A molecular model for the dependence of the osmotic pressure of bovine serum albumin upon concentration and pH.

Expressions derived from the effective hard particle model of Minton and Edelhoch (Biopolymers, 21 (1982) 451) account quantitatively for the combined data of Kanal et al. (Biophys. J., 66 (1994) 153) describing the osmotic pressure of bovine serum albumin as a function of protein concentration (< or = ca. 100 milligrams) and pH (3-8) in buffered 0.1 M NaCl. The best fit of the model yields a molar mass of 68360 and a pH-dependent value of the effective specific volume ranging from a minimum of -0.17 cm3/g at pH 4.6 (the isoelectric point) to maxima of 3.1 cm3/g at pH 3.0 and 2.2 cm3/g at pH 8.0. These values are shown to be consistent with the magnitude of known attractive and repulsive electrostatic interactions between proteins in solution.