REDOX CHEMISTRY OF 3-IODOTYROSINE IN AQUEOUS-MEDIUM

The reactions of 3-iodotyrosine (ITyOH) with the primary . OH/. O-, H ., and e(aq)(-) and selected secondary radicals in irradiated aqueous solutions have been studied. The pK(a) values of ITyOH are 2.1 +/- 0.1 (-COOH), 8.3 +/- 0.05 (Ph-OH), and 10.0 +/- 0.1 (-NH3+), and its reac tivity is pH-dependent, The phenoxyl radical (ITyO .) formed during ox idation exhibits lambda(max) at 275 nm (epsilon = 8200 +/- 400 M-1 cm( -1)) and 405 nm (epsilon = 3300 +/- 270). The rate of oxidation increa ses with the pH; e.g., for the Br-2(-) radical, k = 8.1 x 10(7) M-1 s( -1) at pH 6.7 and 5.0 x 10(8) M-1 s(-1) at pH 11.7. The . OH radical r eaction leads to me formation of the adduct radical ITy(OH)(2) . in 30 % of the cases while 70% lead to rapid formation of ITyO .. Oxygen sho ws negligible reactivity toward ITyO ., but it reacts with ITy(OH)(2) . with k = 1 x 10(7) M-1 s(-1). The respective one-electron reduction potentials (E) for the ITyO ., H+/ITyOH, and ITyO ./ITyO(-) couples ar e 0.82 +/- 0.03 V at pH 7.4 and 0.73 +/- 0.04 V vs NHE at pH 11.5. Red uction by e(aq)(-) (k = 1.2 x 10(10) M-1 s(-1) at pH 6.5, 7.5 x 10(9) M-1 s(-1) at pH 10, and 5.0 x 10(9) M-1 s(-1) at pH 12) follows differ ent paths at different pH. Deamination and deiodination occur at pH < 8, at nearly equal proportions. However, at pH 8-11, the yield of ammo nia decreases whereas the yield of I- increases. The radical produced by deiodination, . TyOH, exhibits lambda(max) at 280 nm with epsilon = 6400 +/- 400 M-1 cm(-1) and reacts with oxygen with k approximate to 10(7) - 10(8) M-1 s(-1). In the absence of oxygen, . TyOH decays by ra dical dimerization reaction with 2k = 5.3 x 10(9) M-1 s(-1). In alkali ne pH, however, the . TyO(-) radical rearranges into TyO .. The latter species oxidizes ITyO(-) to ITyO . with k = 10(8) M-1 s-(1). Formatio n of the semioxidized transient during reduction in alkaline pH is uni que to 3-iodotyrosine and has not been reported for either tyrosine or 3,5-diiodotyrosine. The H . atoms reaction takes place with k = 8 x 1 0(9) and 5 x 10(9) M-1 s(-1) at pH 1.5 and 5, respectively, and leads to the formation of the adduct ITyOH(2) . with gamma(max)nm and epsilo n = 1370 +/- 100 M-1 cm(-1).