Studies of Enthalpy−Entropy Compensation, Partial Entropies, and Kirkwood−Buff Integrals for Aqueous Solutions of Glycine,l-Leucine, and Glycylglycine at 298.15 K

Abstract

Densities and osmotic coefficient measurements for dilute aqueous solutions of glycine, l-leucine, and glycylglycine have been reported at 298.15 K. The partial molar volumes and activity coefficients of solute as well as solvent have been estimated using the density and osmotic coefficient data, respectively. Excess and mixing thermodynamic properties, such as Gibbs free energy, enthalpy, and entropy changes, have been obtained using the activity data from this study and the heat data reported in the literature. The concentration enthalpy-entropy compensation effects have been observed for the studied systems, and the compensation temperatures are reported. It has been observed that the excess free energy change for all the studied systems is almost the same over the studied concentration range, showing that the differences in properties of such solutions are largely decided by the enthalpy-entropy effects. These results, along with partial entropy data, show the effects of the presence of hydrophobic interactions and water structure making effect in the case of aqueous solutions of l-leucine. The application of the Starikov-Norden enthalpy-entropy compensation model yielded information about a "hidden Carnot cycle" and the existence of multiple microphases. Application of the Kirkwood-Buff (KB) theory of solutions for the studied systems yields pair correlation functions between the components. The variation of Kirkwood-Buff integrals with concentration further signifies the concentration dependence of the hydrophobic hydration and interactions in the solution phase. The osmotic second virial coefficients have also been obtained using the KB theory and show good agreement with those obtained using the McMillan-Mayer theory of solutions. The mean square concentration fluctuations is estimated using the KB theory, which gives information about the microheterogeneity in the solution phase, which further reflects the presence of hydration and solute association.

Hydrogen Bonding in Liquid Water and in the Hydration Shell of Salts

“…[41][42][43][83][84][85] In the case of complexforming systems in which strong noncovalenti nteractions dominated, we found that nonlinear EEC effects existed and nonlinearity resulted due to the contribution of heatcapacity. We successfully tested this model experimentally for many systems wherein concentration dependence EEC effects were dominant.…”

“…We successfully tested this model experimentally for many systems wherein concentration dependence EEC effects were dominant. [41][42][43][83][84][85] In the case of complexforming systems in which strong noncovalenti nteractions dominated, we found that nonlinear EEC effects existed and nonlinearity resulted due to the contribution of heatcapacity. [86] In the present case, al inear compensation effect is observed ( Figure 6) with ac ompensation temperature (T comp )o f3 08.9 K and an energetic parametero fÀ5.69 kJ mol À1 .F urthermore, the temperature of 316.5 Ki st he equilibrium point or optimum temperature at which the concentration of fully Hbonded species (ñ = 6300 and 6700 cm À1 Gaussian component) is equal to that of the sum of singly H-bonded and NHB water species( ñ = 7082 and 7368 cm À1 Gaussian component).…”

Thermodynamic Studies of Aqueous Solutions of 2,2,2-Cryptand at 298.15 K: Enthalpy–Entropy Compensation, Partial Entropies, and Complexation with K+ Ions

“…Our previous work was concerned with the measurements of osmotic coefficients, activity and activity coefficient properties of electrolytes and non-electrolytes as well as of surfactants in aqueous solutions of 18-crown-6 and also of aqueous cyclodextrin solutions. − It has been shown with the help of NMR relaxation and diffusion data , that the conformations of these host molecules, their H-bonding with water molecules in the cavities, as well as hydrophobic interaction along with the water structure-making effect govern equilibrium properties in the solution phase. The observed enthalpy–entropy compensation (EEC) effect ,, and application of McMillan–Mayer theory for solution − to obtain osmotic second virial coefficient values for the solute molecules have helped us to understand solute–solvent interactions and solute–solute association in such solutions. The EEC is a very important phenomenon for understanding the molecular interactions in the solution phase of the large numbers of systems ranging from small molecules (alcohols, amino acids, salts, etc.)…”

Entropy-enthalpy compensation may be a useful interpretation tool for complex systems like protein-DNA complexes: An appeal to experimentalists

“…On the other hand, the EEC can in principle be treated as a kind of driving force when studying protein folding and hydration [8,9] (especially, in dealing with the salt and osmolytic effects on the molecular-scale hydrophobic hydration and interactions [10]).Still, as concerns molecular/macromolecular binding (or combined binding-folding/refolding) processes, the role of the EEC phenomenon is not just unambiguously "impeding", as one might immediately conclude after carefully reading, say, the work [7], as clearly demonstrated in the recent papers [1,2,11]. The EEC phenomenon is, furthermore, definitely relevant to enzymatic processes, to the interaction of amino-acid residues in proteins with the water of hydration, in particular, as well as -most probably -to the molecular/supramolecularcrowding-induced self-assembly, in general [12][13][14].Interestingly, several recent works devoted to the EEC phenomenon completely support the standpoint that the EEC is of essential mechanistic significance for the processes involving the host-guest (supra)molecular binding, as well as the physical-chemical events triggered by the latter ones. And, along with all this, the EEC phenomenon ought to be deeply rooted in the thermodynamics [15][16][17][18][19].Remarkably, all the most recent works are completely in line with the above conclusion, for they are demonstrating examples of the correct and successful usage of the EEC concept when trying to explain systematical experimental data obtainable in different fields of physical chemistry [20][21][22][23].When investigating the physical-chemical significance of the enthalpy-entropy compensation principle, it is extremely important to find the detailed connection of the latter to the basics of thermodynamics, as well as to properly refine the sense of the entropy notion.…”

“…Still, as concerns molecular/macromolecular binding (or combined binding-folding/refolding) processes, the role of the EEC phenomenon is not just unambiguously "impeding", as one might immediately conclude after carefully reading, say, the work [7], as clearly demonstrated in the recent papers [1,2,11]. The EEC phenomenon is, furthermore, definitely relevant to enzymatic processes, to the interaction of amino-acid residues in proteins with the water of hydration, in particular, as well as -most probably -to the molecular/supramolecularcrowding-induced self-assembly, in general [12][13][14].…”