Search

The method of molecular dynamics was used to model crystal structures based on dimers and trimers of β-cyclodextrin containing luminescent complexes: naphthalene stacking dimer, exhibiting excimer fluorescence, and phenanthrene–2 tert-butylbenzene heterotrimer, exhibiting room temperature phosphorescence. A novel technique was developed to construct these crystals, which consists in replacing guest molecules in the real crystalline supramolecular systems built from β-cyclodextrin dimers and trimers by a naphthalene dimer or phenanthrene–2 tert-butylbenzene complex. Due to the strength of the natural crystalline cyclodextrin framework, the obtained structures are absolutely stable and allow us to explain the observable luminescence properties of naphthalene and phenanthrene in the solid precipitates of an unknown structure. In crystals consisting of β-cyclodextrin trimers assembled into infinite nanotubes, various positions and orientations of the phenanthrene probe in the cavity of the tubes were studied, and the most promising structure for the main candidate with long-term room temperature phosphorescence was selected. Other cases, in which phenanthrene is more loosely in contact with a pair of tert-butylbenzene molecules and more freely moves within tubes, are related with shorter-term room temperature phosphorescence components. Since the packing of host β-cyclodextrin molecules plays a decisive role in the crystallization of supramolecular complexes, this technique is promising for the prediction of new crystal structures, in which embedded guest molecules have sizes suitable for host cavities, and will make obtaining many important supramolecular crystals from few experimentally measured ones possible. This method is applicable for various cyclodextrins and other cavitands, whose molecules are able to undergo dimerization or trimerization in a crystal and build a strong framework. [ABSTRACT FROM AUTHOR]

Simple Summary: We live in a changing world, and human-induced landscape changes cause shifts and expansions of ranges of living organisms worldwide. During range expansions, animals and plants invade and colonize new areas, which may have dramatic ecological and evolutionary consequences. We studied the genetic consequences of range expansion in the desert rodent, currently colonizing new areas in Kalmykia (Southern Russia) following the desertification of rangelands. We found that genetic diversity in the colonizing population was lower than in the range core. Moreover, the colonizing population exhibited strong spatial structuration and increased frequencies of genetic variants (alleles) rare in the source population—signatures of allele surfing, a phenomenon theoretically predicted but, so far, rarely observed in natural populations. Our findings provide new insights into understanding the genetic consequences of range expansions and are important for managing expanding populations and invasions. Colonizing populations at the leading edge of range expansion are expected to have a reduced genetic diversity and strong genetic structure caused by genetic drift and allele surfing. Until now, few studies have found the genetic signatures of allele surfing in expanding wild populations. Using mtDNA markers, we studied the genetic structure of the population of midday gerbils (Meriones meridianus) expanding their range to the west in Kalmykia (southern Russia) following the new cycle of desertification, re-colonizing areas abandoned in the mid-2010s. In the colonizing population, we found a reduced genetic diversity, the redistribution of haplotype frequencies—in particular, in favor of variants rare in the core population—and strong genetic structure combined with strong differentiation from the core population—patterns suggestive of allele surfing on the wave of expansion. In terms of genetic diversity and spatial structuration, the western edge population sampled in 2008 before its collapse in 2017 occupies the intermediate position between the current colonizing and core population. This suggests that reduced genetic diversity and increased genetic differentiation are general features of marginal populations, enhanced by the founder and allele-surfing effects at the leading edges of expanding ranges. [ABSTRACT FROM AUTHOR]

Inertial effects in the rotational Brownian motion in space of a rigid dipolar rotator (needle) in a uniaxial potential biased by an external field giving rise to asymmetry are treated via the infinite hierarchy of differential-recurrence relations for the statistical moments (orientational correlation functions) obtained by averaging the Euler-Langevin equation over its realizations in phase space. The solutions of this infinite hierarchy for the dipole correlation function and its characteristic times are obtained using matrix continued fractions showing that the model simultaneously predicts both slow overbarrier (or interwell) relaxation at low frequencies accompanied by intermediate frequency Debye relaxation due to fast near-degenerate motion in the wells of the potential (intrawell relaxation) as well as the high frequency resonance (Poley) absorption due to librations of the dipole moments. It is further shown that the escape rate of a Brownian particle from a potential well as extended to the Kramers turnover problem via the depopulation factor yields a close approximation to the longest (overbarrier) relaxation time of the system. For zero and small values of the bias field parameter h, both the dipole moment correlation time and the longest relaxation time have Arrhenius behavior (exponential increase with increasing barrier height). While at values of h in excess of a critical value however far less than that required to achieve nucleation, the Arrhenius behavior of the correlation time disappears. [ABSTRACT FROM AUTHOR]

