Photochemical reactions of hypericin with alcohols and albumins. Kinetics and mechanism of the primary processes

The photochemical reactions of the hypericin sodium salt with alcohols and albumins yielding the corresponding semiquinone radical were studied by means of microsecond and nanosecond pulse photolysis. The decay of the semiquinone radical follows the second-and first-order rate law for alcohol solutions and the NaHY-albumin system, respectively. The lifetime and the yield of the semiquinone radical decrease in the presence of oxygen. In the oxygen-saturated solutions, the lifetime of the semiquinone radical increases with an increase in the albumin concentration in the solution from 1 to 50 mg/ml. This effect is caused by a decrease in the reactivity of the semiquinone radical toward oxygen due to the steric hindrance created by albumin and is not associated with an increase in the solvent viscosity.