Some pulse and gamma radiolysis studies of tyrosine and its glycyl peptides

The only significant product obtained by gamma irradiation of tyrosine in aqueous solution was 3,4-dihydroxyphenylalanine (dopa). G (dopa) was 1.0 in aerated solution, 0.9 in N₂O-saturated solutions, and less than 0.1 in deaerated solution. Pulse irradiation of a N₂O-saturated solution gave G (dopa) 0.6. The absorption spectrum of the initial OH adduct had λ_max at 300 nm (Gϵ = 16,600) and at 330 nm (Gϵ = 18,000). This adduct decayed to give some phenoxyl radical with λ_max at ≈410 nm. The rate of reaction of OH with tyrosine, measured by following formation of the transient, was 6.5±0.3 × 10⁹mol⁻¹ cm³ s⁻¹. Irradiation of the peptides, gly.tyr, tyr.gly, and gly.tyr.gly, gave analogous results; i.e. yields, reaction rates and spectra were all similar strongly suggesting that tyrosine was the main residue attacked. Radiolytic conversion of phenylalanine to tyrosine was examined for comparison. The mechanism of dopa formation is discussed in the light of available results.