Nonlinear transient responses of polar polarizable particles (macromolecules) diluted in a nonpolar solvent to a sudden change in magnitude of a strong external dc field are evaluated using the anisotropic noninertial rotational diffusion model. The relaxation functions and relaxation times appropriate to the transient dynamic Kerr effect and nonlinear dielectric relaxation are calculated by solving the infinite hierarchy of differential-recurrence equations for statistical moments (ensemble averages of the Wigner D functions). The calculations involve matrix continued fractions, which ultimately yield the exact solution of the infinite hierarchy of differential-recurrence relations for the first- and second-order transient responses. [ABSTRACT FROM AUTHOR]

The Kramers theory of the escape rate of a Brownian particle from a potential well as extended by Mel’nikov and Meshkov, [J. Chem. Phys. 85, 1018 (1986)] is used to evaluate the relaxation times and the dynamic susceptibility for the rotational Brownian motion of fixed axis rotators in an asymmetric double-well potential. An expression for the escape rate valid for all values of the dissipation including the very low damping (VLD), very high damping (VHD), and crossover regimes is derived. It is shown that this expression provides a good asymptotic estimate of the inverse of the smallest nonvanishing eigenvalue λ1 of the underlying Fokker-Planck operator calculated by using the matrix-continued fraction method. For low barriers, where the Mel’nikov and Meshkov approach is not applicable, analytic equations for the correlation time τ∥ of the longitudinal dipole correlation function in the VLD and VHD limits are derived and a simple extrapolating equation valid for all values of the damping is proposed. [ABSTRACT FROM AUTHOR]

The transport of charge carriers in disordered organic materials is considered based on the techniques of generalized Langevin equation. We simulate the one-dimensional diffusion of a charge in the ensemble of molecular chains interacting with the acoustic phonon subsystem of bulk environment. The random local charge transitions between chain links are mutually correlated. The full computation of the zero-field charge mobility for the N, N-di(1-naphthyl)-N, N-diphenyl-(1,1-biphenyl)-4,4-diamine (α-NPD) is performed as an illustration. Several models for the probabilities of local transitions are tested. The individual local diffusion constants are randomly varied along a molecular chain within several orders of magnitude. The stationary diffusion regime establishes for every chain the temperature-dependent partial charge mobility as a frequency-dependent complex-valued response function. It is averaged over the chain ensemble. The computational scheme is simple and efficient. The importance of the memory effect depends on specific properties of a given material. This dependence in terms of the system parameters is discussed. [ABSTRACT FROM AUTHOR]

The spatial genetic structure is a topical issue in the studies of various aspects of ecology and evolution. Using the multilocus autocorrelation method with hypervariable microsatellite genetic markers, we investigated a fine‐scale pattern of genetic structure in 5 local populations of the common shrew Sorex araneus separated by distances of 300–1000 m (the Moscow chromosomal race). Spatial genetic autocorrelation analysis based on 5 microsatellite loci (expected heterozygosity >0.79) with 90 alleles revealed a consistent pattern of significant positive genetic structure. By testing the autocorrelation at multiple scales from 25 to 500 m, we found that positive spatial genetic structure is detectable in distance classes of <500 m. The weaker spatial genetic structure positively correlated with a higher ratio of nonresident individuals to residents' activity (number of captures). In contrast to nonresident animals, the residents demonstrated prominent genetic structure. Genetic difference (FST) between the populations was significant (0.016–0.051) and comparable with that between populations of different races analyzed previously (0.016–0.038). FST was not associated with geographic distance. These demographic patterns allowed us to propose a scheme of genetic‐structure dynamics involving periodic appearance of more related local groups and renewal of allelic profiles from а common pool where the alleles are mixed. The scheme predicts fluctuating genetic structure and random similar differences among local populations. [ABSTRACT FROM AUTHOR]

