Photosensitisation with naphthoquinones and binaphthoquinones: EPR spin trapping and optical studies-formation of semiquinone radical and reactive oxygen species on photoillumination

Photodynamic action of two naphthoquinones, 7-methyljuglone (MJ) and plumbagin (PG) and two binaphthoquinones, isodiospyrin (IDP) and diospyrin (DP) extracted from the stem of Diospyros paniculata and roots and root barks of Diospyros montana, are studied by using optical and EPR spin trapping techniques. Efficiency of singlet oxygen generation (by RNO bleaching and EPR spectroscopy) and rate of superoxide generation (by SOD inhibitable ferricytochrome c reduction and EPR spin trapping assays) are determined. In anaerobic medium semiquinone anion of IDP was produced both in dark and in light. On photoillumination of aerobic solution, superoxide radical was generated by all the four quinones via electron transfer to O2. In aqueous solution, the superoxide radical anion formed, rapidly changes to −OH radical via Fenton’s reaction. These findings reveal that the photosensitisation of naphthoquinones and binaphthoquinones may proceed via Type I and Type II mechanisms.