Hydration and interactions in protein solutions containing concentrated electrolytes studied by small-angle scattering

During protein crystallization and purification, proteins are commonly found in concentrated salt solutions. The exact interplay of the hydration shell, the salt ions, and protein–protein interactions under these conditions is far from being understood on a fundamental level, despite the obvious practical relevance. We have studied a model globular protein (bovine serum albumin, BSA) in concentrated salt solutions by small-angle neutron scattering (SANS). The data are also compared to previous studies using SAXS. The SANS results for dilute protein solutions give an averaged volume of BSA of 91 700 Å3, which is about 37% smaller than that determined by SAXS. The difference in volume corresponds to the contribution of a hydration shell with a hydration level of 0.30 g g−1protein. The forward intensity I(0) determined from Guinier analysis is used to determine the second virial coefficient, A2, which describes the overall protein interactions in solution. It is found that A2 follows the reverse order of the Hofmeister series, i.e.(NH4)2SO4 < Na2SO4 < NaOAc < NaCl < NaNO3 < NaSCN. The dimensionless second virial coefficient B2, corrected for the particle volume and molecular weight, has been calculated using different approaches, and shows that B2 with corrections for hydration and the non-spherical shape of the protein describes the interactions better than those determined from the bare protein. SANS data are further analyzed in the full q-range using liquid theoretical approaches, which gives results consistent with the A2 analysis and the experimental structure factor.