Non-monotonic variation with salt concentration of the second virial coefficient in protein solutions

The osmotic virial coefficient $B_2$ of globular protein solutions is calculated as a function of added salt concentration at fixed pH by computer simulations of the ``primitive model''. The salt and counter-ions as well as a discrete charge pattern on the protein surface are explicitly incorporated. For parameters roughly corresponding to lysozyme, we find that $B_2$ first decreases with added salt concentration up to a threshold concentration, then increases to a maximum, and then decreases again upon further raising the ionic strength. Our studies demonstrate that the existence of a discrete charge pattern on the protein surface profoundly influences the effective interactions and that non-linear Poisson Boltzmann and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory fail for large ionic strength. The observed non-monotonicity of $B_2$ is compared to experiments. Implications for protein crystallization are discussed.Comment: 43 pages, including 17 figure

Interaction of Colloidal Particles with Macromolecules: The RISM Integral Equation Theory

A microscopic theory of the mixtures of polymer chains and colloidal particles immersed in a common solvent is developed on the basis of the RISM integral equation technique. The obtained general equations enable the calculation of the correlation functions and thermodynamic characteristics of the system at an arbitrary ratio of the components. The theory is used for a comprehensive study of interaction of small spherical particles with flexible polymer chains in the regimes of weak and strong adsorption of the macromolecules on the surface of the particles. The temperature and concentration regions characterized by different effect of the polymer on stability of the colloidal dispersion are determined. The corresponding temperature–concentration diagrams of state are constructed