(675b) Chemistry of Iron-Sulfur Containing Enzymes

Metals in enzymes perform remarkable chemistry under ambient pressures and temperatures. Among the most common metal cofactors are the iron-sulfur (FeS) clusters, containing between one and eight Fe atoms bridged by S ligands. FeS clusters perform multiple roles in nature, including redox chemistry, electron transfer, and even oxygen sensing, in processes ranging from nitrogen fixation to photosynthesis and respiration.

FeS cluster have multiple low-lying states with differing electronic and magnetic character, which are the key to their rich chemistry. However, determining the details of these states is a highly non-trivial problem. Traditional approaches like Density Functional Theory (DFT), although cheap and usually quite accurate, fail spectacularly when applied to transition metal containing systems. With recent advances in electronic structure theory, in particular with the advent of Density Matrix Renormalizaton Group (DMRG), we now have a method that is able to correctly describe the challenging chemistry of these enzymes. 

In this work we will summarize the results of our DMRG calculations on Fe₂S₂ and Fe₄S₄ containing model complexes.  Our detailed calculations reveal a dense spectrum of electronic states that intriguingly suggests that these complexes behave more like heterogeneous catalysts than simple molecular catalysts. This has led us to revise the canonical models that are the basis of our understanding of the chemistry of these complexes.