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

1995; Elsevier BV; Volume: 92; Issue: 3 Linguagem: Inglês

10.1016/1010-6030(95)04134-7

1873-2666

Faming Jin, Johannes Leitich, Clemens von Sonntag,

Radical Photochemical Reactions

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 × 10−3 while the observed products (quantum yields in parentheses) are 2,2′-bityrosyl (BITYR, 3.1 × 10−3) and 2-amino-4-ethenyl-hex-4-enic acid (AEHEA, 1.6 × 10−3). In N2O-saturated solutions, N2 (1.8 × 10−2), in the presence of 2-propanol, H2 (1 × 10−3) are formed. In air-saturated solutions the tyrosine consumption is increased (9.7 × 10−2); in addition to increased yields of BITYR ({if1.4 × 10−2}) but unaltered yields of AEHEA (1.6 × 10−2), 3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1H-indol-2-carboxylic acid (HOHICA,1.6 × 10−2), 3,4-dihydroxyphenylalanine (DOPA, 3.8 × 10−2) and 2,4-dihydroxyphenylalanine (2,4-DHPhe, 2.3 × 10−3) are observed. The sum of the quantum yields of the O2-dependent products increases with increasing O2 and TYROH concentrations, approaching a plateau value of approximately 7 × 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 (1O2 formation) in the presence of TYROH yields HOHICA (90%) and BITYR (10%). It is concluded that from the excited singlet state (1TYROH) 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 1TYROH 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 (3TYROH) by O2 yields 1O2 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 O2 with 3TYROH possibly yields 1,4- and 3,6-endoperoxides which are the precursors of further HOHICA and of DOPA and 2,4-DHPhe, respectively.