Thermodynamic Properties of Peptide Solutions. Part 17. Partial Molar Volumes and Heat Capacities of the Tripeptides GlyAspGly and GlyGluGly, and Their Salts K[GlyAspGly] and Na[GlyGluGly] in Aqueous Solution at 25 °C

The partial molar volumes, Vo2, and partial molar heat capacities, Cp,2o, at infinite dilution have been determined for the two tripeptides glycylaspartylglycine (glyaspgly) and glycylglutamylglycine (glyglugly), and also for their salts K[glyaspgly] and Na[glyglugly], in aqueous solution at 25 °C. The ionization constants at 25 °C for the aspartyl and glutamyl side-chains have also been determined. These new thermodynamic results have been combined with literature data for electrolytes to obtain the volume and heat capacity changes upon ionization of the acidic side-chains of the peptides. The results are compared with those for other carboxylic acid systems. The partial molar heat capacities and volumes have also been used to calculate the contributions of the acidic amino acid side-chains to the thermodynamic properties.