Heat-capacity changes and partial molal heat capacities of several amino acids in water

Abstract

Integral enthalpies of solution of several amino acids in water at low concentrations have been determined at 25 and 35°C. These data have been used to derive the heat-capacity change ΔC _p ^o on dissolution at 30°C. Partial molal heat capacities C _p2°have been obtained by combining ΔC _p° with C _p2° (heat capacity of pure solid amino acids). The results indicate that the increments in ΔC _p° and C _p2° values per CH₂ group increment in the homologous series of α-amino acids are constant and in agreement with those found for other homologous series of compounds containing monofunctional groups. However, this is not the case with amino acids having the NH ₃ ⁺ group at the terminal position. The present work also indicates that, as the NH ₃ ⁺ group is shifted away from the COO⁻ group, hydrophobic hydration decreases, as indicated by a decrease in ΔC _p° and C _p2° . The results on various isomers of amino acids show that branching of alkyl groups has no effect on ΔC _p° and C _p2° , indicating that hydrophobic hydration is unaffected by branching. The effect of substitution of H by OH and of CH₃ by CH₂-CO-NH₂groups in some amino acids has also been studied and discussed.