Spectral features of the ferrous-CO complex in cytochrome P450: a revisit using TDDFT calculations.

There are different views in the literature regarding how to interpret the observed spectral features of the ferrous-CO complexes in cytochrome P450 enzymes (P450s). In this work, we applied density functional theory (DFT) and time-dependent DFT (TDDFT) calculations at the B3LYP-D3BJ/def2-TZVP level with a CPCM correction to the ferrous-CO models of P450s as well as of proteins that contain a histidine-ligated heme. Our results support the notion derived from a previously reported iterative extended Hückel calculation that the involvement of the sulfur lone-pair orbital (S(n z )) of the axial cysteine ligand in the electronic excitations gives rise to a spectral anomaly. The Q and the shorter-wavelength Soret (B') peaks are primarily due to the electronic transitions from the a 2u - and S(n z )-type molecular orbitals (MOs), generated via an orbital interaction of fragment orbitals, to the near-degenerate e g -type π* MOs, respectively. The transitions from the a 1u -type MO to the e g -type MOs contribute most to the longer wavelength Soret (B) peaks. Both a 2u - and S(n z )-type MOs contribute to the B peaks, but the contribution of the latter is greater. When the axial ligand is histidine, the Q and Soret peaks originate essentially from the excitations from the a 2u - and a 1u -type MOs to the e g -type MOs. The transitions from the b 2u -type MOs to the e g -type MOs play the most significant role in the N peaks of such ferrous-CO complexes. Here, the b 2u -type MOs have a large contribution from the imidazole π orbital.