Prebiotic chemistry on mineral surfaces: Proto-oligopeptide formation on silica and other substrates within depsipeptide forming systems
McKee, Aaron D.
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The chemical origins of life on Earth and perhaps elsewhere in the universe is not utterly unknowable, though the subject is incredibly complex. To endeavor to understand what events brought about life on early-Earth nearly 4 billion years ago, life itself must be broken down into its major processes, which themselves are constructed from increasingly simple and ordinary sub-units, and eventually, examined to describe the building blocks of life and how they might assemble. The presence of amino acids on extra-terrestrial bodies and in prebiotic simulation experiments suggests the plausibility of their existence on early Earth. In contrast to extant biological protein production, abiotic polypeptide formation presents several challenges, such as the thermodynamically disfavored condensation of non-activated amino acids in aqueous solution. Recent work has introduced α-hydroxy acids, a class of molecules found alongside amino acids in prebiotic contexts, into peptide forming systems. This has been shown as a robust route towards proto-polypeptides, producing long mixed-acid oligomers, referred to as depsipeptides. In pursuit of realistic model prebiotic environments, mineral-molecule interactions must be considered, and may facilitate new chemical pathways at interfacial regions. Presented in this thesis is a demonstration that the inclusion of silica and other minerals in hydroxy-acid/amino-acid and related reactions effects the composition of oligomers, resulting in amino acid enrichment relative to a substrate-absent controls. Evidence of surface ester formation suggests that the same ester aminolysis mechanism that proceeds in a homogeneous condition is also able to proceed on the substrate surface as silyl-ester aminolysis, indicating that silica is directly involved in the oligomer growth process and departs from previous studies of mineral catalyzed peptide formation on metal oxides. If depsipeptides are model proto-polypeptides, then surface functionalization of minerals with simple HAs might provide catalytic pathways useful for unraveling plausible routes to the production of complex molecules under early-Earth conditions or on extraterrestrial bodies.