REACTIVITY OF SEMIQUINONE RADICAL-ANIONS OF HYDROXYPERYLENEQUINONE WITH OXYGEN

Hypocrellin B (HB) could be regarded as a 4,9-perylenequinone derivati ve bearing two hydroxy groups at the 3- and 10-positions peri to two c arbonyl groups. The product of demethylation of the 11-methoxy of HE, HB(11-OH), bears three hydroxy groups at the 3-, 10- and 11-positions. Illumination of HB(11-OH) in the presence of an electron donor led to accumulation of the semiquinone radical anion HB(.-)(11-OH). The ESR hyperfine structure was recorded and the UV-visible spectrum was inves tigated. HB.-(11-OH) reacted with oxygen with generation of the conjug ated mono base HB(11-O-) and hydrogen peroxide. The effect of the posi tion of the hydroxy groups on the reactivity of the semiquinone radica l anion with oxygen was investigated by redox potential, acid dissocia tion constant and NMR measurements. It was indicated that the acidity of the 11-hydroxy group in HB(11-OH) was stronger than that of the gro ups at 3- and 10-positions. The reaction of HB.-(11-OH) with oxygen yi elds at first the original quinone HB(11-OH) and O-2(-.), and then HB( 11-OH) transfers the phenolic hydroxy proton at the 11-position to O-2 (.-) to yield the conjugated base HB(11-O-) and HO2. which disproporti onates rapidly to H2O2 and O-2.