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With the technique of pulse radiolysis we have measured the redox midpoint potentials of the tryptophan side chain neutral indolyl radical (1.05 +/- 0.01 V vs NHE, pH 7.0 and 25 degrees C) and the tyrosine side chain neutral phenoxy radical (0.94 +/- 0.01 vs NHE, pH 7.0 and 25 degrees C). These potentials were obtained by using a variety of inorganic reference compounds in both kinetic and equilibrium protocols. We compare these results with others already in the literature, and we also present data useful in establishing a pulse radiolysis redox reference scale over the range 0.42-1.28 V.

The reduction potentials of some purine and pyrimidine bases and the guanine nucleoside and nucleotide at pH values between 7 and 13 were investigated using the techniques of cyclic voltammetry, differential pulse polarography, and pulse radiolysis. The results clearly show that the lowest reduction potentials, in volts vs NHE, at pH 7 are those of xanthine, 0.88 V, and 1-methylguanine, 1.06 V (NHE). The extrapolated value of guanine is ca. 1.0 V. We also studied the one-electron oxidation reaction of the azide radical with the above compounds. We monitored their transient absorption spectra and determined their formation secondorder rate constants. The decay kinetics of these radicals was followed. Radicals derived from the bases decayed via a radical-radical mechanism with a second-order rate constant. However, the guanine nucleosides and nucleotide radicals have shown at all pHs two consecutive processes (first order followed by a second order). The first-order reaction is pH dependent. Formation of a new transient was observed at pH g9 for guanosine, pH g11 for 2′-deoxyguanosine, and at pH 13 for 5′-GMP. The observed new transient spectra were similar to that observed for the oxidized guanine radical. Therefore, we suggest that in these oxidized guanine nucleosides and nucleotide the oxidized guanine radical has been released. As previously suggested our results imply that the radiation-induced base release from nucleosides in alkaline solution is due to the preferred reaction of the nucleoside sugar with O •-Using our results, those of Steenken et al.

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Since its introduction, pulse radiolysis has been an important technique for examining the properties of organic and inorganic radicals, and for enumerating those reactions responsible for cellular damage by ionizing radiation. Biochemists, and biophysicists outside the area of radiation biology appear, perhaps for historical reasons, to have an incomplete appreciation of the technique's potential. Protein chemists in particular, have been only dimly aware of the numerous reports of, and the significant results obtained from pulse radiolysis studies of proteins. Our purpose here is to bring some of these results together in order to emphasize the power and usefulness of pulse radiolysis experiments both for elucidating enzyme reaction mechanisms, and for gaining information on the structure of proteins in aqueous solutions. Reviews containing related, or in part the same material to be covered here have appeared previously; for example, Land (1970), Adams et al. (1972a), Shafferman & Stein (1975), Adams & Wardman (1977). This review updates these earlier works, but more importantly approaches the topic of protein pulse radiolysis with a different emphasis.

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