal involutive basis. The work deals with class of ≻ -divisions which possess many good properties of Janet division and can be considered as its analogs for orderings different from the lexicographic one. Various criteria of constructivity and non-constructivity are given in the paper for these divisions in terms of admissible monomial orderings ≻ . It is proven that Janet division has the advantage in the minimal involutive basis size of the class of ≻ -divisions for which x1 ≻ x2 ≻ ... ≻ xn holds. Also examples of new involutive divisions which can be better than Janet division in minimal involutive basis size for some ideals are given.

Simple one‐particle diffusional model to mimic some properties of the glass transition

A new simplified model is proposed to mimic some properties of the glass transition. The physical system undergoing glass transition is modeled as Brownian particle diffusing in one‐ or two‐dimensional space with obstacles. In one dimension obstacles are points which cannot be crossed by Brownian particles, in two dimensions obstacles are randomly distributed sections of straight lines which are impenetrable for the diffusing particle. The obstacles have a finite lifetime τ. After time τ the obstacle disappears and reappears in some new random position. In another modification of the model the obstacle barrier can be opened for short time and then closed again. Both cases are studied for one‐dimensional diffusion, while in two dimensions only the first modification of the model is considered. The main feature of the model is that the mean lifetime of obstacles τ is connected with the diffusion coefficient of the Brownian particle through the coupling equation Dτ=K, with K being the coupling constant. This...

Theory of elastic light scattering in solutions of semiflexible macromolecules in the region of the liquid-crystalline transition☆

Abstract A study has been made of the theory of elastic light scattering from an isotropic solution of semiflexible persistent macromolecules when approaching the LC transition point. The differential cross-section of scattering per unit volume has been calculated for scattering at an arbitrary angle and with an arbitrary value of the scattering wave vector q . In addition, the degree of depolarization of scattered light was calculated. The analytical results for small q are discussed as well as the accurate results obtained with the aid of a computer. The calculated data are compared with experimental results.