A new time saving numerical method for calculation of equilibrium potential and density distribution of mobile ions around the polyion in a polyelectrolyte system is proposed: the region around the polyion is being divided into two zones-internal and external; in the internal zone all the ions are accounted explicity with the aid of Monte-Carlo procedure; in the external zone the combined Monte-Carlo-self consistent field method proposed earlier is applied, an exchange of ions between regions is being implied. For 1:1 electrolyte the optimal choice of the boundary between the zones has been demonstrated. As an example of a more complicated system calculation for 2:2:1:1 electrolyte was carried out. 相似文献
Abstract Time saving procedures unifying Monte Carlo and self consistent field approaches for the calculation of equilibrium potentials and density distributions of mobile ions around a polyion in a polyelectrolyte system are considered. In the final version of the method the region around the polyion is divided into two zones—internal and external; all the ions of the internal zone are accounted for explicitly in a Monte Carlo procedure, in the external zone the self consistent field approximation is applied with an exchange of ions between regions. Simulations are carried out for cylindrical and spherical polyions in solutions with mono-and divalent ions and their mixtures. The results are compared with Poisson—Boltzmann approximation and experimental data on intrinsic viscosity. 相似文献
BackgroundThe impact of chemical elements on the biosphere is a function of their concentration and chemical form. Elucidation and prognosing of the latters in water basins and soil extracts is of particular significance for the assessment of their bioaccumulation in plants and animals.ObjectivesTrace metals dynamics in the system water – soil–plant–wild rats – Hymenolepis diminuta in two agro-industrial zones (East and West) around Maglizh city, Bulgaria were investigated through experimental studies and thermodynamic modelling of the chemical species.MethodsSamples from surface waters of rivers, their nearby uncultivated soils, meadow uncultivated vegetation (Ranunculus acris and Gramineae) and field rats were collected. In situ measurements and laboratory analyses were performed for the determination of the physico-chemical characteristics and total concentrations of Al, Fe, Mn, Ni, Cu, Zn and Pb. The distribution of their dissolved chemical species in water samples and in the aqueous soil extracts was calculated using a thermodynamic approach. The relationship chemical species - bioaccumulation was discussed.ResultsWaters and soils in the East zone of Maglizh area were found to be more polluted compared to those in the West one, regarding Ni, Mn, Zn, Pb and Cu, while Mn and Cu displayed the highest mobility in West zone soils. Trace metals contents in Ranunculus acris exceed that in Gramineae, since the highest accumulation factors were calculated for Cu and Zn. The highest accumulation in rats was found for Zn followed by Cu, being higher in the West zone. Thermodynamic modelling shows that Mn2+ free ions are dominant in both waters and aqueous soil extracts. Ni2+ and Zn2+ ions followed by metal-organic complexes are dominant in waters of East zone while metal-organic complexes followed by free ions are dominant in waters of West zone and both soil extracts. Metal-organic complexes are dominant for Fe, Cu and Pb in all samples studied, while mainly hydroxy forms (Al(OH)4−) followed by metal-organic complexes are typically for Al depending on pH.ConclusionsExperimentally established bioaccumulation of trace metals in the studied vegetation and rats is a consequence of the total concentration of trace metals in waters and soils, their mobility and chemical species. The dominance of organic complexes of trace metals is a prerequisite for their bioaccumulation in plants. Rats are in direct contact with the soil solution and therefore, of importance is the content of free ions of Mn2+, Ni2+, Zn2+, which are easily absorbed through the skin. The host-helminth system wild rat/H. diminuta could be used as a bioindicator for trace metals pollution. 相似文献
Cuticle ultrastructure of Criconemella curvata and C. sphaerocephala females is presented; males were available only in the second species. Ultrathin sections revealed three major zones: cortical, median, and basal. The cortical zone in the females consists of an external and internal layer. In C. curvata the external layer is trilaminate and at each annule it is covered by a multilayered cap. In C. sphaerocephala the trilaminate layer is lacking and the external cortical layer includes an osmophilic coating. In both species the internal layer consists of alternate striated and unstriated sublayers. The median zone is fibrous with a central lacuna and the zone is interrupted between the annules. The basal zone of the cuticle is striated and narrower between each annule. The cuticle of the C. sphaerocephala male is typical of Tylenchida, except under both lateral fields; the striated layer becomes forked at the first incisure and the innermost two prongs of the fork overlap each other, resulting in a continuous striated band. 相似文献
The range of application of the Manning theory of polyelectrolyte solutions (J. Chem. Phys., 51 , 924 (1969)) is extended to finite concentrations of simple electrolyte by the empirical superposition of excess free energies arising from (i) interactions between mobile ions and polyions and (ii) mutual interactions between mobile ions. A comparison of published results with the modified theory shows excellent agreement over a wide range of concentrations. Results for a polyelectrolyte of low charge density suggest that the effective inter-charge spacing may be less than that of the fully extended polyion. 相似文献
