Exploiting Molecular Symmetry to Quantitatively Map the Excited-State Landscape of Iron-Sulfur Clusters

Cuboidal [Fe₄S₄] clusters are ubiquitous cofactors in biological redox chemistry. In the [Fe₄S₄]¹⁺ state, pairwise spin coupling gives rise to six arrangements of the Fe valences ("valence isomers") among the four Fe centers. Because of the magnetic complexity of these systems, it has been challenging to understand how a protein's active site dictates both the arrangement of the valences in the ground state as well as the population of excited-state valence isomers. Here, we show that the ground-state valence isomer landscape can be simplified from a six-level system in an asymmetric protein environment to a two-level system by studying the problem in synthetic [Fe₄S₄]¹⁺ clusters with solution C₃ symmetry. This simplification allows for the energy differences between valence isomers to be quantified (in some cases with a resolution of <0.1 kcal/mol) by simultaneously fitting the VT NMR and solution magnetic moment data. Using this fitting protocol, we map the excited-state landscape for a range of clusters of the form [(SIMes)₃Fe₄S₄-X/L], (SIMes = 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene; n = 0 for anionic, X-type ligands and n = +1 for neutral, L-type ligands) and find that a single ligand substitution can alter the relative ground-state energies of valence isomers by at least 10³ cm^-1. On this basis, we suggest that one result of "non-canonical" amino acid ligation in Fe-S proteins is the redistribution of the valence electrons in the manifold of thermally populated excited states.