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Zwitterionic Osmolytes Resurrect Electrostatic Interactions Screened by Salt

Journal of the American Chemical Society | 10 Oct 2018

R Govrin, S Tcherner, T Obstbaum and U Sivan
Abstract
Many cells synthesize significant quantities of zwitterionic osmolytes to cope with the osmotic stress induced by excess salt. In addition to their primary role in balancing osmotic pressure, these osmolytes also help stabilize protein structure and restore enzymatic activity compromised by high ionic strength. This osmoprotective effect has been studied extensively but its electrostatic aspects have somehow escaped the main stream. Here, we report that despite their overall neutrality, zwitterions may dramatically affect electrostatic interactions in saline solutions of biological relevance. Using atomic force microscopy, we study the combined effect of osmolytes and salts on electrostatic interactions between two negatively charged silica surfaces in mixtures of 5 salts (NaCl, KCl, CsCl, MgCl2, CaCl2) and 5 zwitterionic osmolytes (betaine, proline, trimethylamine N-oxide, glycine, sarcosine) as a function of solutes concentration and pH. All osmolytes are found to counteract the screening effect of salt on electrostatic repulsion, albeit to a different extent. They do so by both increasing the screening length shortened by added salts, and by desorbing bound protons and cations, hence enhancing the negative surface charge. Both effects are traced to the osmolytes' higher molecular polarizability compared with water. In addition to this direct effect on the solution’s dielectric constant, we identify an osmolytic Hofmeister effect with the more hydrophobic osmolytes desorbing more efficiently weakly hydrated cations from weakly hydrated silica, and less hydrophobic osmolytes desorbing better strongly hydrated cations from strongly hydrated silica. The combined results shed light on Coulomb interactions in a more realistic model of the cytosol, a relatively unexplored territory.
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