Osmotic pressures of aqueous bovine serum albumin solutions at high ionic strength

Using osmometers similar to those described by Vilker et al. [V.L. Vilker, C.K. Colton, K.A. Smith, J. Colloid and Interface Sci. 79 (1981) 548–566] and Amos et al. [D.A. Amos, C.J. Radke, S. Lynn, J. Phys. Chem. B 102 (1998) 2739–2753], osmotic pressures of aqueous bovine serum albumin (BSA) solutions were measured at three pH (4.5, 5.4, 7.4) and at different sodium chloride concentrations (from 1 to 5 M). Experimental data were compared with results calculated from two van der Waals-type models using a potential of mean force including hard-sphere and double-layer repulsions, and van der Waals attraction. In both models, the Carnahan–Starling equation of state represents the contribution of the hard-sphere repulsion to the osmotic pressure. Van der Waals attraction and double-layer repulsion are represented by their contributions to the osmotic second virial coefficient in the first model, or in the second, to the random-phase-approximation (RPA). Although both models give a semi-quantitative description of the osmotic pressures of BSA solutions at various conditions, the first model represents the data slightly better than the second, using the same number of adjustable parameters. Further improvement of modeling requires a more accurate potential of mean force for protein molecules in salt solution.