Abstract: A solution of the infinite hierarchy of differential–recurrence relations for the statistical moments (orientational correlation functions) governing the inertial rotational Brownian motion and the dielectric relaxation of an assembly of spherical top molecules is obtained in terms of a continued fraction. It is shown analytically that the solution based on this approach agrees with that of Sack [Proc. Phys. Soc. Lond. B 70 (1957) 414] (with a corrected misprint) and Fixman and Rider [J. Chem. Phys. 51 (1969) 2425] obtained from the corresponding Fokker–Planck equation. [Copyright &y& Elsevier]

[Display omitted] • The first multi-locus and complete taxon sampling phylogeny of Palaearctic ground squirrels. • Four major clades diverged during the Late Miocene. • The main speciation events took place during the Calabrian stage of the Pleistocene. • A new species from West Siberia is discovered and described. Previous efforts to reconstruct evolutionary history of Palearctic ground squirrels within the genus Spermophilus have primarily relied on a single mitochondrial marker for phylogenetic data. In this study, we present the first phylogeny with comprehensive taxon sampling of Spermophilus via a conventional multilocus approach utilizing five mitochondrial and five nuclear markers. Through application of the multispecies coalescent model, we constructed a species tree revealing four distinct clades that diverged during the Late Miocene. These clades are 1) S. alaschanicus and S. dauricus from East Asia; 2) S. musicus and S. pygmaeus from East Europe and northwestern Central Asia; 3) the subgenus Colobotis found across Central Asia and its adjacent regions and encompassing S. brevicauda , S. erythrogenys , S. fulvus , S. major , S. pallidicauda , S. ralli , S. relictus , S. selevini , and S. vorontsovi sp. nov.; and 4) a Central/Eastern Europe and Asia Minor clade comprising S. citellus , S. taurensis , S. xanthoprymnus , S. suslicus , and S. odessanus. The latter clade lacked strong support owing to uncertainty of taxonomic placement of S. odessanus and S. suslicus. Resolving relationships within the subgenus Colobotis , which radiated rapidly, remains challenging likely because of incomplete lineage sorting and introgressive hybridization. Most of modern Spermophilus species diversified during the Early–Middle Pleistocene (2.2–1.0 million years ago). We propose a revised taxonomic classification for the genus Spermophilus by recognizing 18 species including a newly identified one (S. vorontsovi sp. nov.), which is found only in a limited area in the southeast of West Siberia. Employing genome-wide single-nucleotide polymorphism genotyping, we substantiated the role of the Ob River as a major barrier ensuring robust isolation of this taxon from S. erythrogenys. Despite its inherent limitations, the traditional multilocus approach remains a valuable tool for resolving relationships and can provide important insights into otherwise poorly understood groups. It is imperative to recognize that additional efforts are needed to definitively determine phylogenetic relationships between certain species of Palearctic ground squirrels. [ABSTRACT FROM AUTHOR]

It is shown how the rotational diffusion model of polar molecules (which may be described in microscopic fashion as the diffusion limit of a discrete time random walk on the surface of the unit sphere) may be extended to anomalous nonlinear dielectric relaxation and the dynamic Kerr effect by using a fractional kinetic equation. This fractional kinetic equation (obtained via a generalization of the noninertial kinetic equation of conventional rotational diffusion to fractional kinetics to include anomalous relaxation) is solved using matrix continued fractions yielding the complex nonlinear dielectric susceptibility and the Kerr function of an assembly of rigid dipolar particles acted on by external superimposed dc E0 and ac E1(t)=E1 cos ωt electric fields of arbitrary strengths. In the weak field limit, analytic equations for nonlinear response functions are also derived. [ABSTRACT FROM AUTHOR]

The extension of the Kramers theory of the escape rate of a Brownian particle from a potential well to the entire range of damping proposed by Mel'nikov and Meshkov [J. Chem, Phys. 85, 1018 (1986)] is applied to the rotational Brownian motion of fixed axis rotators in a double well cosine potential. The procedure yields an expression for the Kramers escape rate valid for all values of the dissipation including the very low damping (VLD), very high damping (VHD), and crossover regimes. This equation provides a good asymptotic estimate of the correlation time τ of the longitudinal dipole moment correlation function calculated by solving the underlying Langevin equation using the matrix-continued fraction method. Moreover, for low harriers, where the Mel'nikov and Meshkov approach is not applicable, analytic equations for τ in the VLD and VHD limits are derived and a simple extrapolating equation that is valid for all values of the damping is proposed. [ABSTRACT FROM AUTHOR]

