Iron-Mediated Peptide Formation in Water and Liquid Sulfur Dioxide under Prebiotically Plausible Conditions**

Abstract

Peptides have essential structural and catalytic functions in living organisms. The formation of peptides requires the overcoming of thermodynamic and kinetic barriers. In recent years, various formation scenarios that may have occurred during the origin of life have been investigated, including iron(III)-catalyzed condensations. However, iron(III)-catalysts require elevated temperatures and the catalytic activity in peptide bond forming reactions is often low. It is likely that in an anoxic environment such as that of the early Earth, reduced iron compounds were abundant, both on the Earth's surface itself and as a major component of iron meteorites. In this work, we show that reduced iron activated by acetic acid mediates efficiently peptide formation. We recently demonstrated that, compared to water, liquid sulfur dioxide (SO₂) is a superior reaction medium for peptide formations. We thus investigated both and observed up to four amino acid/peptide coupling steps in each solvent. Reaction with diglycine (G₂) formed 2.0 % triglycine (G₃) and 7.6 % tetraglycine (G₄) in 21 d. Addition of G₃ and dialanine (A₂) yielded 8.7 % G₄. Therefore, this is an efficient and plausible route for the formation of the first peptides as simple catalysts for further transformations in such environments.