Stable Fatty Acid Vesicles form under Low-pH Conditions and Interact with Amino Acids and Dipeptides

Understanding how biopolymers, specifically RNA and proteins, formed prebioticly is important for understanding the origin of life. These bio-molecules are composed of building blocks that would have been widespread and at very low concentrations in early oceans. Here we investigate whether fatty acid membranes, which form spontaneously in water, could have co-assembled with these building blocks, leading to their selection and concentration and to increased membrane stability. Our previous work showed that RNA bases and ribose do bind to and stabilize fatty acid vesicles [Black et al. PNAS 110, 13272 (2013)]. Similar experiments with protein building blocks have been hampered by the extreme sensitivity of vesicle formation to pH and ionic strength at pH 7.5-8.0. Our new experiments show that decanoic acid vesicle solutions produced at pH 6.8-6.9, with phosphate buffer and 0.1 M NaCl, undergo little or undetectable change in pH upon the addition of up to 10 mM of amino acids or dipeptides, and little change in structure upon addition of a further 10 mM NaCl. Using turbidity measurements to assay vesicle formation, we find that some amino acids under these conditions, including glycine and serine, increase vesicle formation, while others, including leucine, do not. Alanine causes only a small increase in turbidity, but adding the same number of alanine residues joined to form a dipeptide doubles the effect. Blocking either the amine or carboxyl end eliminates the dipeptide-induced increase, suggesting that both ends interact with the vesicle surface. Cryo-electron microscopy images indicate that in addition to altering the extent of vesicle formation, alanine-alanine increases contact between lamellae in multilamellar vesicles. Thus prebiotic membranes may have assembled with amino acids and short peptides, which in turn would have altered membrane formation and structure.