The biology of chaotropicity

The biology of chaotropicity John E. Hallsworth Institute for Global Food Security, School of Biological Sciences; Queen’s University Belfast, Medical Biology Centre; 97 Lisburn Road, Belfast; BT9 7BL; Northern Ireland Many of the fermentation products and other microbial metabolites valued by food scientists and industrial microbiologists are solutes/ substances which exhibit chaotropic activity; i.e. they disorder biological membranes, proteins and other types of macromolecular system. Examples of chaotropic solutes are ethanol, butanol, phenol and vanillin (as well as some salts, including MgCl2 and LiCl). Hydrophobic substances, which partition into the hydrophobic domains of biomacromolecular systems (log Poctanol water > 1.95) also disorder macromolecules and so also have a chaotropicity-mediated mode-of-action as stressors of cellular systems, and microbial systems produce both chaotropic solutes and hydrophobic stressors as secondary metabolites. Such compounds are, however, often described using the umbrella terms ‘flavour compounds’ or ‘volatile organic substances’. Many of these chaotropes and hydrophobes play key roles in the natural ecology of microbes, most notably as antimicrobials. Recent work has been carried out to clarify the meaning of, and to quantify, chaotropicity (which can arise from diverse mechanisms). Unlike kosmotropic (macromolecule-structuring) activity, chaotropicity can limit life in industrial systems and Earth’s biosphere. Understanding ways in which microbes respond to chaotropicity-induced stresses (and the associated oxidative stress), via diverse macromolecule-protection systems, is key to mitigating product-induced stresses in biofuel-, wine- and beer-fermentations as well as other industrial systems. Such responses include the production of protein-stabilization proteins, accumulation of compatible solutes, modifications of membrane structure, and increases in energy generation (as well as production of proteins involved in the removal of reactive oxygen species). Other mitigating factors include kosmotropic substances and a reduction of temperature. This said, at low temperatures (from +10 to -20°C), chaotropicity can enhance macromolecular flexibility and thereby enhance cellular metabolism and/or reduce the temperature minimum for growth. Chaotropic substances can exist is inhibitory concentrations in many locations on Earth and, indeed, on other planetary bodies, so chaotropicity has implications for habitability or terrestrial and extraterrestrial environments.