N. L. Smirnova

Amino Acid Solvation in Aqueous Kosmotrope Solutions: Temperature Dependence of the l-Histidine–Glycerol Interaction

We have studied thermodynamics of interaction between the aromatic amino acid L-histidine and glycerol, which is one of the most important stabilizing agents for proteins in water. The pair and triplet interaction parameters have been extracted from enthalpy and solubility data using standard thermodynamic manipulations in a wide temperature range. Our results indicate for the first time that the L-histidine-glycerol pair and triplet interactions are characterized by rather small enthalpy and entropy changes, which do not depend on temperature in either cold or hot water. These temperature-independent enthalpies and entropies of interaction lead to zero heat capacity changes during the amino acid transfer from water to both dilute and rather concentrated aqueous glycerol solutions. We attribute this behavior to a delicate balance between contributions from hydrophobic and hydrophilic fragments in the solute molecules. This unique feature appears to be the major reason that thermodynamics of pair and triplet interactions are nearly identical at standard and physiological temperatures.

Thermodynamics of aqueous L-proline solutions at 273–323 K

Enthalpies of dissolution of L-proline in water were measured calorimetrically at 283–313 K. The experimental temperature of dissolution of proline in water at the studied temperatures was shown to be almost independent of its concentration over the range 0.01–0.11 mol/kg. Standard enthalpies of dissolution and standard heat capacities of dissolution were calculated over this temperature range. The heat capacity of dissolution was ascertained to increase in the row glycine, proline, and alanine. The partial molar heat capacity of proline in water was determined and compared with the values obtained by extrapolation of the apparent heat capacities. The changes in entropy and reduced enthalpy and the Gibbs energy over the temperature range from 273 to 323 K were determined using familiar thermodynamic relations. The data for glycine and alanine were compared.

Temperature and length scale dependence of tetraalkylammonium ion solvation in water, formamide, and ethylene glycol.

This paper focuses on the temperature and length scale dependence of solvation of familiar hydrophobic solutes: tetraalkylammonium ions in highly associated solvents with H-bond networks. Standard enthalpies and heat capacities of solution of symmetrical tetraalkylammonium bromides in formamide (FA) and ethylene glycol (C(2)H(4)(OH)(2)) have been computed using experimental ΔH(m)(sol) values and compared with previously reported enthalpies and heat capacities in pure water. Heat capacities of hydration are found to be large, positive, and increase almost linearly up to tetrapentylammonium bromide. Hydration of tetrahexyl- and tetraheptylammonium bromides is accompanied by much smaller heat capacity changes than would be expected from the length scale dependence observed. The same behavior reveals enthalpies and heat capacities of solute transfer from FA and C(2)H(4)(OH)(2) to water. The results obtained rather indicate that solvation of large tetraalkylammonium ions containing more than twenty carbon atoms is accompanied by much less water orientational ordering than for solutes of a moderate size. It results in smaller fluctuations in the water-water pair energy in a hydration shell and consequently reduces the heat capacity of hydration. The comparison of tetraalkylammonium ion behavior in water and the nonaqueous solvents studied shows that the length scale dependence of solvophobic solvation is strongly pronounced in water but not in nonaqueous media.

Amino acid behavior in aqueous denaturant solutions: temperature dependence of the L-histidine-amide interaction.

We have studied the thermodynamics of the pair interaction between aromatic amino acid-l-histidine and nonelectrolyte denaturing globular proteins-hydrophilic urea (U) and presumably hydrophobic dimethylformamide (DMF) in the temperature range of 288-328 K. Our study does indicate for the first time the anomalous temperature dependence of the enthalpies and entropies of the l-histidine-U and l-histidine-DMF interaction in water, which is consistent with the previously reported results for water-urea (U) and water-U-l-phenylalanine systems. This phenomenon is found to be closely related to the behavior of water, since in all cases, the extrema observed arise in the temperature range of 300-308 K, where the temperature dependence of the heat capacity of pure water passes through the minimum. The amino acid-urea interaction is shown to be accompanied in a wide temperature range by a large negative enthalpy change, which reveals a strong tendency of urea binding with polar and charged groups of proteins.

