Activity coefficients of KCl in highly concentrated protein solutions

As a contribution to the understanding of the thermodynamic state of single salts in living systems, the activity coefficients of KCl were determined in concentrated bovine serum albumin (BSA) solutions. The concentration range studied was 0.01 to 0.5 M KCl and zero to 18% wt BSA, thus amply covering physiological conditions. The activity coefficients of the salt were measured using the EMF method with ion exchange membrane electrodes. Keeping the salt concentration constant, the activity coefficients of the salt decrease linearly with protein concentration, the effect being more pronounced for low salt content. The maximal deviations of the activity coefficients with respect to those in pure salt solution amount to ca. 40% for 0.01 M KCl and 18% wt BSA. The results were interpreted on the assumption of the superposition of three effects i.e. water bound to BSA molecules as non-solvent water, specific Cl^– ion binding and the electrostatic interactions of the polyions with the salt ions. In view of the results it can be concluded that only a small portion of simple intracellular ions are bound, based on the assumption that the cytoplasm of living cells may be regarded as a concentrated protein-salt solution.