Solution electrostatics beyond pH: a coarse grained approach to ion specific interactions between macromolecules

Oblivious to ion specificity, pH has been a key parameter for macromolecular solutions for little more than a century. We here widen the concept by describing the ionization of macromolecules not only via pH, but also pX where X are other binding species. Using binding constants, measured by NMR, of chloride and thiocyanate to amino acid motifs on g-crystallin, we calculate i) titration curves as a function of pH and pX and ii) estimate second virial coefficients using both approximate theory and computer simulations. In agreement with experiment, a Hofmeister reversal for protein-protein interactions is observed when crossing iso-electric conditions. Thiocyanate binding further leads to large charge fluctuations that may trigger intermolecular charge regulation interactions.