The osmotic second virial coefficient, B2, 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 B2 is emphasized, thereby allowing us to model the interaction part of B2 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 B2 for lysozyme at 25°C over pH from 5.0 to 9.0, and 7-60 mM ionic strength. We find that B2 is positive at all solution conditions and decreases with increasing ionic strength, as expected, whereas the interaction part of B2 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 B2 derived from experiment.
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