Photoinduced proton transfer from the first triplet state of hypericin in micellar dispersion.

Hypericin, a natural pigment, possesses light-induced antitumor and antivir al activities and it was suggested that these activities may be related, to some extent, to its ability to undergo a proton transfer reaction in its e xcited states. Thus a laser study of the prototropic equilibrium of triplet hypericin has been performed in a buffered micellar dispersion in order to provide a simplified model of the cellular membrane within which hypericin is thought to partition. Flash photolysis experiments on the microsecond t imescale indicated that hypericin excited in the triplet state is a proton donor, but acid-base equilibrium cannot be completely established during th e lifetime of the excited state. A dynamic analysis was therefore carried o ut in order to obtain the correct value: pK(a)*=7.35. The acidity constant in the triplet state markedly increases compared to that in the ground stat e and the proton dissociation rate is k(d)=1800 s(-1). At pH greater than o r equal to 10 a photoinduced proton transfer between the triplet hypericin and its dianion was observed with a rate constant k(t)=3x10(9) M-1 s(-1); t hus after excitation, a proton could be transferred very quickly from hyper icin to a neighbouring proton acceptor. These results show that a proton tr ansfer reaction in the triplet state has to be considered as one of the pos sible processes responsible for the photosensitizing properties of hyperici n.