Connecting [4Fe–4S] Clusters and Hemes – Towards Modeling the Active Site of Sulfite Reductase



In this paper, we present a new design for biologically inspired models for the active site of assimilatory sulfite and nitrite reductases (aSIR and aNIR), which consists of a siroheme that is directly linked to a [4Fe–4S] cubane cluster. The individual components used here to construct this model are a site-differentiated [4Fe–4S] cluster, a bifunctional bridging ligand, and a metalloporphyrin. We have prepared two new site-differentiated clusters, [Fe4S4(TriS)(SPy)] and [Fe4S4(TriS)(SEtIm)], which contain pyridine and imidazole linkers for the binding to a metalloporphyrin, and characterized these compounds, using UV/Vis, IR, and 1H-NMR spectroscopy, cyclic voltammetry (CV), and mass spectrometry. Titration experiments where then performed by using [Zn(TPP)] (TPP2– = meso-tetraphenylporphyrin dianion) and corresponding fluorinated derivatives to find the best [4Fe–4S]–heme combination for an optimal binding of the two components in solution. Excitingly, our results demonstrate the formation of the desired [4Fe–4S]–heme catalytic arrays in solution with high specificity. The best combination of cubane cluster and metalloporphyrin for future catalyst development corresponds to the complex (Bu4N)2[M(To-F2PP)–{Fe4S4(TriS)(SEtIm)}] {To-F2PP2– = meso-tetra(ortho-difluorophenyl)porphyrin dianion}. The binding between these components with M = Zn2+ was further confirmed by CV. Thus, we have created a new type of biologically inspired model system for the aSIR and aNIR active site that leads to a robust attachment of the individual components in solution.