Tritium-labeled 5-oxo-Pro-Arg-Pro

Conditions of tritium introduction into 5-oxo-Pro-Arg-Pro have been developed. Labelled 5-oxo-Pro-Arg-Pro has been obtained in 75% yield and molar radioactivity of 60 Ci/mmol. Tritium distribution over amino acid residues in the peptide has been determined. 5-Oxo-Pro : Arg : Pro ratio in the labelled peptide has been found to be 1 : 5.2 : 2.8. Nonterminal amino acid has been found to contain the major portion of tritium, about 35 Ci/mmol; i.e., arginine includes more than half of the label. Such a feature can be explained by the effect of readily protonated guanidine group of arginine. The influence of charged guanidine group of arginine seems to increase the shift of electron density in the neighboring C–H bonds of arginine and proton mobility, thus increasing protium–tritium exchange at these carbon atoms.