The osmotic pressure of concentrated protein solutions: Effect of concentration and ph in saline solutions of bovine serum albumin

Abstract

Osmotic pressure measurements are reported as a function of bovine serum albumin (BSA) concentration in 0.15 M sodium chloride at pH 4.5, 5.4, and 7.4. The measured values increased markedly with increasing BSA concentration and with increasing pH (and therefore increasing macroion charge). At a concentration of 450 g/liter solution and a pH of 7.4, osmotic pressure was nearly five atmospheres, which is more than four times the value measured at the same concentration and a pH of 4.5 and about 30 times the value expected for an ideal solution. A semi-empirical analytical expression was developed which gave good agreement between prediction and the experimental data of this and other studies. The data were also compared to the prediction of a three-term virial equation wherein the second and third virial coefficients were calculated by using McMillan-Mayer solution theory. The expression for the potential of mean force was obtained by comparing various contributions to the potential energy of interaction. The terms for electrostatic repulsion and dispersion attraction are the same as those used in the DLVO theory of colloid stability. The predicted curves are of the correct order of magnitude and follow the correct qualitative trend with pH, but they fail to display the strong pH-dependence of the data. The factors responsible for this deficiency are assessed and opportunities for developing a more realistic potential function are identified.