Photosensitisation and photoinduced DNA cleavage by four naturally occurring anthraquinones

Photosensitising properties of four naturally occurring anthraquinones, viz., erythroglaucin (ERG), teloschistin (TEL), 1-hydroxy-2-methylanthraquinone (HYQ) and 1-methoxy-2-hydroxyanthraquinone (MEQ) are studied. Photogeneration of singlet oxygen was monitored by both optical RNO (para-nitrosodimethylaniline) bleaching and electron paramagnetic resonance spectroscopy (EPR)-2,2,6,6-tetramethyl piperidinol (TEMPL) methods. In comparison with rose bengal (RB), singlet oxygen generating efficiencies of ERG, TEL, HYQ and MEQ are determined to be 0.10, 0.14, 0.02 and 0.13, respectively. The formation of O2− was monitored by optical spectroscopy using SOD inhibitable cytochrome c reduction assay. Photolysis of ERG and MEQ in DMSO, in the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) generated a multiline EPR spectrum characteristic of mixture of O2−, OH and OCH3 adducts. Both electron transfer (Type I) and energy transfer (Type II) paths are involved in the photosensitisation of ERG, TEL, HYQ and MEQ. Quantum efficiencies of the quinones exhibit correlation with their half-wave potentials. The photoinduced DNA cleavage by quinones was examined with covalently closed circular plasmid DNA, in vitro. Observed DNA photo strand scission correlates with the reactive oxygen species (ROS) generating efficiencies of the quinones.