Effect of Axial Ligand, Spin State, and Hydrogen Bonding on the Inner-Sphere Reorganization Energies of Functional Models of Cytochrome P450

Using a combination of self-assembly and synthesis, bioinspired electrodes having dilute iron porphyrin active sites bound to axial thiolate and imidazole axial ligands are created atop self-assembled monolayers (SAMs). Resonance Raman data indicate that a picket fence architecture results in a high-spin (HS) ground state (GS) in these complexes and a hydrogen-bonding triazole architecture results in a low-spin (LS) ground state. The reorganization energies (lambda) of these thiolate- and imidazole-bound iron porphyrin sites for both HS and LS states are experimentally determined. The lambda of 5C HS imidazole and thiolate-bound iron porphyrin active sites are 10-16 kJ/mol, which are lower than their 6C LS counterparts. Density functional theory (DFT) calculations reproduce these data and indicate that the presence of significant electronic relaxation from the ligand system lowers the geometric relaxation and results in very low lambda in these SC HS active sites. These calculations indicate that loss of one-half a pi bond during redox in a LS thiolate bound active site is responsible for its higher lambda relative to a sigma-donor ligand-like irnidazole. Hydrogen bonding to the axial ligand leads to a significant increase in lambda irrespective of the spin state of the iron center. The results suggest that while the hydrogen bonding to the thiolate in the SC HS thiolate bound active site of cytochrome P450 (cyp4S0) shifts the potential up, resulting in a negative Delta G, it also increases lambda resulting in an overall low barrier for the electron transfer process.