ISOTHERM OF WATER SORPTION BY HUMAN SERUM-ALBUMIN IN DIOXANE - COMPARISON WITH CALORIMETRIC DATA

The dependence of the amount of water bound to human serum albumin (HSA) suspended in water-dioxane mixtures vs the equilibrium water concentration in the liquid phase was determined by the Fisher method at 298 K. The Langmuir model was used in order to describe the isotherm of the sorption of water by HSA at low water concentrations in the solvent. The calculated equilibrium constant of water adsorption (3.8+/-0.61 mol(-1)) is in good agreement with the adsorption constant obtained earlier from calorimetric data, The comparison of the determined isotherm of water sorption by HSA with the reported enthalpies of suspension formation showed that at low water concentrations in the solvent, water sorption is the only process contributing to the heat effects of the formation of the 'protein + liquid' heterogeneous system, From this comparison, the enthalpy of water adsorption by HSA was evaluated as -11.9+/-1.7 kJ mol(-1). At higher water concentrations in the solvent the amount of water adsorbed by HSA increased considerably, This increase in the amount of water on HSA at water activities above 0.5 is assumed to be due to the enlargement of the protein surface area.