The fractional Klein–Kramers equation describing the fractal time dynamics of an assembly of fixed axis dipoles rotating in a double-well cosine potential representing the internal field due to neighboring molecules is solved using matrix continued fractions. The solution includes both inertial and strong internal field effects which in combination produce a strong resonance peak (Poley absorption) at high frequencies due to librations of the dipoles in the potential, an anomalous relaxation band at low-frequencies mainly arising from overbarrier relaxation and a weaker relaxation band at mid frequencies due to the fast intrawell modes. The high frequency behavior is controlled by the inertia of a dipole, so that the Gordon sum rule for dipolar absorption is satisfied ensuring a return to optical transparency at very high frequencies. Application of the model to the broad-band (0-THz) dielectric loss spectrum of a dilute solution of the probe dipolar molecule CH2Cl2 in glassy decalin is demonstrated. [Copyright &y& Elsevier]

The linear dielectric response of an assembly of symmetric top dipole molecules (each of which is free to rotate in space) is evaluated in the context of fractional dynamics. The infinite hierarchy of differential-recurrence relations for the relaxation functions appropriate to the dielectric response is derived by using the underlying inertial fractional Fokker–Planck (fractional Klein–Kramers) equation. On solving this hierarchy in terms of continued fractions (as in normal rotational diffusion), the complex dynamic susceptibility is calculated for typical values of the model parameters for the particular case of linear molecules. It is shown that the model can reproduce nonexponential anomalous dielectric relaxation behaviour at low frequencies (ωτ≤1, where τ is the Debye relaxation time) and the inclusion of inertial effects ensures that optical transparency is regained at very high frequencies (in the far infrared region) so that Gordon''s sum rule for integral dipolar absorption is satisfied. [Copyright &y& Elsevier]

A simple analytic equation which allows one to obtain a qualitative understanding of the physical processes underlying the behavior of the longitudinal complex susceptibility χ||(ω) of an assembly of noninteracting uniaxial superparamagnetic particles subjected to a strong external uniform DC magnetic field is derived in the context of Brown''s model of magnetic relaxation. It is shown that a knowledge of 3 time constants characterizing the magnetization relaxation, viz. the integral relaxation time τ, the effective relaxation time τef, and the inverse of the smallest eigenvalue λ1 of the Fokker–Planck operator are sufficient to accurately predict the spectrum of χ||(ω) in all frequency ranges of interest as well as the behavior of the equilibrium correlation function of the longitudinal component of the magnetization C||(t) in the time domain at all values of the barrier and external field parameters. [Copyright &y& Elsevier]

The utility of DNA Barcoding for species identification and discovery has catalyzed a concerted effort to build the global reference library; however, many animal groups of economical or conservational importance remain poorly represented. This study aims to contribute DNA barcode records for all ground squirrel species (Xerinae, Sciuridae, Rodentia) inhabiting Eurasia and to test efficiency of this approach for species discrimination. Cytochrome c oxidase subunit 1 (COI) gene sequences were obtained for 97 individuals representing 16 ground squirrel species of which 12 were correctly identified. Taxonomic allocation of some specimens within four species was complicated by geographically restricted mtDNA introgression. Exclusion of individuals with introgressed mtDNA allowed reaching a 91.6% identification success rate. Significant COI divergence (3.5–4.4%) was observed within the most widespread ground squirrel species (Spermophilus erythrogenys, S. pygmaeus, S. suslicus, Urocitellus undulatus), suggesting the presence of cryptic species. A single putative NUMT (nuclear mitochondrial pseudogene) sequence was recovered during molecular analysis; mitochondrial COI from this sample was amplified following re-extraction of DNA. Our data show high discrimination ability of 100 bp COI fragments for Eurasian ground squirrels (84.3%) with no incorrect assessments, underscoring the potential utility of the existing reference librariy for the development of diagnostic ‘mini-barcodes’. [ABSTRACT FROM AUTHOR]

