Belt and Braces: Cryoprotection and Osmoprotection in Zwitterionic Osmolyte Solutions

Cytosol, the liquid compartmentalised by the cell membrane, contains a mixture of salts and organic molecules or “osmolytes”, of which many are zwitterionic. Cytosol composition can be regulated to protect against both salt stress and cold stress through the use of osmoprotecting and cryoprotecting agents – where many zwitterionic osmolytes fulfil both functions. Recently, we investigated the osmoprotecting-effect of the archetypal zwitterionic osmolyte trimethylglycine at room temperature, revealing ordered aggregate formation that was strongly influenced by salt species. Here we will study the cryoprotecting effect of model cytosol solutions by using neutron scattering. By cooling mixtures of trimethylglycine and sodium chloride, we aim to disentangle the osmoprotectant- and cryoprotectant- effect on water structure, ion coordination and aggregation. Previously the impact of cryo- and osmo- protection have largely been considered separately, but the two are inextricably linked: in many cases low (sub-zero) temperatures are accessed through high salt concentration such as in salt lakes, by depression of the freezing point. These results will have significant impact on our understanding of the role of zwitterionic osmolytes in maintaining protein structure and function in the most extreme environments life can survive in.