Understanding Rubredoxin Redox Potentials: Role of H-Bonds on Model Complexes

The energetics of redox states in different models of rubredoxin-like iron-sulfur complexes (ISC) was computed using a combination of density functional and electrostatic continuum theories. In agreement with experiment, the calculated redox potential for the small ISC model [Fe(SCH2CH3)4](1-,2-) in acetonitrile was -813 mV [Galstyan, A. S.; Knapp, E. W. J. Comput. Chem. 2009, 30, 203-211] as compared to the measured value of -838 mV. Surprisingly the experimental values for rubredoxin (Rd) are much higher ranging between -87 and +39 mV. These large variations in redox potentials of ISC models and ISC in Rd are due to specific conformational symmetries adopted by the ligands due to both the protein environment and type and the number of H-bonds, and the dielectric environment. In a dielectric environment corresponding to proteins (epsilon = 20), the computed ISC redox potentials shift positive by about 64 mV for Fe-S center dot center dot center dot H-N and 95 mV for Fe-S center dot center dot center dot H-O H-bonds, correlating well with data estimated from experiments on ISC proteins. In aqueous solutions (epsilon = 80), a positive shift of 58 mV was computed for Fe-S...H-O H-bonds (using a model with the same ISC conformation as in Rd) in agreement with a measured value for Rd with partially solvent exposed ISC. The latter demonstrates the dependence of the ISC redox potentials on the environment (solvent or protein). For a model whose chemical composition is analog to the relevant part of ISC in a specific Rd, the computed redox potential of the model agrees with the measured value in Rd, This study allows to understand redox potential shifts for small ISC models and ISC in proteins.