Chemical evolution of amphiphiles: glycerol monoacyl derivatives stabilize plausible prebiotic membranes

The self-assembly of simple amphiphiles like fatty acids into cell-like membranous structures suggests that such structures were available on prebiotic Earth to support the origin of cellular life. However, the composition of primitive membranes remains unclear because the physical properties of the aqueous environment in which they assembled are relatively unconstrained in terms of temperature, pH, and ionic concentrations. It seems likely that early membranes were composed of mixtures of various amphiphiles in an aqueous medium warmed by geothermal activity prevalent in the Archean era. To better understand the properties of mixed bilayers formed by binary mixtures of single-chain amphiphiles under these conditions, we conducted stability experiments, using membranes composed of various fatty acids having hydrocarbon chain length between 8 and 18 carbons, in mixtures with their glycerol monoacyl amphiphile derivatives (GMAs). The parameters investigated were critical vesicle concentration (CVC), encapsulation, and temperature-dependent stability. We found that hydrocarbon chain length and the presence of GMAs were major factors related to membrane stability. As chain length increased, GMA additions decreased the CVC of the mixtures 4- to 9-fold. Encapsulation ability also increased significantly as a function of chain length, which reduced permeation of small marker molecules. However, long exposures to temperatures in excess of 60 degrees C resulted in a total release of encapsulated solutes and extensive mixing of the membrane components between vesicles. We conclude that GMAs can significantly increase the stability of mixed amphiphile membranes, but further studies are required to establish model membranes that are stable at elevated temperatures.