Thermal calculation of accelerating structures in electron linacs

The temperature distribution in the accelerating structures of standing-wave and traveling-wave electron linacs is calculated. The objects of consideration in the former and latter cases are, respectively, a circular iris waveguide with magnetic coupling and a biperiodic accelerating structure. Calculation data for steady and transient heating conditions are presented. The variation of the cell’s geometry and resonance frequency with the temperature of the structure is also calculated. Calculations are made for copper structures with allowance for microwave wall losses.     

On some properties of nearly conservative dynamics of Ikeda map and its relation with the conservative case

The behavior of the well-known Ikeda map with very weak dissipation (so-called nearly conservative case) is investigated. The changes in the bifurcation structure of the parameter plane while decreasing the dissipation are revealed. It is shown that when the dissipation is very weak the system demonstrates an “intermediate” type of dynamics combining the peculiarities of conservative and dissipative dynamics. The correspondence between the trajectories in the phase space in the conservative case and the transformations of the set of initial conditions in the nearly conservative case has been obtained. The dramatic increase of the number of coexisting low-period attractors and the extraordinary growth of the transient time while the dissipation decreases have been revealed. The method of plotting a bifurcation tree for the set of initial conditions has been used to classify the existing attractors by their structure. Also it was shown that most of the coexisting attractors are destroyed by rather small external noise, and the transient time in noisy driven systems increases still more. The new method of two-parameter analysis for conservative systems was proposed.     

Bistability of Multiwalled Carbon Nanotubes Arranged on Plane Substrates

Physics of the Solid State - Stationary states of multi-walled carbon nanotubes have been studied. The numerical simulation showed that nanotubes become bistable systems at rather large diameters....     

Establishment of Waves Generated by a Pulsating Source in a Finite-Depth Fluid

A correct solution of Sretenskii’s plane problem of a source pulsating in a finite-depth fluid is derived. The solution is found using generalized functions as a limit in the infinite future of a wave regime generated by a source which starts to execute pulsations in a fluid initially at rest at a certain moment of time.     

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.     

Combinatorics of reaction-network posets

Reaction networks are viewed as derived from ordinary molecular structures related in reactant-product pairs so as to manifest a chemical super-structure. Such super-structures then are candidates for applications in a general combinatoric chemistry. Notable additional characterization of a reaction super-structure occurs when such reaction graphs are directed, as for example when there is progressive substitution (or addition) on a fixed molecular skeleton. Such a set of partially ordered entities is in mathematics termed a poset, which further manifests a number of special properties, as then might be utilized in different applications. Focus on the overall "super-structural" poset goes beyond ordinary molecular structure in attending to how a structure fits into a (reaction) network, and thereby brings an extra "dimension" to conventional stereochemical theory. The possibility that different molecular properties vary smoothly along chains of interconnections in such a super-structure is a natural assumption for a novel approach to molecular property and bioactivity correlations. Different manners to interpolate/extrapolate on a poset network yield quantitative super-structure/activity relationships (QSSARs), with some numerical fits, e.g., for properties of polychlorinated biphenyls (PCBs) seemingly being quite reasonable. There seems to be promise for combinatoric posetic ideas.     

Intracule densities in the strong-interaction limit of density functional theory

The correlation energy in density functional theory can be expressed exactly in terms of the change in the probability of finding two electrons at a given distance r(12) (intracule density) when the electron-electron interaction is multiplied by a real parameter lambda varying between 0 (Kohn-Sham system) and 1 (physical system). In this process, usually called adiabatic connection, the one-electron density is (ideally) kept fixed by a suitable local one-body potential. While an accurate intracule density of the physical system can only be obtained from expensive wavefunction-based calculations, being able to construct good models starting from Kohn-Sham ingredients would highly improve the accuracy of density functional calculations. To this purpose, we investigate the intracule density in the lambda --> infinity limit of the adiabatic connection. This strong-interaction limit of density functional theory turns out to be, like the opposite non-interacting Kohn-Sham limit, mathematically simple and can be entirely constructed from the knowledge of the one-electron density. We develop here the theoretical framework and, using accurate correlated one-electron densities, we calculate the intracule densities in the strong interaction limit for few atoms. Comparison of our results with the corresponding Kohn-Sham and physical quantities provides useful hints for building approximate intracule densities along the adiabatic connection of density functional theory.