An extrusion strategy for the FeMo cofactor from nitrogenase - Towards synthetic iron-sulfur proteins

Iron-sulfur clusters are ubiquitous in biological systems, facilitating fun ctions such as electron transfer (rubredoxins, ferredoxins, rieske centres) , isomerization (aconitase) and small molecule activation such as dinitroge n reduction (nitrogenases). Of global importance and recently particular in terest, is the iron - sulfur-containing iron-molybdenum cofactor (FeMoco) c luster that achieves the biological reduction of dinitrogen under mild cond itions. This biologically unique cluster has proved difficult to investigat e due to its extreme air sensitivity and the instability of the cluster's s tructural integrity, outside the protective protein matrix. Here, we report a model iron-sulfur cluster (Roussins black salt (NH4)[Fe4S 3(NO)(7)]) that has been used to achieve the first example of a metal clust er (guest) embedded within a pseudo-protein, cyclodextrin (host). The produ ct formed is supramolecular, that is, it contained no covalent bonds and wa s stabilized by predominantly entropy effects. Formation of a 1 : 1 complex between the host and the guest was established for the iron-sulfur cluster with either seven- or eight-membered cyclodextrins (beta- or gamma -cyclod extrin). A range of techniques was used to characterize the new complexes i n both the solid and solution states. Electrospray mass spectra indicated t he presence of parent ions of the host-guest complexes and electrochemistry was also used to define the redox behavior of the complexes. The iron-sulf ur clusters were significantly more stable in the presence of the host cycl odextrin, as revealed by a negative shift for the reduction potential for t he host-guest product. Using the beta -cyclodextrin as host, the reduction potential of the iron-sulfur cluster shifted more negative by 60 mV; the ef fect was even more dramatic for the larger gamma -cyclodextrin where the re duction potential for the cluster was shifted by 90 mV more negative than t he 'unbound' [Fe4S3(NO)(7)](-) cluster. This is the first example of a metal cluster, stabilized as a supramolecula r complex in a 'host' environment outside of a covalently bonded protein ma trix. Creating such stable environments for metal cofactors or clusters tha t otherwise spontaneously degrade or are catalytically inactive outside the protein matrix could have enormous practical value. Specific implications for the development of extrusion methods for FeMoco from nitrogenase are en ormous, with previously difficult, high-energy molecular transformations, s uch as dinitrogen to ammonia, now more realistically accessible.