Intramolecular H-bonding in the bay region of hypericin: an AM1 study

The semi-empirical quantum chemical method AM1 was employed to study intramolecular H-bonding in the bay region of hypericin (HYP). It was found that three isomers of HYP, which correspond to structures with 0, 1, and 2 H-bonds in the bay region, readily interconvert, and that the isomer with two H-bonds is the predominant species in polar or nonpolar solvents. It was also shown that the formation of intramolecular H-bonding in the bay region facilitates proton dissociation and intramolecular proton transfer, providing an alternative explanation for the experimentally observed transient species produced from HYP