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. Yet the, the low-lying electronic states that are the key to their reactivity and chemical versitility cannot be directly observed.4 way splitting (for single spin state) from HDE model (left inset), versus actual ab-initio spectrum (lowest 150 (S=9) states)  Here, we present the first ever direct quantum calculation of the electronic levels of [2Fe-2S] and [4Fe-4S] clusters free from any model assumptions. Our results highlight the limitations of long-standing models of the electronic structure of these systems. In particular, the widely used Heisenberg-Double-Exchange model completely misses the dense spectrum of the clusters, underestimating the number of low-lying states by 1-2 orders of magnitude. In the [4Fe-4S] cluster, the electronic energy levels of even the same spin states are dense on the scale of thermal and vibrational fluctuations, and this provides a natural explanation for the ubiquity of these clusters in nature for catalyzing reactions. Our calculations further give unprecedented insight into the failure of spectroscopic models, and highlight how such models can be corrected.

Realizing the spectroscopy of iron-sulfur clusters directly from entangled many-particle quantum mechanics

248th National Meeting (2014)

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