SPIN‐TRAPPING AND ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES AND POLYPEPTIDES IN AQUEOUS SOLUTIONS—I. PHENYLALANINE AND RELATED COMPOUNDS

Abstract

Abstract— The direct UV photolysis of l‐Phe and peptides containing l‐Phe in aqueous solutions has been investigated at room temperature. The short‐lived free radicals formed during photolysis were spin‐trapped by t‐nitrosobutane and identified by electron spin resonance. During the photolysis of l‐Phe the decarboxylation and the deamination radicals were spin‐trapped. For N‐formyl and N‐acetyl‐l‐Phe the decarboxylation radicals were observed. For dipeptides containing Phe the decarboxylation radicals were observed and in some cases the deamination radicals from the N‐terminal residue were found. For the tripeptides Gly‐l‐Phe‐l‐Ala and Gly‐Gly‐l‐Phe, the C‐terminal decarboxylation radical was spin trapped; for l‐Phe‐Gly‐Gly only the deamination radical of the N‐terminal residue could be detected. However, for Gly‐l‐Phe‐Gly, five different radicals were identified. The results of the spin‐trapping experiments of the 260 nm photolysis of RNase‐S‐peptide, containing 20 amino acid residues, was interpreted in terms of a chain scission between the alpha carbon of the Phe residue and the adjacent carbonyl group.