Quantum–Chemical Calculations of the Spatial Structure of Bilirubin Molecule Fragments

Abstract

Using the semiempirical methods MINDO/3, MNDO, AM1, and PM3 and the molecular mechanics (MM) method, we have calculated the geometrical structure of eight bilirubin molecule–fragments. For a number of molecules, calculations have also been made by the ab initio method. Effects arising as a result of successive complication of the molecular structure have been analyzed. The electronic spectra of the investigated molecules have been calculated by the CNDO/S and INDO/S methods. It is shown that the semiempirical quantum–chemical methods underestimate the contribution of conjugation effects to the geometrical structure of molecules, whereas the MM method overestimates it. Wave–like instabilities towards nonplanar distortions have been revealed in the five–membered rings A and D of the investigated molecules.