THEORETICAL AB-INITIO AND SEMIEMPIRICAL STUDIES ON BIOLOGICALLY IMPORTANT DIPYRROLIC AND OLIGOPYRROLIC COMPOUNDS - PYRROMETHENONE AND BILIVERDIN

Comparative ab initio (HF/6-31G and MP2/6-31G*) and semiempirical (AM I and PM3) studies of pyrromethenone as a model of the non-hydrogen bo nded part of biliverdin-related compounds have been performed. The str ucture and the relative energies of the conformational isomers formed by rotation around the single and double C-C bonds of the methine brid ge were calculated. It has been found that the geometry of the pyrrome thenone 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 o f 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 e nergies in a vacuum and in a chloroform solution has been performed on biliverdin-related molecules to estimate the energy and structure eff ects at the rotation around the C-5-C-6 Single bond at the methine bri dge. (C) 1998 Elsevier Science B.V.