In a number of biological processes, iron-sulfur clumps play a vital role, where they act as cofactors of enzymes. Research published in applied chemistry now shows that cubic clusters can withstand unusual binding states. This study shows that the cluster is resistant to multiple binding between iron and nitrogen – a structural motif that may be involved in biological nitrogen fixation.
Aggregates made up of iron and sulfur atoms are essential cofactors for a number of enzymes, especially in biological processes involving electron transfer. For example, nitrogen-fixing bacteria use iron-sulfur clumps to convert nitrogen in the air into useful nitrogen compounds. To understand this important biological process, scientists are exploring possible bonding relationships between nitrogen and iron atoms in such groups.
Daniel Suess and his colleagues at the Massachusetts Institute of Technology in Cambridge, USA, have now investigated the cluster’s ability to form unusual bonds between iron and nitrogen. A double bond, which is part of a chemical group called an imide, can play a role in nitrogen fixation.
To build the imide, the team started by producing a cube-shaped cluster of iron-sulfur. The eight corners of the cube are occupied by an alternation of atoms of iron and sulfur; three of the iron atoms are protected by chemical species that act as ligands. These ligands do not bind directly to atoms, but simply protect them instead. The unshielded iron atom remaining in the cluster was bound to a replaceable chloride ligand. Careful selection of reagents allowed the team to exchange the chloride ion and then, by oxidation with a reagent containing nitrogen, the delicate double bond between the single iron atom and the nitrogen atom – and therefore the imide group – was formed. .
The researchers expected that the iron-nitrogen double bond could greatly distort the structure of the cluster. Instead, to their surprise, they only observed minor structural changes. The authors’ spectroscopic studies explain this finding: the electron-rich imide pushes the electron density of neighboring sulfur and iron atoms back, and all of these minor effects are what allow the cluster to host the imide bond. “These results demonstrate a dynamic interaction between iron-nitrogen, iron-sulfur and iron-iron bonds,” the authors state.
The new cluster linked to imido was able to cleave weak carbon-hydrogen bonds from organic reagents. The authors intend to use these studies as a starting point for further study of the reactivity of iron-sulfur clusters bound to imide. “This highlights the promise of harnessing the synergy between the structural robustness and electronic flexibility of these fundamental cofactors,” says Suess.
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