Free Energy Relationships in Aqueous Amino Acid and Peptide Solutions Containing Sodium Chloride

Three systems of the type amino acid or peptide-sodium chloride-water have been investigated over wide solute molality ranges using the isopiestic vapor pressure method. The amino acid employed was L-alpha-alanine, while the peptides were diglycine and triglycine. Equations were obtained for the activity coefficients of these compounds in the salt solutions in terms of the molalities of the solutes. The trace activity coefficients of the peptides were negative at low salt molality and became positive as the salt molality was increased. The limiting interaction parameters were calculated for the systems using the Kirkwood ion-dipole expression and empirical quantities derived from previous work to obtain the salt effect on the nonpolar and amide portion of the molecule. Good agreement was obtained between the calculated values and the experimental results in the case of diglycine, but they diverged in the case of triglycine. The calculated value for L-alpha-alanine is in poorer agreement with the experimental value than for the other amino acids studied previously.