Post-Hartree-Fock study on biologically important di- and tripyrrolic compounds - precursors to the active site of phytochrome

Comparative ab initio (DFT/B3LYP and MP2) studies of pyrromethene and pyrromethanone molecules have been carried out using the standard 6-31G* basis set. The molecular structure, total and relative energies (corrected by ZPE evaluated from the optimized at the HFi6-31G* level geometry) for the conformational isomers formed by the rotation around the single bond of the methine bridge were calculated. Furthermore, the transition structures were located and the rotational barriers between the conformers obtained. A perfect correspondence between the DFT and MP2 data has been shown for both geometric and energetic data. The tripyrroIic molecule has been introduced as a more realistic model of the hydrogen-bonded part of the biliverdin-related molecules. The geometry and relative stability of the conformational isomers arising from rotation around the single bonds of the methine bridges were correlated according to the phenomena of the conformational flexibility of biliverdin-related molecules. The conformational isomers lying in the energy interval of ca. 20 kJ mol(-1) have been determined. The observed difference in the stability patterns of the ZZas and ZZsa conformers of the tripyrrolic molecules has been explained based on their different ability to form intermolecular hydrogen bonds with the H-19 and H-20 atoms of the tripyrrolic molecule. (C) 1998 Elsevier Science B.V. All rights reserved.