The role of autocatalysis on the chemical diversity of the prebiotic ocean of early Earth

The spontaneous formation of catalytic polypeptides of various lengths in a primordial ocean endowed with a source of amino acids from micrometeorites was investigated and found to be sufficient to induce the transformation of potential substrates under the assumption of a high propensity of the environment to catalyse the formation of the peptide bond. This work aims to include in this picture the effect of autocatalysis, i.e. the ability of a polypeptide with a specific length to promote the formation of the peptide bond. Once the formation of an autocatalytic species is attained, the concentrations of the polypeptides, substrates and products of reaction exhibit a time-dependent rate of formation and undergo a catastrophic change. While in the absence of autocatalysis the concentrations of polypeptides are stationary and the formation of reaction products is limited by the proper frequency λ, autocatalysis induces a steady growth of the concentrations of polypeptides and a 100 − 105-fold increase of reaction products at t = ω−1<0.46 Gyr, with a subsequent linear growth in time according to the law u/z0 = 1+s(ω−1+t)/z0, provided the autocatalytic species be active with length fewer than 70 amino acid units. A relationship was found between the catalytic ability of the environment (expressed by the ratio η/ηh of the rate coefficient for peptide bond formation to the corresponding rate coefficient for hydrolysis) and the time of the sharp increase of the concentration of both the polypeptides and their products of transformation. Although the formation of autocatalytic polypeptides is able to rapidly induce a sharp increase in the concentration of both polypeptides and their products of transformation, the crucial formation of the first autocatalytic polypeptides relies on the ability of the environment to promote the formation of the peptide bond. The value of the ratio η/ηh, constrained by the available time for chemical evolution to values bordering the catalytic activity of present-day enzymes, suggests that the correlation between the presence of water and the formation of a complex chemistry should be taken with caution.