Abiotic condensation synthesis of glyceride lipids and wax esters under simulated hydrothermal conditions

Precursor compounds for abiotic proto cellular membranes are necessary for the origin of life. Amphipathic compounds such as fatty acids and acyl glycerols are important candidates for micelle/bilayer/vesicle formation. Two sets of experiments were conducted to study dehydration reactions of model lipid precursors in aqueous media to form acyl polyols and wax esters, and to evaluate the stability and reactions of the products at elevated temperatures. In the first set, mixtures of n-nonadecanoic acid and ethylene glycol inwater, with and without oxalic acid, were heated at discrete temperatures from 150 degrees C to 300 degrees C for 72 h. The products were typically alkyl alkanoates, ethylene glycolyl alkanoates, ethylene glycolyl bis-alkanoates and alkanols. The condensation products had maximum yields between 150 degrees C and 250 degrees C, and were detectable and thus stable under hydrothermal conditions to temperatures < 300 degrees C. In the second set of experiments, mixtures of n-heptanoic acid and glycerol were heated using the same experimental conditions, with and without oxalic acid, between 100 degrees C and 250 degrees C. The main condensation products were two isomers each of monoacylglycerols and diacylglycerols at all temperatures, as well as minor amounts of the fatty acid anhydride and methyl ester. The yield of glyceryl monoheptanoates generally increased with increasing temperature and glyceryl diheptanoates decreased noticeably with increasing temperature. The results indicate that condensation reactions and abiotic synthesis of organic lipid compounds under hydrothermal conditions occur easily, provided precursor concentrations are sufficiently high.