Thermodynamics of diglycine and triglycine in aqueous NaCl solutions: Apparent molar volume, isentropic compressibility, and refractive index

New experimental data for the density, speed of sound, and refractive index of aqueous solutions of diglycine + NaCl and triglycine + NaCl have been reported. The apparent molar volume and apparent molar isentropic compressibility of these peptides at various NaCl concentrations have been calculated from the measured properties. The results show that both peptides exhibit a positive volume transfer to solutions with higher NaCl concentrations and a negative apparent isentropic compressibility in the presence of NaCl. These effects indicate that the apparent volumes of the peptide molecules are larger in solutions with higher NaCl concentrations and that the water molecules around the peptide molecules are less compressible than the water molecules in the bulk solvent. These effects are attributed to the doubly charged nature of the peptides and the interactions between the charged groups and hydrocarbon backbone of peptides with the ions.