Thermochemistry of ethyl acetate solvation in the 1-octanol-N,N-dimethylformamide system

The thermal effects of ethyl acetate (EtOAc), 1-octanol (OctOH), and N,N-dimethylformamide (DMF) solution in a OctOH-DMF model system were measured using a calorimeter of variable temperature with an isothermal shell at 298 K. The standard enthalpies of solution and ester transfer from an alcohol to a binary mixture, and partial mole enthalpies of mixed solvent components were determined. The state of non-electrolyte molecules in OctOH-DMF and OctOH-DMF-EtOAc systems were studied using extended coordination model. It was found that the binary solvent is subjected to microclusterization, since the fraction of the single-type molecules in the solvation sphere of both components of a mixture is significantly higher than in the liquid phase volume. The conclusion was drawn that in the triple system, the ethyl acetate solvation sphere in the whole range of compositions is significantly enriched with amide owing to stronger esteramide dipole-dipole interaction.

The Enthalpies and Heat Capacities of Solution of Tetraethyl- and Tetrabutylammonium Bromides in Methanol, Formamide, and Ethylene Glycol

The heat effects of solution of tetraethyl- and tetrabutylammonium bromides in methanol (MeOH), formamide (FA) and ethylene glycol (EG) were measured at 25 and 40°C at various concentrations. Various calculation methods were used to determine the standard enthalpies of solution. The results were compared with the available data on aqueous systems. The mean standard heat capacities of solution over the temperature range studied were calculated. The behaviors of tetraalkylammonium salts in water and FA were shown to be much less different than in water and alcohols, which was evidence of the possibility of the appearance of solvophobic solvation effects in the solution of nonpolar particles in FA.

Thermodynamic characteristics, structure, and interactions of L-proline in aqueous solutions of alcohols and urea

The enthalpies of dissolution of imino acid L-proline in aqueous solutions of methanol, 2-propanol, ethylene glycol, glycerin, and urea are measured by the calorimetric method at 313.15 K. Enthalpic parameters of the interaction of L-proline with nonaqueous components are calculated and compared with the data at 298.15 K. It is found that the sign of the heat capacity parameter of the pair and ternary interactions depends on whether the nonaqueous solvent component is a destroyer or stabilizer of the water structure. Partial molar heat capacities of proline in mixed solvents are obtained by the integral dissolution heat method. Temperature changes in the reduced enthalpy and entropy of the proline solution are determined at an increase in the temperature from 298 K to 313 K. It is shown that there is entropyenthalpy compensation at temperature changes in the characteristics during dissolution.

Enthalpies and heat capacities of hematoporphyrin solutions in N,N-dimethylformamide and octanol-1

Heat effects of the dissolution of hematoporphyrin tetramethyl ether are measured on a variable-temperature calorimeter for the first time in N,N-dimethylformamide and octanol-1 in the temperature range of 298 to 318 K. Standard enthalpies and heat capacities of dissolution of bioligand are calculated and compared to data obtained earlier for deuteroporphyrin dimethyl ether and ethyl acetate. Partial molar heat capacities of hematoporhyrin are determined at infinite dilution using data from differential scanning calorimetry.

Dissolution enthalpies of L-proline and its interactions with methanol, 2-propanol, ethylene glycol, and glycerin in aqueous solution at 298.15 K

The dissolution enthalpies of L-proline in mixtures of water with methanol, 2-propanol, ethylene glycol, and glycerin were measured calorimetrically at 298.15 K. The enthalpy coefficients of the interaction of proline with alcohols in aqueous solutions were determined. The enthalpy coefficients of the pair interaction of proline and glycine with alcohols were shown to be directly proportional.

Heat properties of Bu4NBr solutions in binary mixtures based on formamide

Heat effects of tetrabutylammonium bromide dissolution in mixtures of formamide with methanol and ethylene glycol at 25°C were determined. Partial molar enthalpies of the components of formamide-ethylene glycol mixtures at 25°C were measured by the calorimetric method, and the mixing enthalpies of this system were determined. Within the limits of the second approximation of the Debye-Hueckel theory the standard enthalpies of dissolution ΔdisH0 were calculated, and the enthalpies of Bu4NBr transfer from formamide in its mixtures with water methanol and ethylene glycol were found on this basis. The enthalpy parameters of Bu4NBr pair interactions with the components of the formamide-water, formamide-methanol, and formamide-ethylene glycol mixtures were calculated. The results obtained were compared with the data for the systems containing N-methylformamide and N,N-dimethylformamide.