Three-Yukawa potential was used to describe the attractive dispersion, soft-sphere repulsion and the screened static-electric repulsion interactions between protein molecules respectively instead of the hardcore two-Yukawa potential. A simple analytical equation of state has been derived based on the Ross perturbation theory to overcome the difficulty exsiting in mean-sphere-approximation (MSA) reported in literature. The equation of state has been applied to study osmotic pressures for aqueous bovine serum albumin (BSA)-NaCl solutions at various values of pH and concentration. The results show that the precision of new theory is superior to the theory recently developed by Lin Yang-Zheng, with only one more independent parameter. And it is shown that the new soft-sphere theory is more reasonable than Lin Yang-Zheng’s one and more appropriate to the protein systems with large molecular weight.

Key words: protein-electrolyte aqueous solution, osmotic pressure, Ross perturbation theory, analytical equation of state, three Yukawa potential