Scheme 1. Schematic Free Energy Profile of the Macroscopic Kspin and the Differences in the Low Spin States Revealed by MCD.
The right side includes the porphyrin HOMOs and the iron d-orbitals with ligand field parameters V and Δ indicated. The nIR charge transfer band energies for different CYPs correspond to different axial ligand field strengths in the relative order shown, with CYP2E1 and CYP125A1 having the strongest and weakest axial ligands, respectively. The lack of LFER between Kspin and these ligand field energies suggests a contribution of the high spin states to isoform-dependent differences in Kspin.