CHARACTERISTICS OF THE REACTION BETWEEN SEMIQUINONE RADICAL-ANION OF HYPOCRELLIN-A AND OXYGEN IN APROTIC MEDIA

Hypocrellin A (HA), a hydroxylperylenequinone derivative, is an effici ent phototherapeutic agent. The reactivity of the semiquinone radical anion of HA (HA(.-)) with O-2 in aprotic medium has been studied. Alth ough 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 pr oton transfer between one of the phenolic hydroxyl groups of HA and O- 2(.-) yielding the conjugated base of HA (HA(b)(-)) and hydrogen perox ide. On the basis of the results obtained from spectrophotometric, ele ctron spin resonance spin-trapping and CCl4-inhibiting experiments, a reasonable reaction pathway was proposed for the oxidation of HA(.-) w ith O-2 in aprotic media.