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.