Abstract:
If the first biomolecules were produced from the interactions between the lithosphere, hydrosphere, and atmosphere, it is likely that greater biocomplexity also evolved in context of a plausible geochemical environment. Both the development of primitive bilayer membranes, or vesicles, and the selective adsorption of organic species onto mineral surfaces may concentrate biomolecules and facilitate the assembly of biopolymers. Vesicles composed of simple fatty acids have been suggested as model "proto-cells". To evaluate the plausibility of fatty acid vesicles as the earliest cells, we characterized the stability of the mid-chain (C10) fatty acid decanoate under varying lipid and salt concentrations. We observed that vesicles composed of decanoate/decanol mixtures remained stable under a wider range of aqueous conditions as opposed to those formed by decanoate alone. Moreover, our addition of rock-forming minerals to decanoate solutions increased the rate of vesicle assembly, possibly through the adsorption of decanoate at the mineral/water interface. We investigated the role of mineral surfaces in concentrating biomolecules in further detail by measuring the adsorption of aspartate on brucite, [Mg(OH)2]. We observed that aspartate attachment almost doubled with CaCl2, and our theoretical calculations with the extended triple-layer model (ETLM) predicted that the Ca2+ ion forms a cooperative complex with aspartate at the brucite surface. This selective adsorption more dramatically occurred when we added aspartate to brucite as part of an equimolar mixture of four other amino acids (glycine, lysine, leucine, and phenylalanine) with CaCl2, and it adsorbed up to 20 times more than the other acids combined. Although both vesicles and mineral surfaces may concentrate biomolecules from a diluting aqueous environment, the dependence of lipid and vesicle assembly on the latter may lead to a scenario in which vesicles form in proximity to mineral surfaces and encapsulate neighboring organic species. A future study of the interactions between mineral surfaces and biopolymers together with vesicles may provide clues toward unraveling the evolution of the first single-celled organism.