Confirmation of free solvent model assumptions in predicting the osmotic pressure of concentrated globular proteins

Previously the osmotic pressure of the concentrated globular proteins immun o-gamma -globulin, bovine serum albumin, hen egg lysozyme, and ovalbumin in moderate-ionic-strength solutions was successfully modeled using a free so lvent model (Yousef et al., J. Colloid Interface Sci. 197,108,1998; 207, 27 3,1998; AIChE J., 2001). This model considered the average solute-solvent a nd microion-solute interactions, represented the hydrated macromolecule and the cooperative interacting water and salts as a single species, and used a mole fraction concentration variable. Although the model assumed no fitte d parameters, the hydration number was regressed on due to its sensitivity and compared with the 1 g H2O/g BSA estimate determined from (H2O)-O-17 mag netic resonance studies of other globular proteins. The resulting average h ydration numbers were in agreement with the reported estimates. These resul ts suggest that solvent-solute interactions are dominant factors when analy zing nonidealities associated with the osmotic pressure data of concentrate d protein solutions in moderate-ionic-strength solutions. However, the cond ition for the applicability of the free solvent model is that long-range in teractions between hydrated macromolecules are screened to reduce solute-so lute interaction. This condition requires a moderately high salt concentrat ion. The objective of this study is to confirm this requirement for a succe ssful model representation of the osmotic pressure of these protein solutio ns at high concentrations. To accomplish this, studies of concentrated solu tions of ovalbumin at low ionic strength 0.01 M at pH 7.0 were carried out and modeled using the free solvent approach. The results showed that no rea sonable free solvent model solution could be obtained. These results streng then the argument that the free solvent model has a physically realizable r epresentation for concentrated protein solutions in moderate salt concentra tions. (C) 2001 Academic Press.