BackgroundGold mining activities in South Africa resulted in contamination of residential environment with uranium-rich wastes from mine tailings. Health of the people living around the mine tailings could be affected by uranium exposure due to its hazardous chemotoxic and radiological properties.MethodsWe conducted a cross-sectional study to assess i) uranium (U) concentrations in individual hair samples of children and adults living in close proximity to mine tailings in Northeast- Soweto in Johannesburg, South Africa, and ii) the association between U concentrations in hair and various factors, including zone of residence, socio-demographic and housing characteristics. Sampling sites were divided into three zones based on the distance between a dwelling and a cluster of mine tailings (zone 1: <= 500 m, zone 2: 2–3 km away, zone 3: 4–5 km away). U concentrations in hair samples were measured using inductively coupled plasma mass spectrometry. To test the association between U concentrations and selected factors we used robust regression models with log-transformed U concentrations.ResultsAmong 128 subjects with available U measurements, 63 (49%) were children (ages 7–15 years) of which 38 were girls, the remaining 65 (51%) were adult females. Mean (median) U concentration in hair samples was 143 (92) µg/kg. In the mutually adjusted analyses, only an inverse association between age and U concentration in hair remained statistically significant, with geometric mean in children being 2.1 times higher compared to adults (P < 0.001). There was no evidence of an association between zones and U concentration (P = 0.42).ConclusionsThere was little evidence of association between U concentration in hair and distance from the mine tailings within the 5 km range, but overall concentrations were elevated compared to general population samples in other parts of the world. Children had statistically significantly higher geometric mean of uranium concentration in hair compared to adults. The results are important for improvement of mining waste policies and implementation of health monitoring and protective measures in populations at risk.Article categoryResearch Article 相似文献
Abstract A new approach to the prediction of secondary RNA structures based on the analysis of the kinetics of molecular self-organisation is proposed herein. The Markov process is used to describe structural reconstructions during secondary structure formation. This process is modelled by a Monte-Carlo method. Examples of the calculation by this method of the secondary structures kinetic ensemble are given. Distribution of time-dependent probabilities within the ensembles is obtained. An effective method for search for the equilibrium ensemble is also suggested. This method is based on the construction of a tree of all possible secondary structures of RNA. By ascribing a probability for each structure (according to its free energy) the Boltzmann equilibrium ensemble can be obtained. 相似文献
Elucidating the mechanical response of diamond is a difficult task due to its ultrahard nature. Here, we applied a molecular dynamics (MD) method to investigate the mechanical response of single-crystal diamond under nanoindentation. There was no obvious “pop in” phenomenon on the load–depth curve, and the elastic modulus deduced from the curve was 1128 GPa, which was similar to the value obtained from experimental measurements. Results from computed tomography (CT) and the coordination number showed that the distribution of the mismatched C atoms around the deformation zone took the form of a ‘double cross.’ The atoms around the indenter tip could be divided into two zones, a translation zone and a lattice distortion zone, based on their movements. Subsequent first-principles calculations revealed that the C-atom displacement barrier varied significantly with direction, which resulted in shear stress between the two zones and the formation of the double-cross splitting.
Summary Sperm of Nucella lapillus was studied by electron microscopy, including the application of a cytochemical silver method. Using silver impregnation a dense precipitation of Ag granules in spermatocyte II nucleoli was seen over the fibrillar component and a slight one in the granular component. On longitudinal sections of the spermatozoon the results demonstrate that argyrophilic proteins are located in the external limiting zone of the acrosome in the anterior portion of the nucleus between the cytoplasmic and the nuclear membranes, in the posterior end of the nucleus and in the terminal portion of the middle region. These data indicate an affinity for silver in areas of the cytoplasm containing microtubules and in zones of transition. 相似文献
IntroductionDistribution pattern and diversity of flora was compared along an altitudinal gradient using the stratified random sampling design for identifying major plant communities of Kedarnath Wildlife Sanctuary of Garhwal Himalaya, India. The reconnaissance of flora is presented, along with the analysis of the distribution of species, genera, and families within five (5) altitudinal zones. Kedarnath Wildlife Sanctuary which is situated in the Indian Himalayas harbours a rich variety of flora and fauna. The Himalayas are recognized for diverse vegetation distributed over a wide range of topographical conditions.ResultsThe analysis of diversity within five (5) altitudinal zones was carried out and a total of 324 plant species, representing 219 genera belonging to 92 families, were found. The dominant family was Asteraceae; the co-dominant family was Rosaceae, followed by Lamiaceae and Ranunculaceae. Eight (8) families were observed in all the altitudinal zones, while forty (40) families were observed in a single altitudinal zone, and the remaining forty-four (44) families were found in more than one (1) altitudinal zone. Most of the tree species were contagiously distributed, but a few of them were randomly distributed in all the altitudinal zones. The shrubs and herbs were contagiously distributed in all the altitudinal zones. The correlation analysis (P < 0.05) between altitude and number of species showed that altitude is negatively correlated with tree (r = −0.96), shrub (r = −0.61), and herb species (r = −0.20). As per the cluster analysis of tree layer, altitudinal zone - III (2450–2650 m) and altitudinal zone - IV (2900–3100 m) were found most similar. Altitudinal zone–V (3350–3550 m) was found to be dissimilar from the other zones for herbs.ConclusionsAlthough species composition varies with altitude, but there is a complex relationship between species richness and altitudinal gradient. A decreasing pattern in both species richness and family richness for trees, shrubs and herbs, was recorded with increasing altitude. The predominant factors underlying this variability in plant species and biogeography appear to be climatic and specific to each taxonomic group. 相似文献
