ELECTRON-TRANSFER INTERACTION BETWEEN HYPOCRELLIN-A AND BIOLOGICAL SUBSTRATES AND QUANTITATIVE-ANALYSIS OF SUPEROXIDE ANION-RADICALS

Hypocrellin A (HA), a hydroxyperylenequinone derivative, is an efficie nt phototherapeutic agent. Laser hash photolysis is used to produce an d investigate the properties of the lowest excited triplet state (T-1) and semiquinone radical anion of hypocrellin A (HA(.-)) at room tempe rature. In the presence of some biological substrates (such as ascorbi c acid and cysteine), the formation and decay of HA radical anion at d ifferent pH, attributed to the electron transfer between triplet HA an d substrates, are observed. Meanwhile, the superoxide radical anion (O -2(.-)) production by photoactivated HA in the presence of biological substrates is examined by using the nitro blue tetrazolium (NBT) trapp ing method in order to elucidate the mechanism of formation of superox ide and to quantify this formation. Specifically, production of O-2(.- ) is demonstrated unequivocally to be by reaction with the superoxide dismutase. The rate of reduction of NBT is dependent not only on the c oncentration of NBT but also on the pH of the system. The relative qua ntum yield of superoxide anion radicals increases considerably in alka line solution: phi(O2)(.-) = 6.97 x 10(-3) at pH 9.0 as compared with phi(O2)(.-) = 2.49 x 10(-4) at pH 5.8 in the presence of ascorbic acid . Based on the experimental results, electron transfer (Type I) mechan ism may play a hitherto unrecognized role in the photodynamic interact ion between HA and some biological substrates.