Prediction of adsorption equilibria of water–methanol mixtures in zeolite NaA by molecular simulation

Predictions for the adsorption of mixtures of water and methanol in zeolite NaA are reported. The pressure dependence of the adsorption properties such as equilibrium amounts of adsorption and isosteric heats of adsorption are calculated at 378 K by molecular simulations using effective pair potential models. These data are also determined for the adsorption from liquid mixtures. The models predict selectivity inversion in the investigated range of pressure. The change in adsorption ratios can partly be explained by the structural characteristics of the system.     

Pyridine derivatives as complexing agents. XIII. The stability of the palladium(II) complexes with pyridine, 2,2′-bipyridyl,and 1,10-phenanthroline

The stability constants of the palladium(II) complexes with pyridine, 2,2′-bipyridyl, and 1,10-phenanthroline have been determined with potentiometric and spectrophotometric measurements. In the case of pyridine, the spectra of solutions of the components with different pH values are used. Otherwise, hydroxyl ion or a tetraamine are exchanged with the diamines in order to investigate equilibria from which the desired stability constants are also obtained. With pyridine, the values of K₁, K₂, K₃, and K₄ are of the expected magnitude. For the diamines, in contrast, a very strong drop from log K₁ to log K₂ is observed; this is due to the steric hindrance between the H atoms in α position with respect to the coordinated N atoms in the 1:2 complexes. Some complex salts of the different ligands have been prepared.     

Dielectric behavior of DNA in water–organic co-solvent mixtures

The radiowave dielectric dispersions of DNA in different water–organic co-solvent mixtures have been measured in the frequency range from 100 kHz to 100 MHz, where the polarization mechanism is generally attributed to the confinement of counterions within some specific lengths, either along tangential or perpendicular to the polyion chain. The dielectric dispersions have been analyzed on the basis of two partially different dielectric models, a continuum counterion fluctuation model proposed by Mandel and a discrete charged site model, proposed by Minakata. The influence of the quality of the solvent on the dielectric parameters has been investigated in water–methanol and water–glycerol mixtures at different composition, by varying the permittivity ?_m and the viscosity η of the solvent phase. The analysis of the dielectric spectra in solvents where electrostatic and hydrodynamic interactions vary with the solvent composition suggests that both the two models are able, in principle, to account for the observed high-frequency dielectric behavior. However, while some certain assumptions are necessary about the polyion structure within the Mandel model, no structural prerequisite is needed within the Minakata model, where the polarization mechanism invoked considers a radial counterion exchange with the outer medium, which is largely independent of the local polyion conformation.     

Computer-augmented modeling studies of Pb(II) and Cd(II) complexes with maleic acid in ethylene glycol–water mixture

Chemical speciation of binary complexes of Pb(II) and Cd(II) ions with maleic acid have been studied pH metrically in the concentration range of 0–50% v/v ethylene glycol (EG)–water mixtures maintaining an ionic strength of 0.16 molL^?1 at 303 K. Alkalimetric titrations were carried out in different relative concentrations of metal and maleic acid. Stability constants of various models of binary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The species detected are ML₂, ML₃, and ML₂H for Pb(II) and Cd(II). The chemical speciation, metal bioavailability, and transportation are explained based on the distribution diagrams.     

Effect of 2,2′-bipyridyl on porphyrin synthesis in etiolated and light-treated barley leaves

The effects of 2,2′-bipyridyl on porphyrin formation differed in illuminated and dark-treated barley leaves. In the dark, bipyridyl treatment increased photoconvertible protochlorophyllide (Pchlide, P650) and decreased the protohaem content. The increase in Pchlide could not be wholly accounted for by a diversion of ‘substrate’ from protohaem synthesis. The rate of Pchlide regeneration was slightly higher in chelator treated leaves which suggests increased δ-aminolaevulinic acid (ALA) synthesis. Only small quantities of Mg-protoporphyrinmonomethylester (Mg-protoME) were detected in etiolated leaves treated with bipyridyl in the dark. Protochlorophyll (P630) synthesis from exogenously supplied ALA was lower in the chelator treatments. The results suggest that only when substantial quantities of ALA are being utilized in dark-grown leaves does a ‘metal’ become limiting in the bipyridyl treated leaves. In the light, bipyridyl inhibited chlorophyll synthesis, again suggesting that when substantial amounts of ALA were being utilized a ‘metal’ becomes rate limiting. Bipyridyl treatment also inhibited ALA production in light-treated leaves. The incorporation of glycine-[¹⁴C] into ALA in the presence of bipyridyl was severely restricted compared to the incorporation of glutamate-[¹⁴C]. The data suggest two pathways for ALA synthesis; the classical ALA-synthetase which utilizes glycine and is operative in dark-grown leaves and a second enzyme system, which uses glutamate, and is of quantitative importance in the light.     

