Reactivity of OH with Tyrosine in Aqueous Solution Studied by Pulse Radiolysis

Article abstract of DOI:10.1021/j150654a030

The specific OH attack on various sites of the tyrosine molecule in neutral aqueous solutions (pH 6-8), saturated with N2O, has been investigated.The main process (ca. 50percent) is the formation of ortho-directed OH adduct (R₁) with k = (7.0 +/- 0.5)E9 dm³ mol^-1 s^-1 (λ_max = 330 nm, ε₃₃₀ = 300 +/- 30 m² mol^-1), which decays by water elimination according to a first-order reaction (k'= (1.8 +/- 0.2)E4 s^-1) under formation of phenoxyl radical, as well as by second order with 2k = (3.0 +/- 1.0)E8 dm³ mol^-1 s^-1.The phenoxyl radical is additionally formed as a primary product (ca. 5percent) with k = (6.0 +/- 1.0)E8 dm³ mol^-1 s^-1.It possesses two absorption maxima, 260 nm (ε₂₆₀ = 600 +/- 50 m² mol^-1) and 405 nm (ε₄₀₅ = 260 +/-20 m² mol^-1), and decays with 2k = (4.0 +/- 1.0)E8 dm³ mol^-1 s^-1).The meta isomer of the OH adducts (R₂) is formed to ca. 35percent with k = (5.0 +/- 0.4)E9 dm³ mol^-1 s^-1, having two absorption maxima at 305 nm (ε₃₀₅ = 280 +/- 30 m² mol^-1) and 540 nm (ε₅₄₀ = 23 +/- 3 m² mol^-1), and disappears with 2k = (2.0 +/- 0.5)E9 dm³ mol^-1 s^-1.The rest of ca. 10percent OH radicals attack most probably the para and to a small extent ipso positions of the phenol ring under formation of the corresponding adducts.The H abstraction from the alanine moiety cannot be excluded.