Results of investigation of phospholipase activity and virulence of pathogenic leptospirae on a solid nutrient medium using the method of agar layers with 1% of L-lecithin in the medium of the second layer are presented. It has been demonstrated that only one zone of translucent medium is formed around the colonies of pathological leptospirae, which is obviously due to the release of phospholipase A. The examined virulent strains of pathogenic leptospirae were found to exhibit greater phospholipase activity (width of the translucent zones being 5.0 +/- 0.34 mm) than the avirulent strains (width of the zones being 1.5 +/- 0.11 mm). 相似文献
Abstract The alkali-ion binding properties of two natural depsipeptide ion carriers, enniatin B (EnB) and valinomycin (VM), are examined and compared by the empirical force field method. While VM has been shown to bind preferentially K+, Rb+, and Cs+ over Na+ in most solvents, EnB is considerably less specific. We find that EnB forms two kinds of complexes, internal and external. In internal complexes, the ion binds to all six carbonyl oxygens, while in external ones, only three oxygens, preferentially those of the D-hydroxy-isovaleryl residues, are bound. The size of the internal cavity is best suited for Na+, while K+ and Rb+ squeeze in asymmetrically by distorting the molecule, and Cs+ not at all. External binding is much less specific. Since internal complexes possess much higher strain energies than external ones, the latter may be at least as stable as the former, even in fairly non-polar solvents. VM is calculated to bind only internally, and with much less strain energy than EnB. The size of its internal cavity is well suited for binding the ions K+, Rb+, and Cs+, but is too big for Na+. The difference between the binding energies of Na+ and K+ is much smaller than that between the corresponding hydration enthalpies, thus explaining the binding preference for the latter ion. 相似文献
This paper describes the ultrastructure of the mature spermatozoon of Heterolebes maculosus. It is the first study of this kind concerning the Opistholebetidae (Platyhelminthes, Digenea). The ultrastructural elements observed in the spermatozoon are: two axonemes with 9+“1” pattern of Trepaxonemata and their attachment zones, two mitochondria, a nucleus, cortical microtubules, external ornamentation of the plasma membrane and spine-like bodies. The number and the disposition of cortical microtubules, the organisation of 11 cortical microtubules disposed in semi-circle around the first mitochondrion in the external ornamentation region and the organisation of the posterior part of the spermatozoon are discussed. Three principal types of posterior part of digenean spermatozoa are proposed. The similarity between the spermatozoon of the Opistholebetidae H. maculosus and Opecoelidae enables us to confirm that these two families are closely related. 相似文献
The growing part of a wheat leaf (the extension zone) is loocatedat the leaf base and following from this it was proposed thatthe absolute leaf extension rate (Ler) can be partitioned intotwo components: the length of the extension zone (Lez) and therelative extension rate of that extension zone (Rez). Rez isan appropriate measure of the efficiency of leaf growth forcomparing different leaves. This model of a wheat leaf was thenused to investigate whether differences in growth rates betweensuccessive leaves on wheat plants were due to differences inhexose sugar or protein concentrations within the extensionzone. Measurements were done in an irrigated field crop suppliedwith 0, 3, 10, or 30 kg N ha1 per week. The mean values of Ler at 15 °C increased with leaf numberand with nitrogen supply as did values for Lez. In contrastRez at 15 °C declined from 0.9 d1 for the first leavesto 0.3 d1 for the flag leaf. Nitrogen supply had littleeffect on Rez. A separate measure of the efficiency of leafgrowth, the responsiveness of Ler to temperature (measured asthe slope of the temperature response curve), also decreasedwith leaf number by the same order as Rez and was similarlyunaffected by nitrogen supply. The protein concentrations in the extension zones of the firstleaves were around 40 mg g1 fr. wt. and this declinedto approximately 20 mg g1 fr. wt for leaves emergingafter tillers emerged and remained low thereafter. Protein concentrationswere not correlated with the external supply of nitrogen. Hexosesugar concentrations followed a reverse pattern of increasingin the later order leaves and these also increased as nitrogensupply decreased. Both Rez and the responsiveness of Ler to temperature were positivelycorrelated with the protein concentration, the relationshipin each case being described by a rectangular hyperbola equation(P < 0.01), and negatively correlated with hexose concentrations.It was concluded that internal competition between growing pointsfor reduced nitrogen caused the observed effects. However, differencesin protein concentrations may not simply reflect differencesin enzyme concentrations; rather these differences may indicatechanges in some other character such as cell numbers. 相似文献