A comparison of the excited-state processes of nearly symmetrical perylene quinones: hypocrellin A and hypomycin B

The excited-state photophysics of two naturally occurring nearly symmetrical perylene quinones are discussed: hypocrellin A and hypomycin B. Hypocrellin A has a hydroxyl group peri to a carbonyl group on either end of its long molecular axis in addition to a hydroxyl group on its seven-membered ring. On the other hand, hypomycin B is unique among this class of known naturally occurring perylene quinones in that it possesses only one hydroxyl group, which is peri to a carbonyl group. These quinones are investigated in different nonionic micellar environments. For hypocrellin A and hypomycin B, a micelle concentration 10 times in excess of that used for hypericin in a previous study, i.e. 100 times the critical micelle concentration, must be employed to obviate aggregation. Under such conditions, the pKa of the peri hydroxyl groups of hypocrellin A have been determined to be 8.9. The pKa of the protonated carbonyl groups could not be measured. A comparable value is estimated for hypomycin B. The differing solubilities and behaviors of hypericin and hypocrellin in micellar environments are briefly discussed in the context of their biological activity. The excited-state processes in hypocrellin A and hypomycin B are compared on a time scale of several hundreds of picoseconds. No deuterium isotope effect is observed for hypomycin B. This result is discussed in the light of the previous assignment of the primary photoprocess in hypocrellin A to hydrogen atom transfer.