Speciation of binary complexes of Co(II), Ni(II) and Cu(II) with L-aspartic acid in propylene glycol–water mixtures

Abstract

Speciation of binary complexes of Co(II), Ni(II) and Cu(II) with L-aspartic acid in (0-60% v/v) propylene glycol-water mixtures was studied pH metrically at 303.0±0.1 K and at an ionic strength of 0.16 mol L^-1. The binary species refined were ML, ML₂, ML₂H₂, ML₂H₃ and ML₂H₄. The stabilities of the complexes followed the Irving-Williams order i.e.Co(II) <Ni(II) < Cu(II). The linear variation of stability constants as a function of dielectric constant of the medium indicated the dominance of electrostatic forces over non-electrostatic forces. Some species were stabilised due to electrostatic interactions and some were destabilised due to the decreased dielectric constant. The order of ingredients influencing the magnitudes of stability constants due to incorporation of errors in their concentrations was alkali > acid > ligand > metal. Equilibria for the formation of binary complexes were proposed based on the forms of the ligand and their existence at different pH values.     

Preferential solvation and association constants of 2-exo and 2-endo norbornyl chlorides in water–acetone mixtures

Molecular dynamics simulations of 2-exo and 2-endo norbornyl chlorides are performed in water–acetone mixtures to investigate the role of preferential solvation on the dissociation process of norbornyl chlorides. Increase in the association constants between carbon (C2) and chloride (Cl) of norbornyl chlorides with an increase in mole fraction of acetone supports the experimental data for the corresponding decrease in the rates of solvolysis for both exo and endo norbornyl chlorides. When the mole fraction of water is increased in water–acetone mixtures, acetone molecules are replaced by water molecules around C2 as well as Cl of norbornyl chloride. Preferential solvation by water is analysed using running coordination numbers and excess coordination numbers around C2 and Cl.     

Binary complexes of Mg(II) and Ca(II) in water–surfactant mixtures

Abstract

Chemical speciation of Mg(II) and Ca(II) complexes of L-histidine in the presence of water–surfactant mixtures in the concentration range 0.0–2.5% w/v CTAB and SDS, 0.0–5.0% v/v TX-100 maintaining an ionic strength of 0.16 mol dm^?3 at 303 K has been studied pH metrically. The active forms of the ligand are LH₃²⁺, LH₂⁺, LH and L^?. The models containing different numbers of species were refined by using the computer program, MINIQUAD75. The predominant species detected were ML₂H₄⁴⁺, ML₂H₃³⁺, ML₂H₂²⁺, and ML₂. The best fit chemical models were arrived at based on statistical parameters. The trend in variation of complex stability constants with change in the composition of the medium is explained on the basis of electrostatic and non-electrostatic forces. The effect of errors in the stability constants was also studied. Chemical speciation was also discussed based on the distribution diagrams.     

Effect of glycerol–water binary mixtures on the structure and dynamics of protein solutions

We have performed 20?ns of fully atomistic molecular dynamics simulations of Hen Egg-White Lysozyme in 0, 10, 20, 30, and 100% by weight of glycerol in water to better understand the microscopic physics behind the bioprotection offered by glycerol to naturally occuring biological systems. The solvent exposure of protein surface residues changes when glycerol is introduced. The dynamic behavior of the protein, as quantified by the incoherent intermediate scattering function, shows a nonmonotonic dependence on glycerol content. The fluctuations of the protein residues with respect to each other were found to be similar in all water-containing solvents, but different from the pure glycerol case. The increase in the number of protein–glycerol hydrogen bonds in glycerol–water binary mixtures explains the slowing down of protein dynamics as the glycerol content increases. We also explored the dynamic behavior of the hydration layer. We show that the short length scale dynamics of this layer are insensitive to glycerol concentration. However, the long length scale behavior shows a significant dependence on glycerol content. We also provide insights into the behavior of bound and mobile water molecules.     

