Competitive photosensitized oxidation of tyrosine and methionine residues in enkephalins and their model peptides

Laser flash photolysis was used to investigate the oxidation of methionine (Met) and tyrosine (Tyr) residues in methionine-enkephalin (MetEnk, TyrGlyGlyPheMet) by the triplet state of 4-carboxybenzophenone (4CB). Quenching of the 4CB triplet by model amino acids and peptides (Tyr, TyrGly, TyrGlyGly, PheMet, TyrMet, MetTyr, MetEnk and TyrGlyGlyPheLeu) occurred with rate constants close to the diffusion-controlled limit, kq = (1 − 3) × 109 M−1 s−1. Experimental transient spectra, that were resolved into components, revealed the presence of various electron-transfer intermediates, i.e. ketyl radicals (CBH) and ketyl radical anions (CB−) of 4CB, tyrosyl radicals (TyrO), and (S∴N)+ radical cations. Based on the concentration profiles obtained from spectral deconvolutions, quantum yields of the transients were determined. For the quenchers containing only the tyrosine residue, the quantum yields of the tyrosyl radicals were found to be close to unity and equal to the sum of the quantum yields for the formation of 4CB ketyl radicals and 4CB ketyl radical anions. For quenchers containing both tyrosine and methionine residues, the formation quantum yields of tyrosyl radicals were decreased from the value of 1 to approximately 0.7–0.8 which was equal to the sum of the quantum yields for the formation of CBH and CB−. The above observations are discussed in terms of competitive oxidation reactions involving Tyr and Met followed by the intramolecular electron transfer from the tyrosine residue to the sulfur-centered radical cation on the methionine residue.