Toward Identification of the Compound I Reactive Intermediate in Cytochrome P450 Chemistry: A QM/MM Study of Its EPR and Mössbauer Parameters

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Abstract
Quantum mechanical/molecular mechanical (QM/MM) methods have been used in conjunction with density functional theory (DFT) and correlated ab initio methods to predict the electron paramagnetic resonance (EPR) and Mössbauer (MB) properties of Compound I in P450cam. For calibration purposes, a small Fe(IV)−oxo complex [Fe(O)(NH3)4(H2O)]2+ was studied. The 3A2 and 5A1 states (in C4v symmetry) are found to be within 0.1−0.2 eV. The large zero-field splitting (ZFS) of the (FeO)2+ unit in the 3A2 state arises from spin−orbit coupling with the low-lying quintet and singlet states. The intrinsic g-anisotropy is very small. The spectroscopic properties of the model complex [Fe(O)(TMC)(CH3CN)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are well reproduced by theory. In the model complexes [Fe(O)(TMP)(X)]+ (TMP = tetramesitylporphyrin, X = nothing or H2O) the computations again account for the observed spectroscopic properties and predict that the coupling of the 5A1 state of the (FeO)2+ unit to the porphyrin radical leads to a low-lying sextet/quartet manifold ∼12 kcal/mol above the quartet ground state. The calculations on cytochrome P450cam, with and without the simulation of the protein environment by point charges, predict a small antiferromagnetic coupling (J ≈ −13 to −16 cm-1; ĤHDvV = − 2JS⃗AS⃗B) and a large ZFS > 15 cm-1 (with E/D ≈ 1/3) which will compete with the exchange coupling. This leads to three Kramers doublets of mixed multiplicity which are all populated at room temperature and may therefore contribute to the observed reactivity. The MB and ligand hyperfine couplings (14N, 1H) are fairly sensitive to the protein environment which controls the spin density distribution between the porphyrin ring and the axial cysteinate ligand.

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