Interaction between hypocrellin A and some biological substrates with emphasis on an electron transfer mechanism

Abstract

Hypocrellin A (HA) is an efficient phototherapeutic agent. Laser flash photolysis was used to produce and investigate the properties of the lowest excited triplet state (T 1) and the semiquinone radical anion of hypocrellin A (HA •−) at room temperature. In the presence of biological substrates, such as ascorbic acid and cysteine, the formation and decay of HA •−, attributed to electron transfer between HA and the substrates, were observed. Production of the superoxide radical anion (O 2 •−) by photoactivated HA in the presence of biological substrates was examined by using the Nitro Blue Tetrazolium (NBT) trapping method in order to elucidate the mechanism of formation of superoxide and to quantify this formation. Specifically, production of O 2 •− was demonstrated unequivocally by reaction with superoxide dismutase. In addition, in weak acidic solution, HA •− could also be produced and observed by laser flash photolysis, but it decays much faster than that in neutral or basic solution. The values of the free energy change for electron transfer between HA* and some biological substrates are less than zero, indicating that this process is permissible thermodynamically. Based on the experimental results, an electron transfer (Type I) mechanism may play a hitherto unrecognized role in the photodynamic interaction between HA and some biological substrates.