An ab initio/hybrid (ONIOM) investigation of biliverdin isomers and metal-biliverdin analogue complexes

Ab initio HF and DFT (BLYP, B3LYP) methods have been applied to the computation of three isomeric forms, diketo, ketoenol and dienol of biliverdin. The fully optimized structures so obtained are compared and the most stable isomer (diketo form) agrees well with the structure determined from X-ray crystallographic studies. Results of the calculations based on three methods, HF, BLYP and B3LYP have been compared with the experimental parameters. B3LYP method results are determined to be closest to the experimental results. Octaethyl dimethyl amido biliverdin as a free ligand while complexed with zinc(II) which forms a metal-biliverdin analogue complex Zn-II(OEBNMe2) has been fully optimized using PM3 and B3LYP methods. Comparison of calculated parameters for this complex with X-ray results has proved the capability of the ab initio method in accurately predicting fine details of the complex. Results obtained by employing a hybrid (ONIOM) calculation method on such system for the first time has established the applicability as well as low computational cost of the technique. (C) 2003 Elsevier B.V. All rights reserved.