UNDERSTANDING NONIDEALITIES OF THE OSMOTIC-PRESSURE OF CONCENTRATED BOVINE SERUM-ALBUMIN

Previously Vilker et al. (J. Colloid Interface Sci. 79(2), (1981)) rep orted the osmotic pressure of concentrated bovine serum albumin (BSA) up to 475 g/L in 0.15 M sodium chloride at pH 4.5, 5.4, and 7.4. The a uthors used a semiempirical model based on Donnan theory to predict th e osmotic pressure with good agreement. However, the formal applicatio n of a three-term virial expansion with the coefficients determined fr om the potential energy of interaction between BSA molecules resulted in poor agreement with their data. In this study, modeling of the osmo tic pressure was reexamined using a free-solvent model that considered average solute-solvent and microion-solute interactions in a mole fra ction concentration variable. The resulting fits were excellent for al l three pH. The model is designed with no fitted parameters; however, the model results were highly sensitive to the selected hydration and microion binding. Therefore the hydration was further regressed from i ts initial estimate of I g H2O/g BSA (based on water-O-17 magnetic res onance studies of other globular proteins) to minimize the least-squar es error between the predicted values and data. The resulting average hydration was determined to be 1.14 +/- 0.03 g H2O/g BSA for all pH va lues. However, the standard error in hydration for each pH was no grea ter than +/-0.0063 g H2O/g BSA. These results demonstrate that solvent -solute interaction and the concentration variable may be critical fac tors when evaluating osmotic pressure data of concentrated protein sol utions. (C) 1998 Academic Press.