Thermodynamic properties of peptide solutions 19. Partial molar isothermal compressions at T = 298.15 K of some peptides of sequence gly-X-gly in aqueous solution

The partial molar isobaric expansions at infinite dilution and T = 298.15 K, E2∘{E2∘=(∂V2∘/∂T)p} were determined recently for some tripeptides of sequence glycyl-X-glycine, where X is one of the amino acids alanine, valine, leucine, isoleucine, phenylalanine, methionine, proline, serine, threonine, asparagine, glutamine, histidine, and tyrosine. These quantities, along with partial molar heat capacity data available from the literature, were used to convert the partial molar isentropic compressions at infinite dilution determined previously for these tripeptides into the partial molar isothermal compressions at infinite dilution and T = 298.15 K, KT,2∘{KT,2∘=-(∂V2∘/∂p)T}. These KT,2∘ results, along with that for triglycine, were used to estimate the contributions of the amino acid side-chains to the partial molar isothermal compressions of peptides. The side-chain contributions have been rationalized in terms of the likely peptide–water interactions.