BINDING OF WATER AND SOLUTE TO PROTEIN-MIXTURE AND PROTEIN-COATED ALUMINA

Extent of water vapour adsorption (n(1)) of gelatin and bovine serum a lbumin and their mixtures in different proportion respectively has bee n measured by isopiestic vapour pressure methods at various values of water activity (a(1)) ranging between zero and unity. Similar measurem ents have also been carried out with gelatin and BSA coated alumina po wder. At a given value of a(1), n(1) for the protein mixture is found to be significantly less than their ideal value obtained from the addi tivity rule. Such decrease is probably due to the protein-protein inte raction as a result of which some of the water binding sites become un available for water vapour adsorption. On the other hand when a protei n is mixed with alumina powder, the water vapour adsorption of the pro tein coated alumina surface at a given water activity is found to be 2 to 3 times larger than its ideal value obtained from the additivity r ule. The standard free energy changes for hydration of protein mixture s and protein-coated alumina have been evaluated using Bull equation. The extent of excess hydration of these proteins and their mixtures as well as protein-coated alumina in the presence of excess neutral salt s and urea respectively have been evaluated using the isopiestic metho d. In all cases, the moles of water and solute respectively bound in a bsolute amount to biopolymers, biopolymer mixtures and protein-coated alumina have been evaluated in the limited range of solute concentrati ons in the medium. Based on the Gibbs-Duhem equations, a rigorous expr ession for the standard free energy change for binding of excess solut e and solvent to biopolymer have been evaluated with reference to unit solute mole fraction as standard state. Free energies of excess hydra tion of different biopolymer systems have been evaluated using this eq uation.