Alexei R. Khokhlov

Collapse of single DNA molecule in poly(ethylene glycol) solutions

The compactization of a single DNA molecule in polyethylene glycol (PEG) solution was investigated both theoretically and experimentally. A theory is proposed taking into account the polyelectrolyte effect and redistribution of PEG within DNA coils. This approach makes it possible to describe the dependence of critical value, c, of PEG concentration at the point of DNA collapse on the degree of PEG polymerization, P, and on the concentration of low‐molecular salt, ns. Observation of single DNA molecule in solution of PEG has been carried out by means of fluorescence microscopy which allows one to observe the conformation of individual DNA directly. Direct evidence that the coil–globule transition of DNA occurs as first order phase transition was obtained. It was confirmed that the critical concentration of PEG decreases with an increase of the degree of PEG polymerization and salt concentration. The width of the coexistence region of coil and globule was found to be dependent on salt concentration and deg...

Liquid-crystalline ordering in the solution of long persistent chains

The orientational ordering in the solution of long persistent macromolecules is studied by means of the Onsager method. It is shown that the liquid-crystalline transition occurs at significantly higher concentrations and that the order parameter at the transition point is much smaller than for the model of freely-jointed segments. It is concluded that the orientational ordering in the solutions of semiflexible chains depends essentially on the flexibility distribution along the chain contour.

Liquid-crystalline ordering in the solution of partially flexible macromolecules

The liquid-crystalline ordering in the solution of persistent chains which length, L, is comparable with the length of the effective Kuhn segment, l, is considered by means of a generalization of the Onsager method. The orientational entropy for this case is calculated using the method proposed by I.M. Lifshitz (for another problem) in 1968. It is shown that a slight flexibility of the persistent chain is sufficient for the complete change in the properties of the liquid-crystalline transition: for example, at Ll∼0.1 these properties are more similar to those which are characteristic for the semi-flexible limit (Ll⪢1), than for the rigid rod limit (Ll⪡1), although at such Ll the geometric form of the macromolecule is much closer to the rodlike one.

Conformational transitions in polymer gels: Theory and experiment

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account. Experimental results demonstrate a good correlation with theoretical analysis.

Hydrophobic aggregation in aqueous solutions of hydrophobically modified polyacrylamide in the vicinity of overlap concentration

The rheological behavior and hydrophobic association of hydrophobically modified polyacrylamide with 0.5 mol% of hydrophobic n-nonylacrylamide units were studied in the vicinity of the overlap concentration. It was shown that the rheological properties of aqueous polymer solutions are determined by the formation of hydrophobic domains playing a role of physical cross-links between polymer chains. It was observed that a pronounced increase of the viscosity above the overlap concentration is accompanied by the increase of the concentration of hydrophobic domains, but the average size of one domain remains constant.

Protein-like copolymers: computer simulation

The notion of protein-like AB copolymers is introduced. Such copolymers can be generated with the help of the “instant image” of a dense homopolymer globule by assigning that the monomeric units closer to the globular surface are of A type, while the core is formed by the B type units. After that the primary structure of the chain is fixed, and one introduces different interaction potentials for A and B units. In doing so, we have in mind mainly aqueous systems and analogy with globular proteins, therefore A units are regarded as hydrophilic, and B units as hydrophobic. By means of Monte Carlo simulation using the bond fluctuation model we study the coil–globule transition for a protein-like copolymer upon the increase of attraction of hydrophobic B units, and compare the results with those for random AB copolymers. From the analysis of the primary structure of protein-like copolymers one can see that the “degree of blockiness” of the protein-like sequence is higher than for random copolymers, therefore the copolymers with the “random-block” primary structure are generated for comparison as well (the average length of A and B sequences being the same as for protein-like copolymers). It is shown that the coil–globule transition in protein-like copolymers occurs at higher temperatures, is more abrupt and has faster kinetics than for random copolymers with the same A/B composition and for random-block copolymers with the same A/B composition and “degree of blockiness”. The globules of protein-like copolymers exhibit a dense micelle-like core of hydrophobic B units stabilized by the long dangling loops of hydrophilic A units. Apparently, a protein-like copolymer “inherits” some of the properties of the “parent globule” which is reflected in the special long-range correlations in primary structure.

