Heat capacities of transfer of some amino acids and peptides from water to aqueous urea solution

Integral enthalpies of solution at low concentrations of several amino acids and peptides in 2 and 6M urea solutions have been determined at 25 and 35°C. These data have been used to derive the enthalpies of transfer (at 25 and 35°C) and heat capacities of transfer (at 30°C) of these amino acids and peptides from water to aqueous urea solutions. Furthermore, the enthalpies of transfer and heat capacities of transfer per CH2 group and per peptide group CONH have also been estimated. These results show that while the enthalpies and heat capacities of transfer per CH2 group are positive and negative, respectively, the reverse is true for CONH group. The implications of these results in the mechanism of the denaturation of proteins by urea are discussed.