Vapor–liquid equilibria and thermophysical behavior of the SPC-HW model for heavy water

The vapor–liquid coexistence curve of the simple point charge heavy-water model (SPC-HW), [J. Chem. Phys., 114, 8064–8067 (2001)] is determined by Gibbs Ensemble Monte-Carlo (GEMC) simulation. The estimated critical conditions of the model based on the Wegner-type expansion for the order parameters and the rectilinear diameter are ρ_c = 0.300 g/cc, T _c = 661 K and P _c = 156 bars. The dielectric constant determined by isothermal–isochoric molecular dynamics is underpredicted along the coexistence curve by 29–44% in comparison with the experimental values. The analysis of the orthobaric temperature dependence of the system microstructure, in terms of the three site–site radial distribution functions, indicates that the first coordination numbers for the oxygen–oxygen and the oxygen–deuterium interactions are ～4.3 ± 0.1 and ～1.9 ± 0.1 at T = 300 K, and decrease by 15 and 55%, respectively, at criticality. The dipole–dipole correlation functions show that the orientational order in heavy water is quickly lost beyond the first oxygen–oxygen coordination shell. The model's second virial coefficient is determined by Monte-Carlo integration and used to aid the interpretation of the predicted phase equilibrium results.     

A comparison of force fields for ethanol–water mixtures

Aqueous ethanol mixtures are studied through molecular dynamics simulations with the focus on exploring how various force field models reproduce the association and its influence on selected thermo-physical properties of these mixtures. The most important conclusion seems to be the inadequacy of all classical force fields to reproduce the very peculiar shape of the excess enthalpy of these mixtures, as a function of the ethanol concentration, neither quantitatively nor qualitatively. The Kirkwood–Buff (KB) integrals calculated using the simulation data follow the same trends as the experimental ones. This suggests complicated correlation of the excess enthalpy with the concentration fluctuation and clustering in these mixtures. The KB force field shows better overall agreement with experimental results than the other studied models.     

Protonation linked equilibria and apparent affinity constants: the thermodynamic profile of the α-chymotrypsin–proflavin interaction

Protonation/deprotonation equilibria are frequently linked to binding processes involving proteins. The presence of these thermodynamically linked equilibria affects the observable thermodynamic parameters of the interaction (K _obs, ΔH _obs⁰). In order to try and elucidate the energetic factors that govern these binding processes, a complete thermodynamic characterisation of each intrinsic equilibrium linked to the complexation event is needed and should furthermore be correlated to structural information. We present here a detailed study, using NMR and ITC, of the interaction between α-chymotrypsin and one of its competitive inhibitors, proflavin. By performing proflavin titrations of the enzyme, at different pH values, we were able to highlight by NMR the effect of the complexation of the inhibitor on the ionisable residues of the catalytic triad of the enzyme. Using ITC we determined the intrinsic thermodynamic parameters of the different equilibria linked to the binding process. The possible driving forces of the interaction between α-chymotrypsin and proflavin are discussed in the light of the experimental data and on the basis of a model of the complex. This study emphasises the complementarities between ITC and NMR for the study of binding processes involving protonation/deprotonation equilibria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Application of kinetic Monte Carlo method to the vapour–liquid equilibria of associating fluids and their mixtures

Canonical kinetic Monte Carlo (C-kMC) simulations have been carried out to assess their feasibility and potential for calculating the vapour–liquid equilibria of various pure components with increasingly strong electrostatic interactions (carbon dioxide, methanol, ammonia and water) over a wide range of temperatures and for methanol/water mixtures at 298 K. The simulation results show that C-kMC is successful as a method for studying phase equilibria and thermodynamic properties. For all the examples investigated, the performance of the C-kMC method is at least as good as that of the conventional Monte Carlo (MC) methods and is efficient at low temperature where these fail. It also provides a route that is superior to the Widom method for the calculation of chemical potential. We recommend this method for this purpose and as an alternative to conventional MC for simulations of strongly associating fluids and at low temperatures.     

