CHARACTERIZATION OF TRANSIENTS PRODUCED IN AQUEOUS-MEDIUM BY PULSE RADIOLYTIC OXIDATION OF 3,5-DIIODOTYROSINE

Using the technique of pulse radiolysis, oxidation studies of 3,5-diio dotyrosine (DTR) with radicals generated in phosphate-buffered aqueous medium are reported. Secondary oxidizing radicals S0(4).(-), Br-., N- 3., Tl(OH)(+), (DMS)(2)(+), CCl3O2., (CNS)(2).(-), Cl-2.(-), and Tl2oxidize DTR in neutral, mildly alkaline, and acidic solutions by an el ectron-transfer process to generate a phenoxyl-type radical transient with peak absorptions observed at 280 and 350 nm and epsilon(350) = 44 00 +/- 300 dm(3) mol(-1) cm(-1). In acidic pH, another transient peak at 310 nm with epsilon(310) = 3500 +/- 150 dm(3) mol(-1) cm(-1) and a pK(a) value of 6.5 is observed due to the simultaneous formation of a radical cation intermediate. The formation rate constant values of the se two transients lie between 5 X 10(8) and 2 x 10(10) dm(3) mol(-1) s (-1) for these secondary oxidizing radicals as well as for the primary .OH/.O- radicals. The .OH radical additionally generates a secondary transient in deoxygenated solutions (k(f) approximate to 1.7 x 10(7) d m(3) mol(-1) s(-1) at pH = 8) with its peak absorption centered at 650 nm, which shows two pK(a) values of 3.9 and 7. It has been assigned t he dimer-type structure arising from the DTR reaction with its hydroxy l radical adduct transient. From the equilibrium studies with dehydroc ysteine (thiyl) as the oxidizing radical, the reduction potential valu e for the couple DTR(+)/DTR has been measured to be +0.78 +/- 0.04 V v s NHE.