Insight into Environmental Effects on Bonding and Redox Properties of [4Fe-4S] Clusters in Proteins

摘要

A classical question about oxidation-reduction potentials in proteins is why the [4Fe-4S] clusters in the high potential iron-sulfur proteins (HiPIPs) have a 1-/2- reduction at 100 to 450 mV vs NHE, while the ferredoxins (Fds) have a 2-/3-reduction at -100 to -645 mV. The differences are generally attributed to hydrogen bonding and electrostatic effects from the surrounding protein and solvent. Recent ligand K-edge X-ray absorption spectroscopy (XAS) experiments find large differences in the Fe-S covalency between HiPIPs and Fds apparently due to hydration and attribute the redox potential differences to electronic effects of water hydrogen bonds based on a correlation between electrochemical redox potentials and metal-ligand bond covalencies in iron-sulfur complexes. However, our combined density functional theory (DFT) and photoelectron spectroscopy (PES) studies indicate hydrogen bond effects are actually primarily electrostatic since the electronic structure and bonding of the clusters are not significantly affected. Here, we present a new mechanism for redox tuning, namely the ligand conformation, which reconciles the seemingly conflicting experimental results.