CONCERNING THE STRUCTURAL COMPLEXITY OF HYPERICIN - THE RELATIVE STABILITIES OF SOME NEUTRAL AND DEPROTONATED TAUTOMERS

The structural complexity of hypericin was investigated with respect t o the three most relevant tautomers and their bay-deprotonation produc ts using nb initio calculations at the 6-31G level. The 1,7- and 1,6-t automers were found to be less stable than the 7,14-tautomer by 45.3 a nd 83.0 kJ/mol. Upon deprotonation, the same order of relative stabili ties was found to be retained. The small differences of absolute stabi lities thus obtained were judged to be too small to account for change s of the tautomeric equilibria upon deprotonation. The dipole moments were also found to be similar for these tautomers, making shifts in ta utomeric equilibria induced by solvent/solute interactions and pH chan ges improbable. The geometrical features calculated for the seven spec ies are discussed with respect to torsional deformation and hydrogen b onding.