Post-Hartree–Fock study on biologically important di- and tripyrrolic compounds—precursors to the active site of phytochrome Leonid Gorb a,c , * , Anatoli Korkin b ,1 , Jerzy Leszczynski a a Department of Chemistry, Jackson State University, Jackson, MS 39217, USA b Quantum Theory Project, 362 Williamson Hall, PO Box 118435, Gainesville, FL 32611, USA c Institute of Water and Colloid Chemistry, National Academy of Sciences of Ukraine, 252680, Kiev-142, Vernadsky av. 42, Ukraine Abstract 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 HF/6-31G* 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 tripyrrolic 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.  1998 Elsevier Science B.V. All rights reserved. Keywords: Pyrromethene; Pyrromethanone; Tripyrrolic compound; Relative stability; DFT and MPT Optimization of Geometry 1. Introduction In the previous papers [1,2], we carried out a detailed quantum chemical investigation of the geo- metry and relative stabilities for the different con- formations of polypyrrolic molecules. All these molecules are precursors to the active sites of phyto- chrome [3–10] which is the photoreceptor of plants. Due to the large size of the studied system in addition to the ab initio Hartree –Fock and MP2 calculations of dipyrrolic molecules (pyrromethene and pyrrometha- none) we have also performed semi-empirical calcu- lations (PM3 and AM1) for the simplest model of plants photoreceptore active sites tetrapyrrolic octamethyl-bilindione, as well as pyrromethene and pyrromethanone molecules. We have found that the geometrical parameters are described relatively well at the AM1, HF and MP2 levels. However, a descrip- tion of the energetic parameters is more complex. We have noticed the best correspondence between the empirical estimation of the value of the hydrogen Journal of Molecular Structure (Theochem) 454 (1998) 217–227 0166-1280/98/$ - see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0166-1280(98)00292-9 * Corresponding author. 1 Present address: Motorola, MD M350, Predictive Engineering Lab, Arizona Technical Laboratory, Semiconductor Products Sec- tor, 2200 W. Broadway Rd., Mesa, AZ 85202, USA