The photolysis (lambda=254 nm) of tyrosine in aqueous solutions in the absence and presence of oxygen. The reaction of tyrosine with singlet oxygen

In the 254 nm photolysis of deoxygenated aqueous solutions of tyrosine (TYROH, 10(-3) mol dm(-3)) TYROH is consumed with a quantum yield of 9.1 x 10(-3) while the observed products (quantum yields in parentheses) are 2,2'-bityrosyl (BITYR, 3.1 x 10(-3)) and 2-amino-4-ethenyl-hex-4-enic acid (AEHEA, 1.6 x 10(-3)). In N2O-saturated solutions, N-2 (1.8 x 10(-2)), in the presence of 2-propanol, H-2 (1 x 10(-3)) are formed. In air-saturated solutions the tyrosine consumption is increased (9.7 x 10(-2)); in addition to increased yields of BITYR (1.4 x 10(-2)) but unaltered yields of AEHEA (1.6 x 10(-3)), 3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro acid (HOHICA, 1.6 x 10(-2)), 3,4-dihydroxyphenylalanine (DOPA, 3.8 x 10(-2)) and 2,4-dihydroxyphenylalanine (2,4-DHPhe, 2.3 x 10(-3)) are observed. The sum of the quantum yields of the O-2-dependent products increases with increasing O-2 and TYROH concentrations, approaching a plateau value of approximately 7 x 10(-2) at high reactant concentrations. In D2O the quantum yields of BITYR, HOHICA and DOPA are increased considerably while those of AEHEA are decreased. Irradiation of Rose Bengal with visible light (O-1(2) formation) in the presence of TYROH yields HOHICA (90%) and BITYR (10%). It is concluded that from the excited singlet state ((1)TYROH) photoionization and splitting of the phenolic O-H bond occurs giving rise to tyrosine-derived phenoxyl radicals (TYRO(.)) which are the precursors of BITYR. Also, from (1)TYROH a 1,3-H-shift of the phenolic O-H and concomitant ring-opening occurs. Addition of water and loss of carbon dioxide in subsequent thermal reactions yields AEHEA as the final product. Quenching of the TYROH triplet state ((3)TYROH) by O-2 yields O-1(2) which reacts with TYROH by H abstraction. The resulting intermediates TYRO(.) and HO2. are the precursors of BITYR and HOHICA. In addition, a reaction of O-2 with (3)TYROH possibly yields 1,4- and 3,6-endoperoxides which are the precursors of further HOHICA and of DOPA and 2,4-DHPhe, respectively.