Combined quantum chemistry and photoelectron spectroscopy study of the electronic structure and reduction potentials of rubredoxin redox site analogues

Iron-sulfur proteins are an important class of electron carriers in a wide variety of biological reactions. Determining the intrinsic contribution of the metal site to the redox potential is crucial in understanding how the protein environment influences the overall redox properties of the Fe-S proteins. Here we combine density functional theory and coupled cluster methods with photodetachment spectroscopy to study the electronic structures and gas-phase redox potentials of the [Fe(SCH₃)₄]^2-/-/0 and [Fe(SCH₃)₃]^-/0 analogues of the rubredoxin redox site. The calculations show that oxidations of [Fe(SCH₃) ₄]^2- and [Fe(SCH₃)₄]^- involve mainly the Fe 3d and S 3p orbitals, respectively. The calculated adiabatic and vertical detachment energies are in good agreement with the experiment for [Fe(SCH₃)₃]^- and [Fe(SCH ₃)₄]^-. The current results further confirm the "inverted level scheme" for the high-spin [1Fe] systems. The redox couple, [Fe(SCH₃)₄]^-/2-. which is the one found in rubredoxin, but cannot be accessed experimentally in the gas phase, was investigated using a thermodynamic cycle that relates it to the [Fe(SCH ₃)₃]^-/0 couple and the ligand association reaction. [Fe(SCH₃)₃]^0/- + SCH₃ ^- → [Fe(SCH₃)₄4]^-/2-. The calculated reduction energy of [Fe(SCH₃)₄]^- (1.7 eV) compares well with the value (1.6 eV) estimated from the calculated bond energies and the experimental detachment energy of [Fe(SCH ₃)₃]^-. Thus, this thermodynamic cycle method can be used to estimate metal-ligand bonding energies and determine intrinsic reduction potentials from photodetachment experiments when the reduced forms are not stable in the gas phase.