Solvent Effects on Tautomeric and Microscopic Protonation Constants of Glycine in Different Aqueous Solutions of Methanol and Ethanol

The acid-base equilibria of glycine have been studied in different aqueous solutions of methanol and ethanol (0?C45?% v/v) using a potentiometric method. In this study, the macro and micro protonation constants of the amino acid and its tautomeric constant have been determined at 25?°C and constant ionic strength (0.1 mol?dm^?3 NaClO₄). The protonation and the tautomeric constants of glycine in different binary mixtures were analyzed in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the constants versus ?? (hydrogen-bond donor acidity), ?? (hydrogen-bond acceptor basicity) and ?? ^? (dipolarity/polarizability) are poor in all solutions. Multi-parameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single- and multi-parameter models. Linear correlation is observed when the experimental protonation constant values are plotted versus the calculated ones when the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on protonation and tautomeric constants.     

Enthalpic Interaction Coefficients of Formamide in Aqueous Methanol and Ethanol Solutions at 298.15 K

The dilution enthalpies of formamide in aqueous methanol and ethanol solutions have been determined using a CSC-4400 isothermal calorimeter at 298.15 K. The homogeneous solution enthalpic interaction coefficients have been calculated over a range of alcohol concentrations according to the excess enthalpy concept. The results show that the enthalpic pair interaction coefficients h ₂ of formamide are negative in aqueous alcohol solutions and pass through a minimum in mixed solvents, whereas the h ₂ coefficients of formamide in aqueous ethanol solutions are more negative than those in aqueous methanol solutions. The results are discussed in terms of solute-solute and solute-solvent interactions.     

Solvent Effect on Protonation Constants of 5, 10, 15, 20-Tetrakis(4-sulfonatophenyl)porphyrin in Different Aqueous Solutions of Methanol and Ethanol

The protonation constants of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H₂tpps⁴⁻, were determined in water–ethanol and water–methanol mixed solvents, using a combination of spectrophotometric and potentiometric methods at 20 °C and 0.1 mol⋅dm⁻³ sodium perchlorate as supporting electrolyte. Two protonation constants, K ₁ and K ₂, were characterized and were analyzed in various media in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the protonation constant K ₁ versus α (hydrogen-bond donor acidity) and π ^* (dipolarity/polarizability) are poor in all solutions, but dual-parameter (α and π ^*) correlation represents a significant improvement with regard to the single- and multi-parameter models. However, the single-parameter correlation of log ₁₀ K ₂ in terms of β (hydrogen-bond acceptor basicity) shows a better result than dual- and multi-parameter correlations. Linear correlation is observed when the experimental log ₁₀ K ₁ and log ₁₀ K ₂ values are plotted versus the calculated ones when the KAT parameters are considered. To evaluate the protonation constants of H₂tpps⁴⁻, the Yasuda-Shedlovsky extrapolation is used to obtain the log ₁₀ K ₁ and log ₁₀ K ₂ values at zero percent organic solvent. Finally, the results are discussed in terms of the effect of the solvent on protonation.     

NaCl(或KCl)-CH3OH-H2O体系的摩尔电导

CHaOH－HzO体系具有非常特殊的性质，同时在工业生产及临床治疗方面有着十分广泛的应用，文献中有大量地从不同角度就这类体系进行研究的报道．然而，就N3CI及KCI－CH30H－H20体系的电导研究而言，文献中仅有关干KCI－CH30H－H20（间隔20％CH30H）的部分研究工作．作为全面、系     

丝氨酸在蔗糖水溶液中的稀释焓

利用LKB 2277生物活性检测仪分别测定了298.15 K时丝氨酸在不同组成的蔗糖水溶液中的稀释焓, 利用McMillan-Mayer理论,计算了丝氨酸在不同组成的蔗糖水溶液中的焓对相互作用系数,并与其在葡萄糖水溶液中的焓对相互作用系数h2进行了比较.结果表明,丝氨酸在蔗糖和葡萄糖水溶液中的焓对相互作用系数h2都是负值,并且随着糖浓度的增加,h2系数的绝对值逐渐减少.根据溶质-溶质相互作用和溶质-溶剂相互作用对结果进行了解释.     

