Thermodynamic Properties of Peptide Solutions. Part 18. Partial Molar Isentropic Compressibilities of Gly-X-Gly Tripeptides (X = Tyr, Pro, Gln, Asp and Glu), and the Peptide Salts K[GlyAspGly], Na[GlyGluGly] and GlyLysGly Acetate in Aqueous Solution at 25 ∘C

The partial molar isentropic compressibilities at infinite dilution, K∘S,2, have been determined for several tripeptides of the sequence glycyl-X-glycine, where X is one of the amino acids tyrosine, proline, glutamine, aspartic acid, glutamic acid and lysine in aqueous solution at 25 ∘C. These results, along with those for triglycine, were used to estimate the contributions of the amino acid side-chains to the partial molar isentropic compressibilities of polypeptides. Values for K∘S,2 have also been determined for aqueous solutions of the two peptide salts K[glyaspgly] and Na[glyglugly]. The K∘S,2 results for the peptides and their salts have been combined with literature data for electrolytes to calculate the changes in isentropic compressibility upon ionization of the acidic side-chains. The results are compared with those for other carboxylic acid systems.