The osmotic second virial coefficient, B(2), obtained by light scattering from protein solutions has two principal components: the Donnan contribution and a contribution due to protein-protein interactions in the limit of infinite dilution. The Donnan contribution accounts for electroneutrality in a multicomponent solution of (poly)electrolytes. The importance of distinguishing this ideal contribution to B(2) is emphasized, thereby allowing us to model the interaction part of B(2) by molecular computations. The model for protein-protein interactions that we use here extends earlier work (Neal et al., 1998) by accounting for long-range electrostatic interactions and the specific hydration of the protein by strongly associated water molecules. Our model predictions are compared with measurements of B(2) for lysozyme at 25 degrees C over pH from 5.0 to 9.0, and 7-60 mM ionic strength. We find that B(2) is positive at all solution conditions and decreases with increasing ionic strength, as expected, whereas the interaction part of B(2) is negative at all conditions and becomes progressively less negative with increasing ionic strength. Although long-range electrostatic interactions dominate this contribution, particularly at low ionic strength, short-range electrostatic/dispersion interactions with specific hydration are essential for an accurate description of B(2) derived from experiment.

We model the hydration contribution to short-range electrostatic/dispersion protein interactions embodied in the osmotic second virial coefficient, B(2), by adopting a quasi-chemical description in which water molecules associated with the protein are identified through explicit molecular dynamics simulations. These water molecules reduce the surface complementarity of highly favorable short-range interactions, and therefore can play an important role in mediating protein-protein interactions. Here we examine this quasi-chemical view of hydration by predicting the interaction part of B(2) and comparing our results with those derived from light-scattering measurements of B(2) for staphylococcal nuclease, lysozyme, and chymotrypsinogen at 25 degrees C as a function of solution pH and ionic strength. We find that short-range protein interactions are influenced by water molecules strongly associated with a relatively small fraction of the protein surface. However, the effect of these strongly associated water molecules on the surface complementarity of short-range protein interactions is significant, and must be taken into account for an accurate description of B(2). We also observe remarkably similar hydration behavior for these proteins despite substantial differences in their three-dimensional structures and spatial charge distributions, suggesting a general characterization of protein hydration.