Solvent Tuning of Properties of Iron-Sulfur Clusters in Proteins

Proteins containing Fe4S4 iron-sulfur clusters are ubiquitous in nature and catalyze one-electron transfer processes. These proteins have evolved into two classes that have large differences in their electrochemical potentials: high potential iron-sulfur proteins (HiPIPs) and bacterial ferredoxins (Fds). The role of the surrounding protein environment in tuning the redox potential of these iron sulfur clusters has been a persistent puzzle in biological electron transfer [1]. Although HiPIPs and Fds have the same iron sulfur structural motif – a cubane-type structure – (Figure 1), there are large differences in their electrochemical potentials. HiPIPs react oxidatively at physiological potentials, while Fds are reduced. Recently, sulfur K-edge x-ray absorption spectroscopy (XAS; measured at SSRL beam line 6-2) has been used to uncover the substantial influence of hydration on this variation in reactivity in a collaborative effort led by Stanford Chemistry and Photon Science researchers Edward I. Solomon, Keith O. Hodgson and Britt Hedman [2].