Protein interactions as seen by solution X-ray scattering prior to crystallogenesis

In a previous work, solubility diagrams of lysozyme and its ability to crystallize in the presence of various ions were established. Inorganic as well as organic anions showed strong effects on lysozyme solubility following the reverse order of the Hofmeister series [Ries-Kautt and Ducruix, J. Biol. Chem. 264 (1989) 745]. In the present paper, we used small angle X-ray scattering (SAXS) to characterize the influence of various salts on the protein-protein interactions of undersaturated lysozyme solutions at constant pH (4.5) and temperature (18 degrees C). The results show that lysozyme in a low ionic strength buffer presents repulsive protein-protein interactions. Addition of increasing concentrations of salts gradually leads from repulsive to attractive interactions demonstrating the ability of a given protein to change its interactive behavior with additives. While cations (Li+, Na+, K+, NH4+, Cs+) all showed similar effects, large differences were observed between anions in their efficiency to modify the interaction potentials. The order of the anions (SCN-, paratoluene sulfonate, NO3-, Cl-, H2PO4-) was found to be the same as that observed for their effectiveness in reducing lysozyme solubility and inducing crystallization.