Ion binding to biomolecules

We investigated specific anion binding to basic amino acid residues as well as to a range of patchy protein models. This microscopic information was subsequently used to probe protein–protein interactions for aqueous lysozyme solutions. Using computer simulations to study both atomistic and coarse grained protein molecules, it is shown that the ion–protein interaction mechanism as well as magnitude is largely controlled by the nature of the interfacial amino acid residues. Small anions interact with charged side-chains via ionpairing while larger, poorly hydrated anions are attracted to nonpolar residues due to a number of solvent-assisted mechanisms. Taking into account ion and surface specificity in a mesoscopic model for protein–protein interactions, we investigated the association of the protein lysozyme in aqueous solutions of sodium iodide and sodium chloride. As observed experimentally, it is found that ‘salting out’ of lysozyme follows the reverse Hofmeister series for pH below the iso-electric point and the direct series for pH above.