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

Abstract

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.

Anisotropic protein–protein interactions due to ion binding

“…This complexity connects to the original Hofmeister series where protein-protein interactions are influenced by the particular type of salt [6]. For example, sodium thiocyanate gives rise to a lower osmotic second virial coefficient for lysozyme than does sodium chloride [5,8,28]. By treating general ion binding to surface sites using a two-state model -as has been done for protons for a centurya more complete electrostatic picture emerges that prove useful in rationalizing ion-specific effects.…”

“…Fig. 2 shows simulated, twodimensional titration curves of the eye-lens protein -crystallin as a function of both pH and salt concentration [8]. In this approach, no specific binding is assumed for chloride, while thiocyanate is able to bind to hydrophobic groups with strengths obtained from experimental data [29].…”

“…For example, recent work shows that trivalent cations such as lanthane and yttrium can reverse the charge of human serum albumin by strong binding to carboxyl groups which in turn influence protein-protein interactions and phase equilibria [2][3][4]. Large ions with a low surface charge density such as thiocyanate or tetra-alkylammonium may bind to hydrophobic surface patches, contributing to the effective electrostatic potential emanating from the protein [5][6][7][8]. That is, pH is merely one of many parameters controlling the surface charge distribution.…”

Faunus– a flexible framework for Monte Carlo simulation

“…We have previously used this implicit titration scheme for constant pH studies of proteins [28] and for more exotic Hofmeister related effects such as thiocyanate and iodide binding to amino acid motifs [29]. The latter was done by defining not only proton binding sites, but also specific thiocyanate binding to the protein backbone and hydrophobic sites using experimental dissociation constants -see Table 4.…”

“…Table 4. Binding constants of protons and thiocyante anions to amino acid motifs [29,30]. These enters the MC acceptance criteria for particle swap moves.…”