Gradual evolution of a homo-l-peptide world on homo-d-configured RNA and DNA.

Modern life requires the translation of genetic information - encoded by nucleic acids - into proteins, which establishes the essential link between genotype and phenotype. During translation, exclusively l-amino acids are loaded onto transfer RNA molecules (tRNA), which are then connected at the ribosome to give homo-l-proteins. In contrast to the homo-l-configuration of amino acids and proteins, the oligonucleotides involved are all d-configured (deoxy)ribosides. Previously, others and us have shown that if peptide synthesis occurs at homo d-configured oligonucleotides, a pronounced l-amino acid selectivity is observed, which reflects the d-sugar/l-amino acid world that evolved in nature. Here we further explore this astonishing selectivity. We show a peptide-synthesis/recapture-cycle that can lead to a gradual enrichment and hence selection of a homo-l-peptide world. We show that even if peptides with a mixed l/d-stereochemistry are formed, they are not competitive against the homo-l-counterparts. We also demonstrate that this selectivity is not limited to RNA but that peptide synthesis on DNA features the same l-amino acid preference. In total, the data bring us a step closer to an understanding of how homochirality on Earth once evolved.