Carbonyl complexes of Rh(I), Ir(I) and Mo(0) containing substituted derivatives of 1,10-phenanthroline and 2,2′-bipyridine

The A₁ symmetry v_CO of the carbonyl complexes [Mo(chel)(CO)₄], [M(chel)(CO)₂] [PF₆] (M=Rh, Ir; chel=bipy, phen and substituted derivatives) are used for determining the electron donor-acceptor properties of the title ligands. The steric hindrance of the methyl groups in positions 2 and 9 of the phenanthroline favours the formation of Rh(I) and Ir(I) pentacoordinated derivatives.     

Distribution and pollution evaluation of fluoride in a soil–water–plant system in Shihezi,Xinjiang, China

Fluoride (F) pollution is a serious environmental problem in some areas of China, but it has yet to be reported in a soil–water–plant system in Shihezi, Xinjiang. This study was undertaken to investigate the distribution and migration rule of F in soil, water, and plants, and to evaluate F pollution of soil. Results showed that the average concentration of total F (T-F) in the topsoil in the northwest, north, and southeast of Shihezi was higher than the national average T-F (478 mg/kg), while it was lower in southwest. The highest T-F contents of the soil profile were detected in the depth of 20 cm. The F content in groundwater in the northwest region was higher than the GB/T 14848–93 (1.0 mg/L), whereas the F contents in other water samples were within the standard. The F contents (1.75?2.81 mg/kg) in plant leaves were higher than the food limits (1.0 mg/kg). The obtained comprehensive pollution index of the soil was 1.86, which means a mild concentration of F in Shihezi. This research has reference value for the study of F pollution and comprehensive control in the northwest oasis with the typical arid and saline conditions.     

Regulation of seedling growth by ethylene and the ethylene–auxin crosstalk

Planta - This review highlights that the auxin gradient, established by local auxin biosynthesis and transport, can be controlled by ethylene, and steers seedling growth. A better understanding of...     

Molecular simulation on self-assembly morphologies of rod–coil polystyrene-b-poly(ethylene glycol) copolymers in aqueous solution

In this work, dissipative particle dynamics simulations were performed to study the self-assembly morphologies of rod–coil block copolymer polystyrene-b-poly(ethylene glycol) (PS-b-PEG) in aqueous solution under different variables. Effect of time evolution on the self-assembly morphology of PS-b-PEG was observed first. Besides, spherical, cylindrical and lamellar structures were obtained at a range of concentrations. In addition, their self-assembly morphologies could also be regulated by the PS chain length. Our simulation results can provide deeper insight into the microstructure of rod–coil block copolymers in aqueous solution, which can be useful to guide the molecular design and experimental preparation of novel rod–coil block copolymers with controlled structures.     

Phase Behavior, Stability, and Mesomorphism of Monostearin–oil–water Gels

This paper is the characterization of a new material comprised of oil, water, monostearin and stearic acid, which can be used as a heart-friendly, low-saturate, trans fatty acid-free spreadable fat and shortening. Oil–water–monstearin mixtures formed a gel above 2% monostearin and 30% water and were stable over a month’s time. An increase in the storage modulus (G′), and peak melting temperature (T _m) was observed over time, which suggests a slow change in structure to a more solid form. Powder x-ray diffraction measurements at temperatures above the Krafft temperature of the monglyceride (57°C) indicated the existence of a lamellar liquid crystalline phase $ {\left( {L_{\alpha } } \right)} This paper is the characterization of a new material comprised of oil, water, monostearin and stearic acid, which can be used as a heart-friendly, low-saturate, trans fatty acid-free spreadable fat and shortening. Oil–water–monstearin mixtures formed a gel above 2% monostearin and 30% water and were stable over a month’s time. An increase in the storage modulus (G′), and peak melting temperature (T _m) was observed over time, which suggests a slow change in structure to a more solid form. Powder x-ray diffraction measurements at temperatures above the Krafft temperature of the monglyceride (57°C) indicated the existence of a lamellar liquid crystalline phase with a (001) reflection occurring at 50 ?. In addition to the 50 ? reflection at small angles, a wide angle reflection at 4.2 ? was observed upon cooling below 60°C, indicating a transition from the to the phase, which upon storage at 22°C for one day converted to the coagel, or β-gel phase.