Studies in isotopic labelling of amino acids and polypeptides

The aromatic protone of amino acids and samll peptides have been exchanged with deuterium under conditions which produced no significant exchange of the non - aromatic protons. A heterogeneous catalyst syrstem was found to be suitable for the deuterium exchange of biologically active molecules, and platinum used in a 1:1 molar ratio with the labelled compound, was the most selective of the G-roup VIII metal catalysts. Substantial exchange of the aromatic protons of phenylalanine, tyrosine, tryptophan and histidine occurred when the reaction was allowed to proceed for 1 day, at 70°C. A similar deuterium distribution was found in the aromatic amino acid residues of a number of model peptides under the same reaction conditions. Substantial incorporation into the diketopiperazine ring of labelled cyclo - dipeptides was fou,nd when iridium, was used to catalyse the reaction. The optical purity and any peptide bonds present in the labelled compounds were preserved through out the exchange reactions.