Theoretical ab initio and semiempirical studies on biologically important di- and oligopyrrolic compounds. Pyrromethenone and biliverdin

Comparative ab initio (HF/6-31G* and MP2/6-31G*) and semiempirical (AMI and PM3) studies of pyrromethenone as a model of the non-hydrogen bonded part of biliverdin-related compounds have been performed. The structure and the relative energies of the conformational isomers formed by rotation around the single and double C-C bonds of the methine bridge were calculated. It has been found that the geometry of the pyrromethenone molecule calculated at the MP2, HF and AMI levels corresponds well with the X-ray data of 3,4-dimethyl-2,2' -pyrromethen-5(1 H)-one. The reliability of the AM 1 method for the calculations of biliverdin-related molecules has been proven. Calculations of the ground state of octamethylbilindione and biliverdin IX alpha molecules as models of open-chain tetrapyrrolic chromophores have been carried out at the AM1 level. An analysis of the rotation barrier heights and the relative energies in a vacuum and in a chloroform solution has been performed on biliverdin-related molecules to estimate the energy and structure effects at the rotation around the C-5-C-6 Single bond at the methine bridge. (C) 1998 Elsevier Science B.V.