Glycoconjugated hypocrellin: photosensitized generation of free radicals (O2•−, √OH, and GHB•−) and singlet oxygen (1O2)

To improve water solubility and specific affinity for malignant tumors, glycoconjugated hypocrellin B (GHB) has been synthesized. Illumination of deoxygenated DMSO solution containing GHB generates a strong electron paramagnetic resonance (EPR) signal. The EPR signal is assigned to the semiquinone anion radical of GHB (GHB•−) based on a series of experimental results. Spectrophotometric measurements show that the absorption bands at 645 nm and 502 nm (pH 8.0) or 505 nm (pH 11.0) arise from the semiquinone anion radical (GHB•−) and hydroquinone (GHBH2) of GHB, respectively. GHBH2 is readily formed via the decay of GHB•− in water-contained solution. The increase of pH value of the reaction media promotes this process. When oxygen is present, superoxide anion radical (O2•−) is formed, via the electron transfer from GHB•−, the precursor, to ground state molecular oxygen. Hydroxyl radical can be readily detected by DMPO spin trapping when aerobic aqueous solution containing GHB is irradiated. As compared with the parent compound, hypocrellin B (HB), the efficiency of O2•− and √OH generation by GHB photosensitization is enhanced significantly. Singlet oxygen (1O2) can be produced via the energy transfer from triplet GHB to ground state oxygen molecules, with a decreased quantum yield, i.e., 0.19. These findings suggest that the new GHB possesses an enhanced type I process and a decreased type II process as compared with hypocrellin B.