Electron-transfer reactions of tryptophan and tyrosine derivatives

Oxidation of tryptophan, tyrosine, and derivatives by oxidizing radicals was studied by pulse radiolysis in aqueous solutions at 20 /sup 0/C. Rate constants for the oxidation of tryptophan derivatives with .N/sub 3/ and Br/sub 2//sup -/. radicals vary from 8 x 10/sup 8/ to 4.8 and 10/sup 9/ M/sup -1/ s/sup -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 potentials for the indolyl and phenoxyl radicals were derived from the measured equilibrium constants by using p-methoxyphenol (E/sub 7.5/ = 0.6 and E/sub 13/ = 0.4 V), bisulfite (E/sub 3/ = 0.84 V), and guanosine (E/sub 7/ = 0.91 V) redox couples as reference systems. Redox potentials of tryptophan derivatives were found to be in dependent on the nature of the side chain and higher than the redox potentials of tryptophan derivatives. Electron transfer from tyrosine to tryptophyl radical was found to be slow in neutral media and is suggested to proceed via multiple steps, one of which is proton transfer from tyrosine to tryptophyl radical followed by electron transfer. 26 references, 2 figures, 4 tables.