Partial molar heat capacities and volumes of Gly-X-Gly tripeptides in aqueous solution: model studies for the rationalization of thermodynamic parameters of proteins.

The thermodynamics of protein unfolding can be rationalized if the temperature dependence of the partial molar volumes and heat capacities of their constituent groups are known reliably. Despite many experimental and theoretical studies there are still several inconsistencies in the published thermodynamic data. We have investigated some of these inconsistencies by applying high sensitivity scanning densimetry and microcalorimetry to aqueous solutions of tripeptides of the structure Gly-X-Gly, where X is one of the amino acids Met, Asn, Gly and Ile. For these side-chains either no direct data have been determined or serious discrepancies exist between the values published by different laboratories. Partial molar heat capacities and volumes have been determined for the peptides in pure water, in water adjusted to pH = 4 and in 0.5 M sodium acetate buffer at pH = 4. The results obtained are critically compared with those in the literature.