Roles of oxygen and photoinduced acidification in the light-dependent antiviral activity of hypocrellin A

Hypocrellin A displays photoinduced Antiviral activity, in particular against the human immunodeficiency virus (HIV), as does its counterpart, hypericin. Although hypocrellin A, like hypericin, executes an excited-state intramolecular proton transfer, it differs from hypericin in two important ways. Unlike hypericin, hypocrellin A absolutely requires oxygen for its Antiviral activity. Also, whereas we have previously demonstrated that hypericin functions as a light-induced proton source, we do not observe that hypocrellin A acidifies its surrounding medium in the presence of LIGHT. These results are discussed in the context of the ground- and excited-state photophysics of hypericin and its mechanisms of photoinduced virucidal activity.