Studies on the photoinduced sulfonation of hypocrellins

Hypocrellin A (HA) and hypocrellin B (HB) are efficient phototherapeutic agents. Irradiation of HB with visible light in the presence of sodium sulfite in water-pyridine or water-N,N-dimethylformamide (1:1, v/v) solution gives hypocrellin B-5-sulfonic acid and hypocrellin B-5,8-disulfonic acid. HA reacts similarly under the same conditions. Electron spin resonance and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin trapping studies indicate that this photoreaction is initiated by the photoinduced electron transfer between HB and sodium sulfite to produce the semiquinone radical anion of HB (HB^.?) and the sulfur trioxide radical anion (·SO₃^-). The intermediates (HB^.? and 5-SO₃^?-HB^.?) produced from the photolysis process have also been observed in absorption spectra. The attack of ·SO₃^? on HB at the 5 and/or 8 positions to form the sulfonated products has been verified by quenching experiments. The effects of air and pH on the sulfonation reaction have been investigated. On the basis of the experimental evidence, the reaction pathway for the photoreaction is proposed.