Photodynamic action of damnacanthal and nordamnacanthal

Photodynamic properties of two anthraquinones, damnacanthal (DAM) and nordamnacanthal (NDAM) are studied. Photogeneration of singlet oxygen is monitored by both optical, and EPR methods. In comparison with rose bengal (Φ1O2 for RB=0.76), singlet oxygen generating efficiencies of DAM and NDAM are determined to be 0.29 and 0.10, respectively. Rate of RNO bleaching is found to be retarded by specific 1O2 quenchers such as DABCO and NaN3, confirming the involvement of 1O2 as an active intermediate. Photolysis of DAM and NDAM in DMSO, in the presence of spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) generates 12-line EPR spectra, characteristic of O2− adduct. Photogeneration of O2− is also monitored by optical spectroscopy using SOD inhibitable cytochrome c reduction assay. Our results indicate that DAM possesses high ability to generate reactive oxygen species. Both Type I and Type II paths are involved in the photosensitisation of DAM as well as NDAM. The quantum mechanically calculated lowest unoccupied molecular orbital (LUMO) energies of DAM and NDAM are correlated with the experimental redox potential.