On the ground state energy hypersurface of blepharismins and oxyblepharismins

AM1 calculations on blepharismins and oxyblepharismins, which are related p hotosensory pigments of certain protists, revealed that the accessory subst ituents of the natural pigments do not lead to a change of the tautomerism and conformational states of the fundamental systems. The valence tautomeri sm possible in principle for the blepharismins yielding a cycloheptatriene- norcaradiene system was found to reside completely on the side of the cyclo heptatriene. With respect to proton tautomerism, the strong predominance of the mese-type 7,15- and 7,14-dioxo tautomers was established in both serie s. Whereas the conformation of the fundamental condensed aromatic ring syst em of the oxyblepharismins remains comparable to that of hypericin, the con formational situation of the blepharismins was found to be unique with the phenyl group in an ende-position and dihedral twisting at the unperturbed b ay-site only.