Swelling and impregnation of polystyrene using supercritical carbon dioxide

Abstract Swelling of polystyrene (PS) in supercritical carbon dioxide was studied in situ. Experiments were carried out in a high-pressure cell furnished with an optical channel. This allowed to measure the dynamics of diffusion front propagation in the polymer samples during the swelling process by the use of an optical microscope supplied with a digital video camera. The measurements of this dynamics provided information about diffusion coefficients of CO 2 molecules in PS. Diffusion coefficients and equilibrium sorption degrees of the PS samples were also calculated by the use of a gravimetric method. The data were compared with the results of in situ technique. Impregnation of PS films wish ethyl-2-cyano-3(4′-dimethylaminophenyl)acrylate as a model substance for UV-filters and electrographic materials was carried out.

Peptide nanofibrils boost retroviral gene transfer and provide a rapid means for concentrating viruses

Inefficient gene transfer and low virion concentrations are common limitations of retroviral transduction. We and others have previously shown that peptides derived from human semen form amyloid fibrils that boost retroviral gene delivery by promoting virion attachment to the target cells. However, application of these natural fibril-forming peptides is limited by moderate efficiencies, the high costs of peptide synthesis, and variability in fibril size and formation kinetics. Here, we report the development of nanofibrils that self-assemble in aqueous solution from a 12-residue peptide, termed enhancing factor C (EF-C). These artificial nanofibrils enhance retroviral gene transfer substantially more efficiently than semen-derived fibrils or other transduction enhancers. Moreover, EF-C nanofibrils allow the concentration of retroviral vectors by conventional low-speed centrifugation, and are safe and effective, as assessed in an ex vivo gene transfer study. Our results show that EF-C fibrils comprise a highly versatile, convenient and broadly applicable nanomaterial that holds the potential to significantly facilitate retroviral gene transfer in basic research and clinical applications.

Structure of collapsed persistent macromolecule: Toroid vs. spherical globule

We studied theoretically the behavior of a collapsed persistent macromolecule in poor solvent as a model of collapse transition of single double-stranded DNA chain, and constructed the diagram of states in the variables with contour length of a macromolecule and quality of the solvent. We found that the state of toroidal globule exists as an intermediate state between the states of elongated coil state and the spherical globule. Our theoretical result suggests that a single linear macromolecule with a high degree of polymerization can form a toroidal globule. However, the range in which the toroidal structure is stable decreases as the macromolecule length increases. Experimental observation with transmission electron microscopy has been performed to study the globular structure of single DNA chain (bacteriophage T4 DNA, λ-DNA) collapsed by hexammine cobalt (III) at different concentrations. We found that an extremely long chain of T4 DNA (166 kbp), with a contour length of 56 μm, actually forms a toroidal globule, and that isotropic spherical globule appears at higher hexammine cobalt concentration.

Structural organization of water-containing Nafion: The integral equation theory

Using the mutually consistent variant of the integral equation polymer reference interaction site model (RISM) theory and the chemically realistic rotational isomeric state (RIS) model, we perform a molecular level modeling of the specific structural organization of perfluorosulfonic acid ionomer (Nafion) with certain amount of physisorbed water. Our results establish molecular scale information necessary for understanding the equilibrium structure and thermodynamics of water-containing Nafion as well as water distribution and ionic (molecular) transport phenomena in hydrated Nafion membranes. As a first step in this direction, semi-empirical, quantum mechanical calculations on the molecular structures and energies of the polymeric backbone and pendant chains of Nafion with and without additional water molecules are carried out. These data are used in the single-polymer RIS Monte Carlo simulation, in which the short-range intramolecular interactions are taken into account via appropriate matrices of statistical weights. The local structure and morphology of the entire bicomponent water-containing system is calculated consistently on the basis of the RISm theory at different contents of absorbed water. We find that the addition of even a relatively small amount of water leads to the strong intensification of aggregation processes observed for polar sulfonic acid (SO 3 H) groups. Water molecules and polar SO 3 H groups form mixed aggregates with a three-layer structure. Incorporation of water molecules inside the aggregates results in an increase of their stability and leads to the increase of the number of associating groups in a stable aggregate. With increasing the number of incorporated solvent molecules, the average size of mixed aggregates increases. The results obtained in the present study support the concept of an irregularly shaped cluster surfaces. Such geometries are favorable to the formation of long channels of connected water-containing aggregates providing the unique permeability characteristics of Nafion membranes. In addition, we investigate the clustering and continuum percolation in the water phase, using the formalism of pairconnectedness correlation functions. The mean cluster size found theoretically is in reasonable agreement with the corresponding experimental estimates existing in the literature.