LABELING OF AMINO ACIDS AND PEPTIDES WITH ISOTOPIC OXYGEN AS FOLLOWED BY 17O‐N.M.R.*

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Abstract
17O was introduced into the respective .alpha.- and .gamma.-COOH groups of Boc-Gly and Boc-Glu [BOC = tert-butyloxycarbonyl] by saponification of the corresponding O-methyl esters with 1 N NaOH in H2 17O. Other 17O enriched Boc-amino acids were prepared by acid catalyzed exchange into the amino acid .alpha.-COOH group followed by tert-butyloxycarbonylation with tert-butyl S-4, 6-dimethylpyrimidin-2-ylthio carbonate. Final enrichment, by approximately 3 orders of magnitude over natural abundance, was 60-100% of the possible maximum. The synthesis of [17O]-Gly-Ala, [17O]-Gly-Leu and [17O]-Gly-Glu by dicyclohexylcarbodiimide/1-hydroxybenzotriazole mediated coupling of Boc-Gly-[17O]-.alpha.-COOH with amino acid-O-tert-butyl esters followed by deprotection with HCl/EtOAc proceeded without undue loss of the isotope. Boc-[17O]-Pro-Leu-Gly-NH2 was prepared by a similar procedure. [Tyr2-17O]-, [Pro7-17O]- and [Gly4-17O]-oxytocin were synthesized using solid phase support. 17O-chemical shifts of synthetic intermediates and of the final products were as expected for each functional group. Linewidth data correlate with the MW of the compounds prepared.