SPECTROSCOPIC STUDIES OF A WATER-SOLUBLE DERIVATIVE OF HYPOCRELLIN-A AND ITS ONE-ELECTRON AND 2-ELECTRON REDUCTION PRODUCTS

In this study the spectroscopic characteristics of a water-soluble der ivative of hypocrellin A (HA), 14-dehydroxy-15-deacetyl-hypocrellin A- 13-sulfonate(13-SO3Na-DDHA),and its one- and two-electron reduction pr oducts have been investigated. From the changes in absorbance with pH it was observed that the two phenolic hydroxy groups at C-3 and C-10 p ositions of 13-SO3Na-DDHA or HA dissociated stepwise with increase of pH values. The pKa values for 13-SO3Na-DDHA and HA were determined usi ng an effective method established in this study. Attempts were also m ade to use absorption and ESR spectroscopies to study the photoreducti on of 13-SO3Na-DDHA. It was found that 13-SO3Na-DDHA was directly redu ced to its two-electron reduction product in buffered aqueous solution (pH 7.7). However, in DMF-buffer (1:1/ v:v, pH 7.7), it proceeded wit h one-electron reduction to generate its semiquinone radical anions. T he semiquinone radical anions decayed according to second-order kineti cs, indicating that the termination went through protonation and dispr oportionation of the radical anions. Despite of four phenolic hydroxy groups in the molecule of the two-electron reduction product only two hydroxy groups seem to dissociate with increase of pH. This reflects t hat strong intramolecular hydrogen bonds are probably formed in the mo no- and dianionic forms of the two-electron reduction product.