A numerical study of a binary Yukawa model in regimes characteristic of globular proteins in solution

The main goal of this paper is to assess the limits of validity, in the regime of low concentration and strong Coulomb coupling shigh molecular chargesd, of a simple perturbative approximation to the radial distribution functions sRDF’sd, based upon a low-density expansion of the potential of mean force and proposed to describe protein-protein interactions in a recent small-angle-scattering sSASd experimental study. A highly simplified Yukawa sscreened Coulombd model of monomers and dimers of a charged globular protein sb-lactoglobulind in solution is considered. We test the accuracy of the RDF approximation, as a necessary complementary part of the previous experimental investigation, by comparison with the fluid structure predicted by approximate integral equations and exact Monte Carlo sMCd simulations. In the MC calculations, an Ewald construction for Yukawa potentials has been used to take into account the long-range part of the interactions in the weakly screened cases. Our results confirm that the perturbative first-order approximation is valid for this system even at strong Coulomb coupling, provided that the screening is not too weak si.e., for Debye length smaller than monomer radiusd. A comparison of the MC results with integral equation calculations shows that both the hypernetted-chain sHNCd and Percus-Yevick closures have a satisfactory behavior under these regimes, with the HNC being superior throughout. The relevance of our findings for interpreting SAS results is also discussed.