EPR studies of trapped singlet oxygen (1O2) generated during photoirradiation of hypocrellin A.

Hypocrellin A, a peryloquinone derivative, has recently been isolated from the sacs of the fungus Hypocrella bambusae. This pigment, in combination with phototherapy, has been used in human medicine to cure various skin diseases. The generation of singlet oxygen during photoirradiation of Hypocrellin A (HA) was detected as an oxidation product of a sterically hindered amine (tetramethylpiperidine oxide; TEMPO) by electron paramagnetic resonance (EPR) spectroscopic techniques. Azide inhibited the EPR signal intensity in a dose-dependent manner with a quenching rate constant of 3.86 x 10(8) M-1s-1 in ethanol. Deuterated solvents, known to increase the lifetime of singlet oxygen, augmented the EPR signal intensity. The rate of production of singlet oxygen was dependent not only upon the concentration of HA and the time of irradiation but also on the oxygen content of the reaction mixture. The hyperfine splitting constant (aN = 16.3 G) and g-value (g = 2.0056) of the photoproduct of TEMP-singlet oxygen and TEMPO were found to be identical. This indicates that the nitroxide species detected by EPR spectroscopy generated by reacting TEMP with photogenerated 1O2 is TEMPO. The rate constant (kT) for the reaction of singlet oxygen with TEMP to form TEMPO radical was found to be 5.3 x 10(5) M-1s-1.