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

Abstract

The differences between the one-electron reduction potentials at pH 7 (E 7 1) 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 E 7 1 (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 E 7 1 values than the 5-nitro-analogues, and 4-nitroimidazoles are still weaker oxidants. Substitution with electron-withdrawing groups in the side chain can increase E 7 1 by amounts greater than the difference between 2- and 5-nitroimidazoles. These values are based on E 7 1 (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 (BV 2+), assuming E 7 1 (BV 2+ /BV +) = -354 ± 6 mV. The usefulness of viologens as redox indicators in this type of study is discussed.