Iron-mediated peptide formation in water and liquid sulfur dioxide under early Earth conditions

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 harsh conditions and the catalytic activity in peptide bond forming reactions is 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. Here we show, that reduced iron activated by acetic acid mediates efficiently peptide formation. We observed, both in water and liquid sulfur dioxide, up to four amino acid/ peptide coupling steps and the formation of up to decamers. In 21 days 4.1% triglycine G3 and 11.3% tetraglycine G4 were formed. Addition of G3 and dialanine A2 yielded 26.0% G4. Therefore, this is a highly efficient and plausible route for the formation of the first peptides as simple catalysts for further transformations in such environments.