Cytochrome P450: An Investigation of the Mössbauer Spectra of a Reaction Intermediate and an Fe(IV)=O Model System

We have carried out a series of density functional theory (DFT) calculations to predict the ⁵⁷Fe Mössbauer quadrupole splittings (ΔE_Q) and isomer shifts (δ_Fe) 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 ΔE_Q 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 ΔE_Q and δ_Fe 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).