Исследование активационного барьера кристаллизации метастабильной жидкости методом метадинамики

The crystallization of Lennard-Jones fluid under weak supercooling was studied in molecular dynamics simulation with the metadynamics method. The Steinhardt parameter Q6 for a group of particles around a random atom and the system's potential energy were chosen to describe phase transition. The change in collective variables was observed in the process of overcoming the activation barrier of crystallization. The critical nucleus formation data were obtained.

Multiband coupling and nuclear softness in optical model calculations for even-even and odd-A actinides

A new dispersive multiband coupled channels optical model with soft-rotator “effective” deformations is proposed to describe nucleon scattering on even-even and odd-A actinides. The impact of the introduction of axial and non-axial dynamical deformations that describe nuclear softness is discussed. Softness and multiband coupling are shown to change compound-nucleus formation cross section by up to ≈ 10% for incident neutron energies below 1 MeV.

The symmetry origin of the austenite-cementite orientation relationships in steels

The connection between austenite/cementite orientation relationships and crystal structure of both phases has been established. The nucleus formation mechanism at the mutual transformation of austenite and cementite structures has been proposed. Mechanism is based on the interpretation of the considered structures as crystallographic tiling onto triangulated polyhedra, and the said tiling can be transformed by diagonal flipping in a rhombus consisting of two adjacent triangular faces. The sequence of diagonal flipping in the fragment of the initial crystal determines the orientation of the fragment of the final crystal relative to the initial crystal. In case of the mutual austenite/cementite transformation the mutual orientation of the initial and final fragments is coinciding to the experimentally observed in steels Thomson-Howell orientation relationships: ${\left\{ {\bar 103} \right\}_{\rm{C}}}||{\left\{ {111} \right\}_{\rm{A}}};{\rm{}} < {\kern 1pt} 010{\kern 1pt} { > _{\rm{C}}}{\rm{||}} < {\kern 1pt} 10\bar 1{\kern 1pt} { > _{\rm{A}}};\; < {\kern 1pt} 30\bar 1{\kern 1pt} { > _{\rm{C}}}\;||\,\, < {\kern 1pt} \bar 12\bar 1{\kern 1pt} { > _{\rm{A}}}{\rm{}}$ The observed orientation relationship between FCC austenite and cementite is determined by crystallographic group-subgroup relationship between transformation participants, and non-crystallographic symmetry which is determining the transformation of triangulated clusters of transformation participants.

Swi5–Sfr1 stimulates Rad51 recombinase filament assembly by modulating Rad51 dissociation

Eukaryotic Rad51 protein is essential for homologous-recombination repair of DNA double-strand breaks. Rad51 recombinases first assemble onto single-stranded DNA to form a nucleoprotein filament, required for function in homology pairing and strand exchange. This filament assembly is the first regulation step in homologous recombination. Rad51 nucleation is kinetically slow, and several accessory factors have been identified to regulate this step. Swi5–Sfr1 (S5S1) stimulates Rad51-mediated homologous recombination by stabilizing Rad51 nucleoprotein filaments, but the mechanism of stabilization is unclear. We used single-molecule tethered particle motion experiments to show that mouse S5S1 (mS5S1) efficiently stimulates mouse RAD51 (mRAD51) nucleus formation and inhibits mRAD51 dissociation from filaments. We also used single-molecule fluorescence resonance energy transfer experiments to show that mS5S1 promotes stable nucleus formation by specifically preventing mRAD51 dissociation. This leads to a reduction of nucleation size from three mRAD51 to two mRAD51 molecules in the presence of mS5S1. Compared with mRAD51, fission yeast Rad51 (SpRad51) exhibits fast nucleation but quickly dissociates from the filament. SpS5S1 specifically reduces SpRad51 disassembly to maintain a stable filament. These results clearly demonstrate the conserved function of S5S1 by primarily stabilizing Rad51 on DNA, allowing both the formation of the stable nucleus and the maintenance of filament length.