Heat capacity of proteins. I. Partial molar heat capacity of individual amino acid residues in aqueous solution: hydration effect.

The partial molar heat capacities of various peptides and various organic compounds that model the amino acid side-chains or their parts in aqueous solution have been determined by precise scanning microcalorimetry in the temperature range from 5 to 125 degrees C. This provides an estimate of the partial molar heat capacity of the peptide--CHCONH--group and the side-chains of all amino acid residues. The values obtained are compared with the values found for these substances in the gaseous phase, in order to define the hydration effect. It has been shown that the partial heat capacity of the non-polar groups is positive at low temperature (5 degrees C) and decreases with increasing temperature, while for the polar and charged groups it is negative at low temperature, becomes zero at room temperature and increases further with increasing temperature. This leads to a hydrophobicity scale of the amino acid side-chains based upon the temperature dependences of their heat capacities. Due to the observed specificity in the temperature dependence, at room temperature, the heat capacities of amino acid side-chains correlate well with the non-polar surface areas.