Concerning the deprotonation of the photooxidized 3-hypericinate ion

By means of AM1 calculations it was established that the radical produced by photooxidation of the 3-hypericinate anion (((3))1(.)) possesses strong acidic properties and is at least as acidic as hypericin itself. Dissociation of ((3))1(.) yields a proton which stems from the 6-peri- and/or 4-bay-hydroxyl group (which seem to have comparable acidities). The peri-hydroxyl groups of ((3))1(.) at positions 1,8, are 13 are significantly less capable of deprotonation. This result sheds new light on the interpretation of a recent experimental finding involving intermolecular proton transfer following excitation of hypericinate and several of its alkyl derivatives. On the basis of observing a gradual change of proton transfer from these derivatives one could conclude that the proton might stem from the bay- and/or the peri-region hydroxyl groups.