Photosensitization with anthraquinone derivatives: optical and EPR spin trapping studies of photogeneration of reactive oxygen species

Photodynamic properties of three synthetic anthraquinone derivatives are studied. Efficiency of singlet oxygen generation (by N,N-dimethyl-4-nitrosoaniline bleaching assay and electron paramagnetic resonance (EPR)-2,2,6,6-tetramethyl-4-piperidinol assay) and rate of superoxide generation (by superoxide dismutase inhibitable ferricytochrome c reduction assay and EPR spin trapping assay) of 2-(1-hydroxyethyl)quinizarin (HQ), 2-acetylquinizarin (AQ) and quinizarin (QZ) are determined. The singlet oxygen generating efficiency follows the order: HQ>AQ>QZ. Irradiation in the presence of specific 1O2 quenchers such as 1,4-diazabicyclo[2,2,2]-octane and sodium azide lends supportive evidence for the formation of 1O2. Photolysis of HQ in dimethyl sulphoxide in the presence of 5,5-dimethyl-1-pyrroline-N-oxide generates a multiline EPR spectrum characteristic of mixture of spin adducts corresponding to superoxide anion and hydroxyl radical. Electron donors such as reduced nicotinamide adenine dinucleotide (NADH), ethylenediaminetetra acetic acid and diethyltriaminopenta acetic acid are found to enhance the yield of O2− EPR study further reveals that in aqueous solution, the superoxide radical anion formed rapidly transforms to hydroxyl radical. Enzymatic reduction of HQ, AQ and QZ in dark by incubation with NADH-cytochrome c reductase generates O2− and the efficiency follows the order: HQ>AQ>QZ. Cyclic voltammetric studies indicate a correlation between this order and the redox potential of the quinones.