The effective hard particle model provides a simple, robust, and broadly applicable description of nonideal behavior in concentrated solutions of bovine serum albumin and other nonassociating proteins.

Published data on the concentration dependence of osmotic pressure of solutions of bovine serum albumin in 0.15 M NaCl at concentrations up to greater than 400 g/L are shown to be described to within experimental uncertainty by a simple one-parameter model in which protein molecules are represented by effective hard spherical particles. The volume of the effective hard particle reflects both steric and electrostatic repulsion and thus varies with pH and ionic strength. The pH dependence of the effective volume is shown to agree well with that previously obtained from analysis of the concentration dependence of sedimentation equilibrium and static light scattering. (c) 2007 Wiley-Liss, Inc.