Zwitterionic Osmolytes Employ Dual Mechanisms for Resurrection of Surface Charge under Salt-stress

Salt imbalance in cells is a major detrimental abiotic stress which causes ionic toxicity and disrupts important cellular functions. To counter this saline stress, cell often produces low molecular weight cosolutes, known as osmolytes, which have the ability to resuscitate homeostasis. Here we combine atomistic computer simulation, contact angle measurements and Raman spectroscopic analysis to identify the mechanistic role of multiple osmolytes (glycine, TMAO and betaine) in modulating the electrostatic interaction under salt stress, a slowly emerging aspect of osmoprotection. By utilising a pair of negatively charged silica surfaces in a ternary mixture of osmolyte and KCl solution as a proxy of charged surface of biomacromolecule, our investigation reveals that all three osmolytes are able to resurrect the electrostatic interaction between the two surfaces, which had been otherwise charge-screened by excess salt. The joint venture of experiment and simulation discover dual and mutually exclusive mechanisms of recovering charge interaction by zwitter-ionic osmolytes. However, the relative ability and the underlying mechanism of revival of electrostatic interactions are found to be strongly dependent of chemical nature of osmolyte. Specifically, glycine was found to competitively desorb the salt-ions from the surface via its direct interaction with the surface. On the other hand, TMAO and betaine counter-act salt stress by retaining adsorbed cations but partially neutralising their charge-density via ion-mediated interaction. We believe that the access to dual and mutually alternative modes of osmolytic actions, as elucidated here, would provide the cell the required adaptability in combating salt-stress.