MMX and AM1 methods were employed to calculate heat of formation of hypericin(HYP) in the process of structure change.It was found that (i) Hydroxyls of HYP at position 1,2 could form intramolecular hydrogen bond (IHB) through crossing a barrier of about 20kJ/mol and rotating around C--O bond, and the hydrogen bond energy was estimated to be approximately 10kJ/mol.(ii) By rotating other four hydroxyls, the conformation of HYP could be changed in similar processes. The barriers were about 26kJ/mol, and the corresponding hydrogen bond energies were about 20kJ/mol. (iii) Isomers of HYP generated through intramolecular hydrogen transfer were not thermallystable. In addition, the influences of forming IHB between hydroxyls at position 1,2 on photosensitivity of HYP were investigated as well。

Key words: CONFORMATION, HYDROGEN BONDS, ANTITUMOR DRUGS, MOLECULAR MECHANICS