Sodium 4,5-dihydroxybenzene-1,3-disulfonate (tiron) has been reported to be an efficient chelator of certain metal ions, and a substrate in several enzyme reactions. Its small size facilitates cell entry and therefore modulates intracellular electron transfer reactions as an antioxidant by scavenging free radicals. Its reduction by electrochemical and enzymatic methods gives identical products; a semiquinone detectable by EPR spectroscopy. In a test of its use as a spin trap, in comparison with DMPO, tiron does not form a molecular spin-adduct but proves more functional as an electron trap. Electron addition to tiron is more facile than reduction of dioxygen as observed by the non-formation of DMPO-OOH spin-adduct in the system XO/HPX/O2/DMPO/tiron. Rather, it is the tiron semiquinone radical which is formed quantitatively with increasing concentration of hypoxanthine independent of oxygen concentration. These results offer explanation for the action of tiron and its suitability for measuring electron release in hypoxic conditions, and also for mitigating redox-induced toxicity in drug regimes by acting as an electron scavenger.

Radiation damage to proteins is a topic of intense interest to those involved in radiation effects in biology, and also to those involved in radiotherapy. Although it has been widely studied, fundamental processes in protein damage are very hard to specify because of the complexity of the final damage products. But the non-invasive technique of electron-spin resonance is ideally suited to the task of detecting and identifying the primary and secondary products as these are expected to contains unpaired electrons (that is, free-radicals) and such species are uniquely detected by this sensitive form of spectroscopy. Our present study shows that a major radical species formed by electron loss in a range of proteins is the backbone amido radical, -N.(CO)-, characterized by hyperfine coupling to one 14N nucleus. These centres are efficiently trapped in proteins at low temperatures. In contrast, the expected backbone electron-capture centres, -NH(CO.-)-, are not readily trapped and electron transfer occurs until the ejected electron is trapped by some electrophilic centre. Such electron mobility was in fact established in our previous work on oxyhaemoglobin (FeO2----FeO2-), superoxide dismutase (Cu(II)----Cu(I] haemocyanin (Cu(II)O2Cu(I)----Cu(I)O2Cu(II] and various proteins containing S-S bonds (-S-S-)----(-S.-S-) (refs 1-4 respectively). This is strongly supported by our observation that such electrons are captured by DNA molecules, giving T.- centres, when nucleohistones are irradiated, and that Fe(CN)3-(6) ions readily scavenge such electrons from proteins which are devoid of highly electrophilic centres.

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