Estimation of the hydration of polar groups of α-amino acids by differential scanning calorimetry

The heat capacity of hydration of zwitterions derived from aliphatic amino acids depends linearly on the surface area of the amino acid side radicals accessible to water molecules with the slopeb = 2.35±0.11 J mol−1 K−1 A−2 at 298 K. The linear correlation between hydration heat capacities of zwitterions of aliphatic amino acids and the corresponding aliphatic alcohols with a coefficient of approximately unity confirms the assumption that hydrophobic hydration does not depend on the nature of the surrounding groups. Using the assumption that the hydration of hydrocarbon radicals is independent of the neighboring groups, theb value has been used to calculate the contributions of polar groups. The contributions of OH, COON, and CONH groups of the side radicals in polar amino acids in the zwitterion form are close to zero; in the case of organic nonionic molecules, these contributions are negative. The increments for polar groups obtained for the zwitterions can be used for the calculation of the heat capacities of proteins and polypeptides incorporating charged amino acid residues. The difference between hydrophilic and hydrophobic hydration mechanisms is manifested not only as different magnitudes and signs of heat capacities and temperature coefficients but also in the fact that the neighboring polar (charged) groups have an effect on hydrophilic hydration but have no effect on hydrophobic hydration.