E. Allahyarov¹, H. Lowen¹, A.A. Louis², J.P. Hansen^2
1Institute Theory II
²Institut für Theoretische Physik II, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany

The effective Coulomb interaction between globular proteins is calculated as a function of monovalent salt concentration cg, by explicit Molecular Dynamics simulations of pairs of model proteins in the presence of microscopic c_s and counterions. For discrete charge patterns of monovalent sites on the surface, the resulting osmotic virial coefficient B₂ is found to be a strikingly non-monotonic function of c_s. The non-monotonicity follows from a subtle Coulomb correlation effect which is completely missed by conventional non-linear Poisson-Boltzmann theory and explains various experimental findings.