A theoretical study for the linear free energy relationship of CH bond activation and the role of the axial ligand in cytochrome P450 model complexes

Hydrogen abstraction is essential for CH bond activation by Compound I in cytochrome P450 and is influenced by various factors, including spin states, bond dissociation energies of the CH and FeOH bonds, axial ligands, and quantum mechanical tunneling. The role of axial ligands has been extensively studied, but it is still not fully understood. To explore their role, we used density functional theory calculations to determine whether a linear free energy relationship is established for the hydrogen transfer reaction, according to changes in axial ligands. The B3LYP* functional exhibits a strong linear correlation, but the slopes are inconsistent with the characteristics of the transition state. Natural bond orbital analysis reveals no direct orbital interaction between axial ligands and the reaction center of hydrogen transfer. The electron-donating orbitals of the axial ligands weaken the FeO bond, lowering the energy barrier, but they do not directly participate in the intrinsic hydrogen transfer. During the reaction, the FeO bond length increases significantly before the hydrogen transfer itself, generating an asynchronous shift in the bond orders, and most of the activation energy is used for the increase in the FeO bond rather than the hydrogen transfer itself. This study may explain why there is no apparent correlation between the rate constants and the FeO bond strength.