Characteristics of the reaction between semiquinone radical anion of hypocrellin A and oxygen in aprotic media

Hypocrellin A (HA), a hydroxylperylenequinone derivative, is an efficient phototherapeutic agent. The reactivity of the semiquinone radical anion of HA (HA(.-)) with O-2 in aprotic medium has been studied. Although the standard redox potential of the HA(.-)-HA couple (-360 mV vs. a normal hydrogen electrode (NHE)) is less negative than that of the O-2(.-)-O-2 couple (-500 mV vs. NHE), it appears that HA(.-) can still be oxidized with oxygen. The electron transfer is accompanied by a proton transfer between one of the phenolic hydroxyl groups of HA and O-2(.-) yielding the conjugated base of HA (HA(b)(-)) and hydrogen peroxide. On the basis of the results obtained from spectrophotometric, electron spin resonance spin-trapping and CCl4-inhibiting experiments, a reasonable reaction pathway was proposed for the oxidation of HA(.-) with O-2 in aprotic media.