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 specicity 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.