Stream chemistry in permafrost regions is regulated by a variety of drivers that affect hydrologic flowpaths and watershed carbon and nutrient dynamics. Here we examine the extent to which seasonal dynamics of soil active layer thickness and wildfires regulate solute concentration in streams of the continuous permafrost region of the Central Siberian Plateau. Samples were collected from 2006 to 2012 during the frost-free season (May–September) from sixteen watersheds with fire histories ranging from 3 to 120 years. The influence of permafrost was evident through significantly higher dissolved organic carbon (DOC) concentrations in the spring, when only the organic soil horizon was accessible to runoff. As the active layer deepened through the growing season, water was routed deeper through the underlying mineral horizon where DOC underwent adsorption and concentrations decreased. In contrast, mean concentrations of major cations (Ca 2+ > Na + > Mg 2+ > K +) were significantly higher in the summer, when contact with mineral horizons in the active zone provided a source of cations. Wildfire caused significantly lower concentrations of DOC in more recently burned watersheds, due to removal of a source of DOC through combustion of the organic layer. An opposite trend was observed for dissolved inorganic carbon and major cations in more recently burned watersheds. There was also indication of talik presence in three of the larger watersheds evidenced by Cl − concentrations that were ten times higher than those of other watersheds. Because climate change affects both fire recurrence intervals as well as rates of permafrost degradation, delineating their combined effects on solute concentration allows forecasting of the evolution of biogeochemical cycles in this region in the future. [ABSTRACT FROM AUTHOR]

Abstract: An approach for preparing MoS2 films in mild conditions is described, which is based on producing the suspensions of nanocrystalline MoS2 particles in organic solvent via exfoliation of MoS2 crystals, followed by the deposition of these suspensions onto substrate. According to X-Ray diffraction data, acetonitrile suspensions yield highly oriented films with the basal planes of MoS2 crystallites being mainly oriented in the plane parallel to the substrate. Atomic force microscopy examination revealed changes in the film surface topography under variation of the film thickness, which varied in the range of 0.03–2.2μm. The optical absorption spectra of the obtained MoS2 films were found to resemble those of thin natural MoS2 single crystals. Dark conductivity of the films with the thickness of d =0.12–2.2μm was determined to be ~10−3 S∙cm−1 at 300K. Experimental data on the conductivity in the temperature range of 10–300K were fitted to Mott function for 3D variable-range hopping. The synthesized MoS2 films were found to be photosensitive in the range of 300–800nm. They provided the value of reduced photocurrent density of about 7∙10−4 (A∙cm)/(WV) under photoexcitation at 575nm. [Copyright &y& Elsevier]

Abstract: The longitudinal relaxation time and the complex dielectric polarizability of rod-like molecules with dipole moment parallel to the long axis in a biaxial nematic liquid crystal are calculated using as model the rotational Brownian motion in a mean field potential so reducing the problem to a solution of a set of linear differential-recurrence relations for statistical moments (the appropriate equilibrium orientational correlation functions). The solution of this set is obtained by matrix continued fractions. Moreover, simple analytic equations (based on the exponential separation of the time scales of the intrawell and overbarrier (interwell) relaxation processes), allowing one to understand the qualitative behavior of the system and accurately predicting the longitudinal complex polarizability for wide range of the barrier height and anisotropy parameters, are proposed. [Copyright &y& Elsevier]

Abstract: The relaxation time of the magnetization of single-domain ferromagnetic particles with cubic and triaxial (i.e., orthorhombic) magnetocrystalline anisotropy is estimated for all dissipation regimes, i.e., very low damping (VLD), intermediate-to-high damping (IHD), and turnover, using the method of Coffey et al. (Adv. Chem. Phys. 117 (2001) 483; Phys. Rev. E 63 (2001) 021102). Their method generalizes the Mel’nikov–Meshkov approach (J. Chem. Phys. 85 (1986) 1018) for bridging the VLD and IHD Kramers’ escape rates for mechanical Brownian particles (the Kramers’ turnover problem) to the analogous magnetic turnover problem. It is shown that the simple asymptotic formulae for the greatest relaxation time so obtained are in complete agreement with the relaxation time calculated from the infinite hierarchy of linear differential-recurrence equations for the statistical moments. This hierarchy, which governs the relaxation of the magnetization of an individual particle, is derived by averaging the underlying stochastic Landau–Lifshitz–Gilbert equation over its realizations. The exact solution of the system of moment equations is obtained by matrix continued fractions. [Copyright &y& Elsevier]