One- and two-electron reduction of quinizarin and 5-methoxyquinizarin: a pulse radiolysis study

Absorption characteristics of the semiquinone free radicals formed by one-electron reduction of quinizarin (QH2), 5-methoxyquinizarin (MQH2) and quinizarin 2-sulphonate (QSH2) have been studied by pulse radiolysis in a mixed solvent system consisting of water, isopropyl alcohol and acetone. Second-order rate constants have been determined for the reactions of (CH3)2COH with the quinones, of the semiquinones with O2 and of the semiquinones with each other. The one-electron reduction potentials (vs. NHE) are E71 = -269 mV for QH2, -333 mV for MQH2 and -298 mV for QSH2. They vary with pH in accordance with the pKa values of the parent quinones and the semiquinones. The radicals are stable within the approximate pH range 5-11. The stability constant is highest at pH 8.5 (K2 ≅ 0.09) for QH2, at pH ≅ 9.5 for QSH2 (Ks ≅10) and pH ≅ 10.8 for MQH2 (Ks ≅ 4.8), respectively. The one-electron reduction potentials of the semiquinones and the two-electron reduction potentials of the quinones are calculated to be E72 = -188, -192 and -216 mV, and E7m = -229, -263 and -257 mV for QH2, MQH2 and QSH2, respectively. The effect of solvent on the properties of the semiquinones is discussed. (author)