SPECTROSCOPIC STUDY ON THE PHOTOREDUCTION OF HYPOCRELLIN-A - GENERATION OF SEMIQUINONE RADICAL-ANION AND HYDROQUINONE

Hypocrellin A (HA) is an efficient phototherapeutic agent. Illuminatio n of HA in dimethylsulphoxide (DMSO) or DMSO-buffer (1:1 by volume, pH > 6.7) generated a strong electron paramagnetic resonance (EPR) signa l. This EPR signal was intensified in the presence of reductants. The EPR spectrum obtained was assigned to the semiquinone radical anion of HA (HA(.-)) based on a series of experimental results. Decay of HA(.- ), attributed to a radical-radical reaction, follows second-order kine tics. In acidic DMSO-buffer (1:1 by volume) solution, no EPR signal of HA(.-) was detected in the absence or presence of reductant. This was explained by a fast disproportionation of radicals, facilitated by pr otonation of radicals. The spectrophotometric measurements indicated t hat on illumination HA was directly reduced to its two-electron reduct ion product, i.e. hydroquinone, in acidic solution. The absorption max imum of the hydroquinone of HA which is at 496 nm at pH 5.8 shifts bat hochromically with increase in pH of the medium. In DMSO or DMSO-buffe r (1:1 by volume, pH > 6.7) solutions the semiquinone radical anion of HA was also observed spectrophotometrically. The absorption maximum o f HA(.-) is at around 628 nm. Strong intramolecular hydrogen bonding w as considered to exist in the chromophore of HA(.-).