Environmental constraints on the origin of life based on membrane formation: the role of salinity

Abstract The environmental conditions for the origin of life are still not well-constrained, but membrane-bound structures must have been key to the origin of life. Membranes composed of fatty acids are promising candidates due to their simplicity and plausible prevalence in prebiotic environments. To assess the stability of membranes composed of fatty acids with tail lengths ranging from 12 to 16 carbons at different temperatures and sodium chloride concentrations that may have existed on the early Earth, we conducted all-atom molecular dynamics (MD) simulations. In the absence of salt (freshwater), none of the fatty acids exhibited bilayer formation, whether below or above their chain melting temperature. However, elevating the salt concentration from 0.15 M (saline solution), 0.5 M (seawater), 1 M (seawater tide pools), 3 M (salty tide pools) and 5 M (Dead Sea) resulted in the formation of stable bilayers. The 16-carbon fatty acid required lower salt concentration, while shorter, 12-carbon chain necessitated higher salt levels. Increasing the salt concentration led to three main effects: (1) increased bilayer thickness, (2) reduced area per fatty acid and (3) elevated deuterium order parameter of the chains, resulting in more robust membranes. Our simulations indicated that the salt cations aggregated on the bilayer surfaces, effectively mitigating repulsive interactions among hydrophilic fatty acid head groups. These findings suggest that fatty acid bilayers are more likely present in ancient waters connected to saltwater reservoirs, or seawater tide pools with elevated salt concentrations.