Calculations on the structure and spectral properties of cytochrome c551 using DFT and ONIOM methods

The active site geometry of cytochrome (Cyt) c(551) and its mutated form containing Fe(II) and Fe(III) ions have been calculated using density functional theory (DFT)-based Becke's three-parameter hybrid exchange and Lee-Yang-Parr correlation (B3LYP) method. In addition, calculations have also been carried out using hybrid meta DFT-based M06 functional. The effect of the protein milieu on the active site geometry has also been probed using two-layer via our own N-layered integrated molecular orbital + molecular mechanics (ONIOM) method. Evidence from the calculations reveal that the active site geometry is not significantly affected by the oxidation state of metal ion. The difference in the geometry of the active site and that of the same with the entire protein environment is only minimal, which shows that the protein milieu does not influence the structure of the active site. The calculated electronic transition energies from the time-dependent DFT (TDDFT) calculations are in close agreement with the experimental values. Although there are no significant variations in the active site geometry upon oxidation, the changes in the electronic transition energies have been attributed to the reduction in the overlap of metal ion with the ligand orbitals. In addition, it is found that mutation does not influence the active site geometry and the electronic transition energies. Nevertheless, mutation leads to the formation of more compact structure than the native Cyt c(551).