The participation of a tryptophan residue in the binding of ferric iron to pyrocatechase

The fast reaction technique of pulse radiolysis was used to produce free radicals in aqueous solution from alcohols, deoxyribose, cytosine, uracil, thymine, dihydrothymine and histidine. The electron transfer reactions from these radicals to p-benzoquinone was observed from the formation kinetics of the semiquinone anion ·BQ^? at 430 nm and the efficiency was found to be as high as 90% or more, with k～5×10⁹ M^?1sec^?1. In acid or neutral solutions in the presence of oxygen the peroxy radicals ·O₂RH formed do not essentially transfer an electron to BQ, and the efficiency is <10%. The significance of these results in the fixation of radiation damage in photobiology and radiation biology are indicated. The reactions of the superoxide ·O₂^? radical with BQ are also presented and discussed.