Density of states in random lattices with translational invariance

We propose a random matrix approach to describe vibrations in disordered systems. The dynamical matrix M is taken in the form M = AA ^T , where A is a real random matrix. It guaranties that M is a positive definite matrix. This is necessary for mechanical stability of the system. We built matrix A on a simple cubic lattice with translational invariance and interaction between nearest neighbors. It was found that for a certain type of disorder acoustical phonons cannot propagate through the lattice and the density of states g(ω) is not zero at ω = 0. The reason is a breakdown of affine assumptions and inapplicability of the macroscopic elasticity theory. Young modulus goes to zero in the thermodynamic limit. It reminds of some properties of a granular matter at the jamming transition point. Most of the vibrations are delocalized and similar to diffusons introduced by Allen, Feldman, et al., Phil. Mag. B 79, 1715 (1999). We show how one can gradually return rigidity and phonons back to the system increasing the width of the so-called phonon gap (the region where g(ω) ∝ ω²). Above the gap the reduced density of states g(ω)/ω² shows a well-defined Boson peak which is a typical feature of glasses. Phonons cease to exist above the Boson peak and diffusons are dominating. It is in excellent agreement with recent theoretical and experimental data.     

Non-monotonic temperature dependence of the alignment of a nematic mixture on a ferroelectric substrate

Polarization optic techniques have been applied to study specific features of the anisotropic interaction between a dye-doped eutectic mixture of nematic liquid crystals p-methoxybenzylidene-p-n-butylaniline and p-ethoxybenzylidene-p-n-butylaniline and a polar surface of a ferroelectric triglycine sulphate crystal over the temperature range including the substrate Curie point T _c. It has been found that the temperature-induced structural changes in the nematic layer in the vicinity of T _c are related to the changes in the orientational part of the tensor order parameter Q_ik . The temperature dependence of the director angle θ¯, averaged over the nematic layer, has been obtained from the effective dichroism values of solute absorption. The experimental data were interpreted using the model, in which the anisotropic part of the surface energy has two terms with orthogonal easy axes.     

Temperature-induced orientational transition of nematics on the surface of a ferroelectric crystal

A temperature-induced orientational transition is investigated in a mixture of nematic liquid crystals on the surface of a cleavage of a ferroelectric triglycine sulfate crystal. The transition has been observed by the variation of the polarized absorbance components of a dye introduced into the nematic matrix with increasing temperature. The reorientation of molecules in the liquid crystal volume confined by solid walls is due to competition between dispersion and polar forces at the surface and the decrease in the electric field of the substrate up to its complete disappearance at the ferroelectric Curie point.     

Preparation of α-Adamantyl-substituted Aliphatic Ketones

Russian Journal of Organic Chemistry -     

Atomic dynamics of lead embedded into nanoporous glass

The thermal atomic vibration spectrum of lead nanostructured in porous glass with an average pore size of 7 nm and the thermal vibration spectrum of conventional bulk lead (taken for comparison) are measured using inelastic neutron scattering. The density of states in the phonon spectrum of lead nanoparticles is found to exceed the density of states in the spectrum of bulk lead at both low (E < 2.5 meV) and high (E > 9.5 meV) energies. These data are used to propose a model for the structure of a porous glass-lead nano-composite.     

Diffusion of vibrations in disordered systems

We consider diffusion of vibrations in random lattices with translational invariance. Above the frequency ω_IR corresponding to the Ioffe-Regel crossover (and depending on the strength of disorder), phonons cannot propagate through the lattice and transfer energy. At the same time, most of the vibrations in this range are not localized. We show that these delocalized excitations are similar to diffusons introduced by P. B. Allen, J. L. Feldman, J. Fabian, and F. Wooten (see, e.g., Phil. Mag. B 79, 1715 (1999)) to describe heat transport in glasses. In this range the energy in the lattice is transferred by means of diffusion of vibrational excitations. We have calculated the diffusivity of the modes D(ω) using both the direct numerical solution of Newton equations and the Edwards-Thouless formula. It is nearly constant above ω_IR and goes to zero at the localization threshold.     

Rational Design of Practically Important Enzymes

Majority of native enzymes are poorly applicable for practical usage: that is why different methods of enzyme modification are used to obtain the biocatalysts with appropriate characteristics. Development of genome sequencing and various modern approaches in protein engineering allow one to identify protein of interest and to improve the enzyme properties for a particular process. This review describes the results on development of novel biocatalysts based on bioinformatics and rational design. New genes encoding formate dehydrogenase (FDH) from bacterium Staphylococcus aureus, yeasts Ogataea parapolymorpha and Saccharomyces cerevisiae and moss Physcomitrella patens (SauFDH, OpaFDH, SceFDH and PpaFDH, respectively), have been cloned. New FDHs were produced in the active form and characterized. SauFDH was shown to have at least 2-fold higher catalytic constant than other known FDHs. OpaFDH has catalytic parameters as good as those for soy FDH mutant forms, and in addition, is more thermostable. Apo- and holo-forms of SauFDH have been crystallized. Mutation of two Cys residues in Pseudomonas sp.101 enzyme (PseFDH) yields enzyme preparations with improved kinetic parameters and enhanced thermal and chemical stability. New generation of PseFDH preparations with the coenzyme specificity changed from NAD⁺ to NADP⁺ have been obtained. The effect of ionic liquids on the catalytic properties and thermal stability of six wild-type recombinant FDHs, and a number of their mutants, have been studied. In case of D-amino acid oxidase (DAAO), single-point mutations have been combined to create multi-point mutants. The introduced amino acid replacements have been shown to exert an additive effect, improving both kinetic parameters and increasing thermal and chemical stability. DAAO genes from Hansenula polymorpha yeast have been cloned. α-Amino acid ester hydrolase (AEH) gene has been cloned and expressed in the active form in E. coli. Structural modeling has been performed and the effectiveness in amino beta-lactams synthesis studied. The structure of a single-chain penicillin acylase from Alcaligenes faecalis (scAfPA) has been modeled and two variants of scAfPA gene was generated by PCR. Both variants have been expressed in E. coli, isolated and characterized. Catalytic properties of scAfPA were slightly better than those of its natural heterodimer.     

Adamantyl-containing fluorinated 1,3-diketones

A convenient one-step procedure has been developed for the synthesis of fluorine-containing 2-(adamant-l-yl)-1,3-diketones by reaction of fluorinated 1,3-diketones with 1,3-dehydroadamantane. The products can be used as starting compounds for the preparation of new fluorinated adamantyl-containing heterocyclic compounds.