o-Tyrosine hydroxylation by OH radicals. 2,3-DOPA and 2,5-DOPA formation in γ-irradiated aqueous solution

The reactions of radiolytically generated hydroxyl radicals (OH) with dl-o-tyrosine (o-Tyr) in 10−3 mol dm−3 aqueous solutions, saturated with N2O/O2 gas mixture (4:1, v/v), were studied in the dose range between 30 and 270 Gy. Two isomeric o-Tyr hydroxylation products, namely, 2,3-dihydroxyphenylalanine (2,3-DOPA) and 2,5-dihydroxyphenylalanine (2,5-DOPA) have been identified. The chemical structure of both products was confirmed by an electrospray ionization mass spectrometry, UV and NMR-1H spectroscopy. The quantitative analysis with the use of an electrochemical and a spectrofluorimetric detection showed that the concentration of 2,3-DOPA and 2,5-DOPA increased and the content of substrate decreased linearly with increasing radiation dose. The ratio of the yields of 2,3-DOPA and 2,5-DOPA was equal to 1.3 for the whole range of radiation dose applied. The total yield of 2,3- and 2,5-DOPAs was approximately 50% of the level of o-Tyr decomposition.