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 parenthese s) 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 forme d. 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-dihydroxyphenylalani ne (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-depe ndent products increases with increasing O-2 and TYROH concentrations, approaching a plateau value of approximately 7 x 10(-2) at high react ant concentrations. In D2O the quantum yields of BITYR, HOHICA and DOP A are increased considerably while those of AEHEA are decreased. Irrad iation of Rose Bengal with visible light (O-1(2) formation) in the pre sence of TYROH yields HOHICA (90%) and BITYR (10%). It is concluded th at from the excited singlet state ((1)TYROH) photoionization and split ting of the phenolic O-H bond occurs giving rise to tyrosine-derived p henoxyl radicals (TYRO(.)) which are the precursors of BITYR. Also, fr om (1)TYROH a 1,3-H-shift of the phenolic O-H and concomitant ring-ope ning occurs. Addition of water and loss of carbon dioxide in subsequen t thermal reactions yields AEHEA as the final product. Quenching of th e TYROH triplet state ((3)TYROH) by O-2 yields O-1(2) which reacts wit h 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 t he precursors of further HOHICA and of DOPA and 2,4-DHPhe, respectivel y.