LIGHT-INDUCED ACIDIFICATION BY THE ANTIVIRAL AGENT HYPERICIN

The naturally occurring polycyclic quinone hypericin possesses light-i nduced antiviral activity against the human immunodeficiency virus (HI V) and other closely related enveloped lentiviruses such as equine inf ectious anemia virus (EIAV). We have previously argued that hypericin undergoes a fast proton transfer reaction in its singlet state (J. Phy s. Chem. 1994, 98, 5784). We have also presented evidence that the lig ht-induced antiviral activity of hypericin does not depend upon the fo rmation of singlet oxygen (Bioorg. Med. Chem. Lett. 1994, 4, 1339). It is demonstrated here that steady-state illumination of a solution con taining hypericin effects a pH drop. When hypericin and an indicator d ye, 3-hexadecanoyl-7-hydroxycoumarin, are both imbedded in vesicles, h ypericin transfers a proton to the indicator within a time commensurat e to its triplet lifetime. Proton transfer to the indicator is not obs erved when the indicator is protonated or when the system is oxygenate d. Since hypericin is known to form triplets and to generate singlet o xygen with high efficiency, this latter result is taken to confirm tri plet hypericin as a source, but not necessarily the only source, of pr otons.