Cytochrome P450: an investigation of the Mössbauer spectra of a reaction intermediate and an Fe(IV)[double bond]O model system.

We have carried out a series of density functional theory (DFT) calculations to predict the 57Fe Mössbauer quadrupole splittings (DeltaEQ) and isomer shifts (deltaFe) for an Fe(IV)=O model compound ([Fe(O)(TMC)(NCCH3)](OTf)2, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, OTf = CF3SO3-) as well as a cytochrome P450 reaction intermediate (P450-RI). The DFT predictions on the model compound are in almost exact agreement with experiment. The same DFT methods did not enable the prediction of the experimental DeltaEQ results for P450-RI when using the experimental protein crystal structure and seven different spin/charge/protonation state combinations, but did permit good predictions of both DeltaEQ and deltaFe when using a geometry optimized structure having a porphyrin dianion, a protonated cysteine, and S = 1 (the same as that found from previous ESR studies).