Apparent Molar Volumes of Proline-Leucine Dipeptide in NaCl Aqueous Solutions at 318.15 K

Density measurements of pseudo-binary solutions of proline-leucine dipeptide in aqueous NaCl solutions with molality ranging from 0 to 1 mol-kg^–1 have been performed at 318.15 K. Apparent molar volumes, V_{, 2}, of proline-leucine were calculated from the measured data. Limiting partial molar volumes, V₂, and limiting partial molar volumes of transfer from water to aqueous NaCl solutions, _tr V₂, were derived and interpreted in terms of ion-dipeptide interactions and changes in the characteristics of the hydration shell around the biomolecules as well.     

Improved precision and efficiency of free energy calculations for small systems using lambda-scaled atomic masses and separating conformational and transformational sampling

We present results showing the importance of appropriate treatment of atomic masses in molecular dynamics (MD)-based single topology free-energy perturbations (FEPs) on small molecule systems. The reversibility of gas phase simulations is significantly improved by scaling the atomic mass of mutated atoms with the lambda variable normally used for the scaling of energy terms. Because this effect is less pronounced for solvated systems, it will not cancel in estimates of the relative hydration free energy difference. The advantage of mass scaling is demonstrated by a null mutation of ethane to ethane and the calculation of the relative hydration free energy difference between ethane and n-propane. Furthermore, it is found that the simulation time necessary for converged MD/FEPs is prohibitively large for relative hydration free energy calculations on cyclic alkanes. Therefore, we explore an alternative free energy pathway including strongly constrained conformations to improve convergence in FEP simulations of flexible molecules.     

Thermodynamic properties of peptide solutions: 14. Partial molar expansibilities and isothermal compressibilities of some glycyl dipeptides in aqueous solution

The partial molar volumes at infinite dilution have been obtained for a series of glycyl dipeptides in aqueous solution at 15, 30, and 35°C. These results have been combined with data obtained at 25°C, that were reported earlier, to evaluate the partial molar expansibilities at infinite dilution for the dipeptides at 25°C. These quantities, along with the partial molar heat capacities and isentropic compressibilities at infinite dilution that were reported in previous studies, were used to derive the partial molar isothermal compressibilities at infinite dilution for the glycyl dipeptides at 25°C. The results obtained are rationalized in terms of the hydration of the constituent groups of the dipeptides.     

Intradiffusion coefficients for perchlorate ions in zinc perchlorate and zinc chloride solutions at 25°C: Comparing transport properties of zinc chloride and zinc perchlorate systems

Intradiffusion coefficients for³⁶ClO ₄ ^– have been measured in solutions of zinc perchlorate of concentration 0.1 to 3 mol dm^–3 at 25°C by the diaphragm cell technique. In addition, intradiffusion coefficients for perchlorate ions in zinc chloride solutions have been measured over a concentration range at 25°C. The results confirm previous work on the effect of complexation on diffusion in zinc chloride solutions above a salt concentration of 0.1M. The present data, together with literature data for diffusion coefficients of the other species present in the zinc perchlorate electrolyte system, have enabled a simple analysis of the hydration around the zinc ions to be carried out. This indicates that the water diffusion data are consistent with the zinc ions having an effective hydration sphere of 11 (±2) water molecules. This is in keeping with values obtained for other simple divalent electrolytes using the same model. The model is extended here to allow analysis of water diffusion in zinc chloride solutions taking into account the presence of complexed chloro-zinc species. The experimental data are consistent with the effective hydration of the chloro-zinc complexes being independent of the number of chloride ligands and equal to 18±3 over a concentration range of 0 tol mol-dm^–3. This postulate is discussed in terms of its consequences on the water ligand dynamics for the complex equilibria.     

离子水化模型和水化能的计算

本文根据静电理论和晶体场理论将金属离子的水化分为水合和水溶剂化两个过程,水化能主要决定于这两个过程的能量变化,并由此推得水化能的计算公式。计算结果与实验值基本相符,从而证明这一模型是合理的。     

NH_4~+，尿素和聚乙二醇对蒙脱土的抑制膨胀作用

通过膨胀仪与粒度分布仪对NH_4~+, K~+，尿素和聚乙二醇（PEG）对蒙脱土／ 水悬浮体中土颗粒水化、膨胀及分钐的抑制作用进行了系统研究。发现NH_4~+与 K~+一样可通过嵌入蒙脱土晶体的六角网孔而产生相似的水化抑制作用。尿素随着 含量的升高对粘土水化产生更强的抑制作用，但存在一个最高点。PEG与K~+和 NH_4~+可形成复合物，产生协和的抑制作用，而尿素与PEG/K~+和PEG/NH_4~+的复 合是目前发现的最强的抑制蒙脱土水化分散与膨胀的体系。     

Thermochemical, volumetric and spectroscopic properties of lysozyme–poly(ethylene) glycol system

Investigations of lysozyme–polyethylene glycol system were made by differential scanning calorimetry, fluorescence and density techniques. The values of unfolding enthalpies, ΔH_N^U, unfolding temperatures, T_m, excess molar heat capacities, ΔC_p, and apparent molar volumes, V_Φ , were determined as functions of PEG concentration. The three PEGs of average molecular mass (MW) 6000, 10000, 20000 were used as macromolecular crowding agents. The concentration of polymers was changed in the range 0–30% mass per volume (w/v). The values of ΔH_N^U remained constant with no dependence on PEG concentration, while PEG addition to buffered lysozyme solutions caused linear decrease of T_m. The values of ΔC_p and V_Φ of lysozyme dramatically changed in the range of 8–10% of PEG concentration. The fluorescence spectroscopy was used in order to investigate the polymer influence on possible solvent–lysozyme interactions. The electrical properties of polymer–water and polymer–buffer systems, the dielectric constants of solutions were determined with use of impedance spectroscopy.     

Temperature dependences of heat capacity and volumetric characteristics of solutions of dipeptides in a water-dimethyl sulfoxide mixture

Temperature dependences of thermodynamic parameters characterizing intermolecular interactions in solutions of peptides are considered. It was observed that only in the case of glycylglycine do small additions of DMSO not cause a significant change of heat capacity volumetric characteristics of solutions. The nature of this effect depends on the structure of the peptides and the size of the side radicals. For the peptides -alanylglycine and -alanyl--alanine the values ofC _p2 ⁰ decrease significantly, and for peptides containing side alkyl radicals (DL--alanyl-DL--alanine andDL--alanyl-DL-valine) the addition of DMSO changes sharply the values of the partial molar volumes at infinite dilution.Institute of Chemistry of Nonaqueous Solutions, Russian Academy of Sciences, Ivanovo 153045. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 805–812, April, 1992.     

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. IX. Enthalpies of hydration of 9-methyl-8-alkyladenines and 6,9-dimethyl-8-alkyladenines

Enthalpies of solution in water, H _sol ^o , and enthalpies of sublimation, H _sub ^o , were determined experimentally for a number of crystalline derivatives of adenine: 6,8,9-trimethyladenine; 6,9-dimethyl-8-ethyladenine; 6,9-dimethyl-8-propyladenine; 6,9-dimethyl-8-butyladenine; 8,9-dimethyl-adenine and 9-methyl-8-ethyladenine. Standard enthalpies of hydration, H _hydr ^o , derived from these data were calculated. The latter were discussed together with the values for variously alkylated adenines, determined previously. The data obtained show that the dependence of enthalpy of hydration on the number of methylene groups added upon substitution with 8-n-alkyl groups of 9-methyladenine and 6,9-dimethyladenine is nonlinear.