Electron transfer interaction between hypocrellin A and biological substrates and quantitative analysis of superoxide anion radicals

Abstract

Hypocrellin A (HA), a hydroxyperylenequinone derivative, is an efficient phototherapeutic agent. Laser flash photolysis is used to produce and investigate the properties of the lowest excited triplet state (T₁) and semiquinone radical anion of hypocrellin A (HA˙^–) at room temperature. In the presence of some biological substrates (such as ascorbic acid and cysteine), the formation and decay of HA radical anion at different pH, attributed to the electron transfer between triplet HA and 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) trapping method in order to elucidate the mechanism of formation of superoxide 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 concentration of NBT but also on the pH of the system. The relative quantum yield of superoxide anion radicals increases considerably in alkaline solution: ϕ_O2;˙^– = 6.97 × 10^–3 at pH 9.0 as compared with ϕ_O2;˙^– = 2.49 × 10^–4 at pH 5.8 in the presence of ascorbic acid. Based on the experimental results, electron transfer (Type I) mechanism may play a hitherto unrecognized role in the photodynamic interaction between HA and some biological substrates.