Density functional theory calculations of redox properties of iron–sulphur protein analogues

Abstract

A central issue in understanding redox properties of iron–sulphur (Fe–S) proteins is determining the factors that tune the reduction potentials of the Fe–S clusters. Studies of redox site analogues play an important role, particularly because individual factors can be examined independently of the environment by combining calculations and experiments of carefully designed ligands for the analogues. For iron–sulphur analogues, our study has shown that broken-symmetry density functional theory gives good energetics when the geometry is optimised using B3LYP with a double-ζ basis set with polarisation functions, and the energies of these geometries are calculated using B3LYP with additional diffuse functions added to the sulphurs. A comparison of our calculated energies for redox site analogues in the gas phase against electron detachment energies measured by a combination of electrospray ionisation and photoelectron spectroscopy (EI–PES) by Wang and co-workers has been essential because the comparison is for exactly the same molecule with no approximation for the environment. Overall, the correlation of our B3LYP/ 6-31(++)_SG**//B3LYP/6-31G** detachment energies with EI–PES experiments is excellent for a wide variety of analogues. Moreover, our calculations at this level have provided insight into a wide variety of properties of iron–sulphur proteins.

Keywords:

• density functional theory
• iron–sulphur protein
• reduction potential
• inner-sphere free energy
• reorganisation energy

Acknowledgements

This study was supported by a grant from the National Institutes of Health (GM-45303). The calculations were performed at the EMSL, a national user facility sponsored by the US DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, operated for DOE by Battelle, under the grant GC3565, GC20901 and EMSL38793. Additional computational resources were provided by the William G. McGowan Foundation.