298.15 K甘氨酰二肽在乙酸钠水溶液中的体积性质

测定了298.15 K三种甘氨酰二肽(甘氨酰甘氨酸、甘氨酰-L-缬氨酸和甘氨酰-L-亮氨酸)在0.5, 1.0, 1.5和2.0 mol•kg－1乙酸钠水溶液中的密度, 计算了这些肽在乙酸钠水溶液中的表观摩尔体积, 标准偏摩尔体积, 标准偏摩尔转移体积, 理论水化数和体积相互作用参数. 结果表明: 甘氨酰二肽的标准偏摩尔体积和标准偏摩尔转移体积均随乙酸钠浓度的增加而增大, 溶液中离子与肽带电基团/甘氨酰基团(CH2CONH)之间的相互作用大于离子与肽的非极性基团间的相互作用, 乙酸钠和甘氨酰二肽之间主要是对相互作用. 利用共球交盖模型对所研究的肽与乙酸钠之间的体积相互作用进行了解释. 利用氨基酸的标准偏摩尔体积值, 对二肽的标准偏摩尔体积进行了估算, 发现计算值与实验值一致.     

Excess heat capacities at T=298.15 K and excess enthalpies at T=293.15 and 298.15 K of aqueous solution of 2-methoxyethanol

Excess isobaric heat capacities of mixture (2-methoxyethanol+water) were measured at T=298.15 K and excess enthalpies at T=293.15 and 298.15 K. Excess enthalpies were extremely exothermic, up to -1290 J mol^-1 atT=293.15 K and -1240 J mol^-1 at T=298.15 K. Excess isobaric heat capacities were positive and very large, approximately 9 J K^-1 mol^-1 at the maximum. In contrast to the data reported by Page and coworkers, the excess heat capacity data were positive in the entire composition range and there was no change in their signs. Consistently, no crossing was found between the curves of excess enthalpies at T=298.15 and 293.15 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enthalpic Interactions of Amino Acids with Glucose in Aqueous Solutions at 298.15 K

The enthalpies of mixing of aqueous glucose solutions and six kinds of aqueous amino acid solutions (glycine, L-alanine, L-serine, L-valine, L-proline, and L-threonine) and their respective enthalpies of dilution have been determined at 298.15 K using flow microcalorimetry. The experimental data have been analyzed in terms of the McMillan–Mayer model to obtain the heterotactic interaction coefficients. The results have been interpreted from the point of view of solute–solute interactions.     

丙氨酸在葡萄糖和蔗糖水溶液中的体积性质

利用精密数字密度计分别测定了丙氨酸在不同组成的葡萄糖和蔗糖水溶液中的密度,计算了丙氨酸的表观摩尔体积、极限偏摩尔体积和理论水化数,根据结构水合作用模型讨论了迁移偏摩尔体积的变化规律,并与乙二醇-水和丙三醇-水等多羟基体系作了比较.结果表明,丙氨酸分子在多羟基化合物-水体系中体积效应的大小与多羟基化合物所含OH基数目有关.     

Thermodynamics of Aqueous Solutions of 18-Crown-6 at 298.15 K: Enthalpy and Entropy Effects

We reported previously activity and activity coefficient data for aqueous solutions of 18-crown-6 (18C6) in the concentration range of 0.1–2.0 mol-kg⁻¹ at 298.15 K. The results were interpreted in terms of the binding of four water molecules (two bridged and two singly H-bonded) inside the 18C6 cavity having a D_3d conformation. In this work, we report our thermodynamic analysis of the Gibbs energy and enthalpy data (obtained using enthalpy virial data from literature) in aqueous solutions of 18C6 at 298.15 K. The excess enthalpy and Gibbs energy parameters are computed and further used to obtain excess entropies of solutions as a function of 18C6 concentration. The same data are utilized to compute the partial molar entropies of solvent and solute at finite, as well as at infinite, dilution of 18C6 in water. It is observed that ΔG_mix, ΔH_mix and TΔS_mix values are all negative, whereas ΔG^E values show a slightly positive variation as a function of the 18C6 concentration. The partial molar excess entropy of water, ( , decreases (becomes negative) whereas that of 18C6, ( , increases with a increase in the 18C6 concentration. These results are explained in terms of various effects, which include water structure making, incorporation of water molecules in the crown cavities and crown–crown hydrophobic interactions, which persist even at the lowest concentration studied.     

水溶液中氨基酸与甲脲的焓相互作用

用LKB-2277精密微热量计测定了298.15 K时甘氨酸、L-丙氨酸、L-丝氨酸、L-缬 氨酸、L-苏氨酸和L-脯氨酸六种α-氨基酸分别与甲脲分子在水溶液中的混合过程焓变及 这些溶质分子在水溶液中的稀释焓.实验数据根据McMillan-Mayer理论进行回归分析,得到 各级交叉焓相互作用系数.讨论了不同氨基酸与甲脲分子的相互作用机制.结果表明,氨基酸 的两性离子部分、α-碳上的非极性脂肪侧基、极性的羟基侧基和五元吡咯环结构对交叉焓 对作用系数hxy具有不同贡献.与尿素相比,甲脲分子中-CH3基团的引入明显增强了分子的 疏水性.     

