Solvation of amino acid residues in water and urea-water mixtures: Volumes and heat capacities of 20 amino acids in water and in 8 molar urea at 25°C

The limiting partial molar volumes V^o and heat capacities C_p^o of 20 amino acids have been determined in water and in 8 molar urea at 25.0°C using flow calorimetry and flow densimetry. The side chain contributions to V^o and C_p^o were obtained as the difference between the properties of the various amino acids and those of glycine, both in water and in 8M urea. The solvent accessible surface area of the amino acid residues were obtained using a method developed by Hermann, and the total surface areas were separated into their hydrophobic A_Hb and hydrophilic components. In water, C_p^o values for the various residues C_p^o(R) were found well correlated with A_Hb, though much less so in the urea solution. Hence, C_p^o(R) values, in water yield a good estimate of side chain hydrophobicity, but the (waterurea) transfer heat capacities appear strongly affected by specific solvation effects in the urea solution.Presented at the sixth Italian meeting on Calorimetry and Thermal Analysis (AICAT) held in Naples, December 4–7, 1984.