Amino Acid Behavior in Aqueous Denaturant Solutions: Temperature Dependence of the l-Histidine−Amide Interaction

Abstract

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.