Enthalpic Interaction of Amino Acids with 2-Chloroethanol in Aqueous Solutions at 298.15 K

The enthalpies of mixing have been determined for five kinds of aqueous amino acids solutions (glycine, L-alanine, L-valine, L-serine, and L-proline) with 2-chloroethanol by an LKB-2277 Bio Activity Monitor at 298.15 K. In addition, the enthalpies of dilution at 298.15 K of aqueous solutions containing the five kinds of amino acids and 2-chloroethanol have been obtained. The heterotactic enthalpic pairwise interaction coefficients of the virial expansion of excess enthalpy were evaluated and interpreted from the point of view of solute–solute interactions. In comparison with ethanol, 2-chloroethanol shows a stronger exothermic interaction with amino acids because of its hydrophilic Cl atom and a more acidic –OH group. Using the additivity groups concept by Savage and Wood (SWAG), contributions of each of functional group of the amino acids and ethanol and 2-chloroethanol have been estimated.     

The Enthalpies of Solution and Solvation of L-Serine in Aqueous Alcoholic Solutions at 298.15 K

The integral enthalpies of solution (Δ_sol H ^m ) of L-serine in water-alcohol (ethanol, n-propanol, isopropanol) mixtures were measured over the range of alcohol concentrations up to 0.32 mole fractions. The standard enthalpy of solution (Δ_sol H°), enthalpy of transfer of L-serine from water into a mixed solvent (Δ_tr H°), and enthalpy of solvation (Δ_solv H°) were calculated. The dependences of Δ_sol H°, Δ_solv H°, and Δ_tr H° on the composition of mixtures had extrema. The calculated enthalpy coefficients of the pair interactions of L-serine with alcohol molecules were positive and increased along the series ethanol, n-propanol, isopropanol. The data obtained were interpreted in terms of different types of interactions in solutions and the influence of the nature of amino acid residues on the thermochemical solution characteristics. Original Russian Text ? I.N. Mezhevoi, V.G. Badelin, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 4, pp. 789–791.     

甘氨酸、L-丙氨酸和L-丝氨酸在尿素水溶液中的体积性质

蛋白质的折叠与解折叠、稳定性、变性行为和酶的活性等都受到环境中其它各种物质影响.作为蛋白质模型分子,氨基酸在混合溶液中的热力学研究近年来引起了广泛重视.尿素在生物体系中的独特地位主要表现在:它是水结构的破坏者,同时又是许多球状蛋白的变性剂.然而,尿素对球状蛋白的变性作用尚未达成共识.     

On The Effect of Dielectric Constant on Formation and Stability Constants of Vanadium(v) with Leucine in Mixed Solvent of Methanol + Water at 298.15 K

Abstract

Equilibria in aqueous solutions in the system leucine + VO₂ ⁺ has been studied by a combination of potentiometric and spectrophotometric methods, in the p^H range 3–4.4 with high ligand-to-metal ratios. An equilibrium model, MY, is assumed, where M and Y represent the metal ion and fully dissociated amino acid anion, respectively. The results showed that formation and stability constants of complex system increased with decrease of dielectric constants. Linear relationships were observed when log K_P was plotted vs. 1/D, where K_P and D represent stability and dielectric constants of the system, respectively.     

Effect of Solvent Composition on Protonation Constants of Some Glycine Dipeptides

Stoichiometric protonation constants (log₁₀ K ₁ and log₁₀ K ₂) of some aliphatic dipeptides (Gly–Tyr, Gly–Phe, Gly–Val, Gly–Leu, Gly–Thr, Gly–Met and Gly–Pro) were determined potentiometrically in 20, 40, and 60 % (v/v) 1,4-dioxane–water and dimethyl sulfoxide–water mixtures at 25.0 (±0.1) °C with an ionic strength of 0.10 mol·L^?1 sodium chloride. The protonation constants were calculated with the computer program PKAS and selection of the best fit chemical models is based on the statistical parameters. The effects of solvent composition on these protonation constants are discussed to determine the factors which control these processes. It has been observed that, while the correlation between log₁₀ K ₁ and log₁₀ K ₂ with the percentages of dimethyl sulfoxide in the dimethyl sulfoxide–water mixtures are not linear, these values linearly increase as the concentration of 1,4-dioxane increases in the solvent mixtures.     

An Empirical Relation for the Evaluation of the Dielectric Constants of Some Miscible Aqueous-Organic Solvent Mixtures at 298.15 K

Abstract

Dielectric constants of some miscible aqueous-organic solvent mixtures at 25°C have been analyzed by an equation analogous to Tamura and Kurata equation for viscosity. The hydration numbers n_h found out from the plots of Δ? vs. x₂ are explained in terms of H-bonding both in water-rich and alcohol-rich medium.