Understanding Rubredoxin Redox Sites by Density Functional Theory Studies of Analogues

Determining the redox energetics of redox site analogues of metalloproteins is essential in unraveling the various contributions to electron transfer properties of these proteins. Since studies of the [4Fe–4S] analogues show that the energies are dependent on the ligand dihedral angles, broken symmetry density functional theory (BS-DFT) with the B3LYP functional and double-ζ basis sets calculations of optimized geometries and electron detachment energies of [1Fe] rubredoxin analogues are compared to crystal structures and gas-phase photoelectron spectroscopy data, respectively, for [Fe­(SCH₃)₄]^0/1–/2–, [Fe­(S₂-o-xyl)₂]^0/1–/2–, and Na⁺[Fe­(S₂-o-xyl)₂]^1–/2– in different conformations. In particular, the study of Na⁺[Fe­(S₂-o-xyl)₂]^1–/2– is the only direct comparison of calculated and experimental gas phase detachment energies for the 1–/2– couple found in the rubredoxins. These results show that variations in the inner sphere energetics by up to ∼0.4 eV can be caused by differences in the ligand dihedral angles in either or both redox states. Moreover, these results indicate that the protein stabilizes the conformation that favors reduction. In addition, the free energies and reorganization energies of oxidation and reduction as well as electrostatic potential charges are calculated, which can be used as estimates in continuum electrostatic calculations of electron transfer properties of [1Fe] proteins.