Zwitterionic Osmolytes Resurrect Electrostatic Interactions Screened by Salt

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 mainstream. 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 five salts (NaCl, KCl, CsCl, MgCl2, and CaCl2) and five zwitterionic osmolytes (betaine, proline, trimethylamine N-oxide, glycine, and 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 more efficiently desorbing weakly hydrated cations from weakly hydrated silica and the less hydrophobic osmolytes better desorbing 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.

Polyol and sugar osmolytes can shorten protein hydrogen bonds to modulate function

“…Considering that the h-bond change is relatively small, the major osmolyte effect on protein stability may come from the denatured state destabilization. Recent studies have also shown that zwitterionic osmolytes can perturb electrostatic interactions 56 – 58 . It becomes clear that osmolytes are more than just inert crowders.…”

Unifying the Ambivalent Mechanisms of Protein-Stabilising Osmolytes

“…Moreover, the current work's demonstration that protein-stabilizing osmolyte may have ambivalent interactions with the proteins, can have possible implication in their mechanistic action in modulating protein-protein interactions. 29,[43][44][45] Recent experiments in these directions have shown that osmolyte and non-osmolytes may have a distinct role in perturbing charge-mediated inter-protein interactions 43 and that zwitterionic osmolytes can resurrect the electrostatic interactions among charged surfaces, with low temperature playing an important role. 44,46,47 Future investigation in the possible molecular mechanism of osmolyte-induced protein-protein interaction and its possible modulation of liquid-liquid protein phase separation 48 can provide interesting insights.…”

Zwitterionic polymers in drug delivery: A review

“…86 Zwitterionic polymeric structures can be efficiently modulated to play distinct functions in different environmental factors such as ions, pH, and functional groups tagged on the outer walls of proteins. 87 Armes et al synthesized a series of pHresponsive phosphobetaine nanocarriers and successfully used them as carriers for delivering plasmid DNA, antibodies, and therapeutic drugs without undesirable systemic toxicity. [88][89][90][91] For targeted codelivery of two anticancer drugs, pH-responsive poly(MPC-DPA) polymersome was prepared.…”