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The structure and spin-states of some Fe(III) mimics of nitrile hydratase, studied by DFT and ONIOM(DFT:PM3) calculations

Density functional methods have been successful in studying the electronic structure of molecular systems containing transition metal atoms, such as metalloenzymes. However, the treatment of large systems is still very computationally demanding, and is definitely not practical for many systems of interest, due to their size. In this paper we assess the use of these methods both alone, and when combined with the semi-empirical PM3 method within an ONIOM scheme, for determining the structure and spin-dependent energetics of a series of Fe(III) model complexes that have been synthesized to mimic the active site of nitrile hydratase, a metalloenzyme that catalyses the conversion of a wide variety of nitriles to their corresponding amides. We have found that geometry optimizations employing B3LYP generally give a good description of the structure and spin-states of these complexes and that when combined with PM3 in the framework of ONIOM, the multilevel method also performs well for some of them, suggesting that the ONIOM(B3LYP:PM3) approach offers the possibility for improvement in future calibration studies. We also find that DFT and ONIOM(DFT:PM3) calculations at the experimental geometries using the BP86, PW91PW91 and B3LYP functionals can also describe the spin-state energetics of these model complexes, with DFT performing the best.