L. Gorb et al., Post-Hartree-Fock study on biologically important di- and tripyrrolic compounds - precursors to the active site of phytochrome, THEOCHEM, 454(2-3), 1998, pp. 217-227
Comparative ab initio (DFT/B3LYP and MP2) studies of pyrromethene and pyrro
methanone molecules have been carried out using the standard 6-31G* basis s
et. The molecular structure, total and relative energies (corrected by ZPE
evaluated from the optimized at the HFi6-31G* level geometry) for the confo
rmational isomers formed by the rotation around the single bond of the meth
ine bridge were calculated. Furthermore, the transition structures were loc
ated and the rotational barriers between the conformers obtained. A perfect
correspondence between the DFT and MP2 data has been shown for both geomet
ric and energetic data. The tripyrroIic molecule has been introduced as a m
ore realistic model of the hydrogen-bonded part of the biliverdin-related m
olecules. 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 energ
y interval of ca. 20 kJ mol(-1) have been determined. The observed differen
ce in the stability patterns of the ZZas and ZZsa conformers of the tripyrr
olic molecules has been explained based on their different ability to form
intermolecular hydrogen bonds with the H-19 and H-20 atoms of the tripyrrol
ic molecule. (C) 1998 Elsevier Science B.V. All rights reserved.