A new set of peptide-based group heat capacities for use in protein stability calculations

The partial molar heat capacities of the tripeptides of the sequence glycyl-X-glycine, where X is one of the amino acids leucine, threonine, glutamine, phenylalanine, histidine, cysteine, proline, glutamic acid or arginine, and of the two tetrapeptides tetraglycine and glycyltryptophanylglycylglycine in aqueous solution over the temperature range 10-100 degrees C have been determined using high sensitivity scanning microcalorimetry. These results were used to derive the partial molar heat capacities of the various amino acid side-chains. This report completes our programme to derive reliable side-chain heat capacities for all 20 amino acids of proteins over a wide temperature range using the tripeptides Gly-X-Gly as realistic model compounds. Included in the study is a summary of the partial molar heat capacities of all 20 amino acid side-chains. These results, along with the heat capacity of the peptide backbone group, were used to calculate the partial molar heat capacities of some oligopeptides and of the random coil form of some unfolded proteins in water. The calculated heat capacities of the proteins obtained using this new set of heat capacities for the constituent groups are consistent with the heat capacities of the denatured state determined experimentally.