One-electron reduction potentials of substituted nitroimidazoles measured by pulse radiolysis

Abstract

The differences between the one-electron reduction potentials at pH 7 (E¹₇) of a quinone couple (Q/Q^–), and seventeen nitroimidazole couples (S/S^–) were estimated for S = substituted 2-, 4-, and 5-nitroimidazoles, by measuring the equilibrium constant K_c of the reaction S^–+ Q ⇌ S + Q^–. The radical-ions S^– and Q^– were generated by pulse radiolysis, and their concentrations measured spectrophotometrically after the equilibrium was attained (typically 1–50 µs after the pulse). Estimated values of E¹₇(S/S^–) range from –243 mV for S = 5-formyl-1-methyl-2-nitroimidazole; –398 mV for 1-(2-hydroxyethyl)-2-nitroimidazole; –486 mV for 1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole to ⩽–527 mV for 4-nitroimidazole (all ± 10 mV). In the absence of other factors, 2-nitroimidazoles have more positive E¹₇ values than the 5-nitro-analogues, and 4-nitroimidazoles are still weaker oxidants. Substitution with electron-withdrawing groups in the side chain can increase E¹₇ by amounts greater than the difference between 2- and 5-nitroimidazoles. These values are based on E¹₇(Q/Q^–)=–244 ± 7 mV for duroquinone, and –375 ± 8 mV for 9,10-anthraquinone-2-sulphonate. These reference potentials were determined by measuring K_c for one-electron transfer equilibria between the quinones and 1,1′-dibenzyl-4,4′-bipyridylium ion (BV²⁺), assuming E¹₇(BV²⁺/BV⁺)=–354 ± 6 mV. The usefulness of viologens as redox indicators in this type of study is discussed.