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

摘要

Using the isopiestic vapour pressure technique, the magnitudes of excess binding of water and NaCl per mole of twenty different poly-L-amino acid residues, respectively in the presence of different bulk molefractions (Xsub2/sub) of NaCl have been evaluated from the mathematical expressions for the Gibbs surface excesses. At certain high ranges of NaCl concentration, the plot of -?sub1/subsup2/sup vs. Xsub1/sub/Xsub2/sub becomes linear, so that moles of water and NaCl, respectively bound per mole of amino acid residue can be evaluated. ?sub1/subsup2/sup is the excess moles of Hsub2/subO per mole of amino acid residue and Xsub1/sub and Xsub2/sub stand for mole fractions of the water and NaCl, respectively in the sample system. Also, using the integrated form of the Gibbs absorption equation, the values of standard free energy change (ΔGo) for the excess adsorption of NaCl 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 ΔGosubhy/sub (equal to -ΔGo) is negative due to spontaneous excess hydration of polyamino acid in the presence of a salt.