Photogeneration of the free radicals and singlet oxygen by chrysophanol from rheum

In the current work, chryosphanol (MHAQ) was isolated and purified from rheum, and the photosensitization activities were studied. In nitrogen-saturated DMSO solution, irradiation of chryosphanol (MHAQ) with visible light (>430 nm) produced the semiquinone radical anions (MHAQ^.?), which was intensified significantly by an electron donor NADH, suggesting electron transfer between the excited and ground state photosensitizer molecules. In an air-saturated DMSO solution, superoxide radical anions (O₂ ^.?), trapped by DMPO, were detected. It was proved that O₂ ^.? was not originated from the singlet oxygen (¹O₂) but dependent on DMPO, chrysophanol, oxygen and the irradiation time. The singlet oxygen and^.OH could also be produced by the photosensitization of the photosensitizer. These suggest that chrysophanol possesses the photosensitization activity via Type I mechanism and Type II mechanism. To evaluate the photosensitization activities of MHAQ, emodin and mixed anthraquinone derivatives extracted from rheum, the relative productivities of O₂ ^.? were estimated to be 1.8, 1.1 and 1.0 respectively and the quantum yields for singlet oxygen to be 0.36, 0.53 and 0.14 respectively. Therefore, these low-cost pigments may be potentially used as phototherapeutic medicine for some kind of vascular diseases or photo-activated pesticides.