Interaction of water vapour with twenty different poly-L-amino acids and their excess hydration in presence of sodium chloride

Using the isopiestic vapour pressure technique, the magnitudes of excess binding of water and NaC1 per mole of twenty different poly-L-amino acid residues, respectively in the presence of different bulk molefractions (X 2 ) of NaCI have been evaluated from the mathematical expressions for the Gibbs surface excesses. At certain high ranges of NaCI concentration, the plot of -r 1 2 vs. X 1 /X 2 becomes linear, so that moles of water and NaCI, respectively bound per mole of amino acid residue can be evaluated. r 1 2 is the excess moles of H 2 O per mole of amino acid residue and X 1 and X 2 stand for mole fractions of the water and NaCI, respectively in the sample system. Also, using the integrated form of the Gibbs absorption equation, the values of standard free energy change (ΔG°) for the excess adsorption of NaCI per kg of poly-L-amino acids have been evaluated. These values are all positive as a result of positive excess hydration of polyamino acids. The standard free energy of excess hydration AG h y (equal to -ΔG°) is negative due to spontaneous excess hydration of polyamino acid in the presence of a salt.