Patchy Proteins, Anions and Hofmeister Series

Abstract. We investigate specific anion binding to a range of patchy protein models and use our results to probe protein-protein interactions for aqueous lysozyme solutions. Our molecular simulation studies show that the ion-protein interaction mechanism and strength largely depends on the nature of the interfacial amino acid residues. Via direct ion-pairing, small anions interact with charged side-chains while larger anions are attracted to non-polar residues due to several solvent assisted mechanisms. Incorporating ion and surface specificity into a mesoscopic model for proteins-protein interactions we calculate the free energy of interaction between lysozyme molecules in aqueous solutions of sodium chloride and sodium iodide. In agreement with experiment, we find that “salting out ” follows the reverse Hofmeister series for pH below the iso-electric point and the direct series for pH above. 1