Determining the Diastereoselectivity of the Formation of Dipeptidonucleotides by NMR Spectroscopy.

Proteins are composed of l-amino acids, but nucleic acids and most oligosaccharides contain d-sugars as building blocks. It is interesting to ask whether this is a coincidence or a consequence of the functional interplay of these biomolecules. One reaction that provides an opportunity to study this interplay is the formation of phosphoramidate-linked peptido RNA from amino acids and ribonucleotides in aqueous condensation buffer. Here we report how the diastereoselectivity of the first peptide coupling of the peptido RNA pathway can be determined in situ by NMR spectroscopy. When a racemic mixture of an amino acid ester was allowed to react with an 5'-aminoacidyl nucleotide, diastereomeric ratios of up to 72 : 28 of the resulting dipeptido nucleotides were found by integration of 31 P- or 1 H-NMR peaks. The highest diastereomeric excess was found for the homochiral coupling product d-Ser-d-Trp, phosphoramidate-linked to adenosine 5'-monophosphate with its d-ribose ring. When control reactions with an N-acetyl amino acid and valine methyl ester were run in organic solvent, the diastereoselectivity was found to be lower, with diastereomeric ratios≤62 : 38. The results from the exploratory study thus indicate that the ribonucleotide residue not only facilitates the coupling of lipophilic amino acids in aqueous medium but also the formation of a homochiral dipeptide. The methodology described here may be used to search for other stereoselective reactions that shed light on the origin of homochirality.