(132b) Processes and Systems in the Origin of Life: Thermodynamics and Kinetics

The origin of life on Earth four billion years ago is not well understood. One approach to studying the origin of life is to design and construct minimal systems that demonstrate key principles associated with living systems, and that lie within the constraints of early Earth environments. Biopolymers are a key component of living systems, as nucleic acid polymers store information and proteins fold and catalyze reactions. Because RNA can store information and also catalyze reactions as ribozymes, the RNA World hypothesis provides one construct in which to frame origins of life research—that RNA was the first biopolymer, with subsequent evolution incorporating DNA, proteins, and other components. However, nucleotides are much more difficult to synthesize, compared to amino acids. The Miller-Urey experiments of the 1950’s established a first prebiotic pathway to the synthesis of amino acids, providing the monomers. However, the abiotic formation of the polymers (peptides) in a Peptide World faces several challenges. Drying reactions have been used to form amide bonds between amino acids, limiting loss via hydrolysis, but the formation of the cyclic dimer hinders further growth. Depsipeptides provide a possible route to further peptide elongation, as a co-polymer of hydroxy acids and amino acids. Ester bonds form first between hydroxy acids, followed by ester-amide exchange to form the amide bond. The resulting hydroxl-terminated peptides cyclize but the reaction is reversible. With continued elongation, functional peptides could emerge and undergo selection. System-level modeling of the depsipeptide system under cyclic environmental conditions demonstrates that the reversible polymerization can be understood via elementary reactions and classical mass transfer, providing a possible prebiotic pathway to the abiotic synthesis of peptides on the early Earth.