Electron Transfer Reactions of Tryptophan and Tyrosine Derivatives

Article abstract of DOI:10.1021/j100400a039

Oxidation of tryptophan, tyrosine, and their derivatives by oxidizing radicals was studied by pulse radiolysis in aqueous solutions at 20 deg C.Rate constants for the oxidation of tryptophan derivatives with .N3 and Br2^-. radicals vary from 8x10⁸ to 4.8 x 10⁹ M^-1 s^-1 and oxidation goes to completion; no pH dependence was observed.Oxidation rate constants for tyrosine derivatives increase upon deprotonation of the phenolic residue at higher pH.Redox potential for the indolyl and phenoxyl radicals were derived from the measured equilibrium constants by using p-methoxiphenol (E_7.5 = 0.6 and E₁₃ = 0.4 V), bisulfite (E3 = 0.84 V), and guanosine (E₇ = 0.91 V) redox couples as reference systems.The redox potential of the tryptophyl radical was measured by pulse radiolysis and laser photolysis and found, by both techniques, to be E =0.64 V at pH 7.Redox potentials of tryptophan derivatives were found to be dependent on the nature of the side chain possibly due to interaction of the side chain with the nitrogen atom in the pirolle ring.Redox potentials of tyrosine derivatives were found to be independent of the nature of the side chain and higher than the redox potentials of triptophan derivatives.E = 0.85 V and E₁₃ = 0.65 V were measured for the tyrosine/phenoxylradical redox couple at pH 7 and 13, respectively .Electron transfer from tyrosine to tryptophyl radical was found to be slow in neutral media, k = 5x10⁵-1.3 x 10⁶ M^-1 s^-1, and is suggested to proceed via multiple steps, one of which is proton transfer from tyrosine to tryptophyl radical folowed by electron transfer.