Water vapour adsorption by L-amino acids at different water activities

Abstract

Experimental studies on the hydration of all 20 L-amino acids by isopiestic vapour pressure method have been carried out in detail. The water vapour adsorption curves of these amino acids resemble type It or type III BET isotherms. The maximum water uptake (Delta n(1)(0)) of the amino acids at relative humidity equal to unity and values of secondary hydration (n(1)(s)) of these amino acids at p/p(o) equal to 0.92 have been compared critically with each other. Values of Delta n(1)(0) and n(1)(s) for each amino acid has been found to be different from that of the corresponding amino acid residue obtained front our earlier studies. It appears that many amino acids having hydrophobic side chain may form crystals with strong ionic bonds so that they resist hydration at 0.92 relative humidity. The standard free energies of hydration of all amino acids at p/p(o) equal to unity have been evaluated using the Bull equation. The activity coefficients of several amino acids having low solubilities at p/p(o) close to unity has been evaluated when hydrated mixtures remain in gel state.