Elucidating different aspects of protocell emergence on the early Earth

Abstract

The origin of life on Earth likely followed a fundamental step of protocell formation. This process would have been influenced by interactions between different co-solutes that would have been present in a heterogeneous prebiotic soup. Towards this, we studied the effect of amphiphiles as a co-solute on abiotic peptide synthesis, under prebiotically-pertinent wet-dry cycling conditions. While doing so, we made a serendipitous discovery of N-acyl amino acids (NAAs) formation. NAAs are a prebiotically unexplored class of single-chain amphiphiles that contain amino acid and fatty acid linked via an amide linkage. We showed that NAAs are readily synthesized by the reaction between amino acids and ester bond-containing amphiphiles (like phospholipids/monoglycerides) via an ester-amide exchange process. It demonstrates a new prebiological route for NAA synthesis, which strongly supports their potential availability on the early Earth. Subsequently, the properties of NAAs to assess their potential as a new model protoamphiphile system were characterized. Protoamphiphiles are prebiotically-relevant amphiphiles that might have served as membrane components of early protocells. Notably, NAAs were found to self-assemble into vesicles at acidic pH, in contrast to fatty acids (conventional model protoamphiphiles) that form vesicles at neutral to alkaline pH. Furthermore, mixed systems of NAAs and monoglycerides were shown to form vesicles over a broad pH range, and these vesicles were also stable in the presence of metal ions, thereby circumventing the two significant challenges that conventional protocell membranes typically face. Pertinently, pH-stable membranes were also generated by blending NAAs and fatty acid systems. Also, NAAs acted as a substrate for peptide chain growth under wet-dry cycling conditions to generate different lipopeptides that could have aided in the functionalization of protocell membranes. Overall, these results illustrated the role of NAAs as a highly promising model protoamphiphile system, while also highlighting their putative role in shaping the emergence and evolution of functional protocell membranes on primitive Earth. Finally, to get a realistic understanding of prebiotic membrane assembly in a natural environment as opposed to stringently controlled laboratory conditions, we studied vesicle formation by fatty acid-based and NAA-based amphiphile systems in different hot spring water samples. Pertinently, terrestrial hot springs are also considered a plausible niche that assisted life’s emergence. In these studies, we showed how these natural settings are indeed supportive of vesicle formation by both the above-mentioned systems. We also demonstrated how certain amphiphilic systems might have had a selective advantage toward vesicle formation under these conditions.