Demonstration of alternative spin states in clusters containing the biologically relevant [Fe4S4]1+ core

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Controlling Substrate Binding to Fe4S4 Clusters through Remote Steric Effects

The extraordinary reactivity exhibited by many Fe-S enzymes is due in large part to the influence of the protein scaffold on substrate binding and activation. In principle, the coordination chemistry of synthetic Fe-S clusters could similarly be controlled through remote steric effects. Toward this end, we report the synthesis of 3:1 site-differentiated [Fe₄S₄] clusters ligated by N -heterocyclic carbene (NHC) ligands with variable steric profiles: IMes (1,3-dimesitylimidazol-2-ylidene) and I ⁱPr^Me (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). Treatment of (IMes)₃Fe₄S₄Cl with NaBAr^F₄ in ethereal solvents (Et₂O and THF) leads to the formation of an ether adduct, [(IMes)₃Fe₄S₄(solv)][BAr^F₄]; solvent can be displaced by addition of ^tBuNC to form the unusual monoisocyanide adduct [(IMes)₃Fe₄S₄(CN ^tBu)][BAr^F₄]. Carrying out the same reactions with the less sterically encumbered cluster (I ⁱPr^Me)₃Fe₄S₄Cl results in more typical reactivity: undesired ligand redistribution to form the homoleptic cluster [(I ⁱPr^Me)₄Fe₄S₄][BAr^F₄] and generation of the triisocyanide adduct [(I ⁱPr^Me)₃Fe₄S₄(CN ^tBu)₃][BAr^F₄]. The increased steric profile of the IMes ligands disfavors ligand redistribution and defines a binding pocket at the apical Fe, thereby enabling the generation of a coordinatively unsaturated and substitutionally labile Fe site. This method of controlling the coordination chemistry at the apical Fe site by modifying the sterics of ligands bound to adjacent Fe sites complements existing strategies for generating site-differentiated Fe-S clusters and provides new opportunities to direct reactivity at cuboidal metalloclusters.

Stabilization of 3:1 site-differentiated cubane-type clusters in the [Fe(4)S(4)](1+) core oxidation state by tertiary phosphine ligation: synthesis, core structural diversity, and S = 1/2 ground states

An extensive series of 3:1 site-differentiated cubane-type clusters [Fe(4)S(4)(PPr(i)(3))(3)L] (L = Cl(-), Br(-), I(-), RO(-), RS(-), RSe(-)) has been prepared in 40-80% yield by two methods: ligand substitution of [Fe(4)S(4)(PPr(i)(3))(4)](1+) in tetrahydrofuran (THF)/acetonitrile by reaction with monoanions, and reductive cleavage of ligand substrates (RSSR, RSeSeR, I(2)) by the all-ferrous clusters [Fe(8)S(8)(PPr(i)(3))(6)]/[Fe(16)S(16)(PPr(i)(3))(8)] in THF. These neutral clusters are stable and do not undergo ligand redistribution reactions involving charged species in benzene and THF solutions. X-ray structural studies confirm the cubane stereochemistry but with substantial and variable distortions of the [Fe(4)S(4)](1+) core from idealized cubic core geometry. Based on Fe-S bond lengths, seven clusters were found to have compressed tetragonal distortions (4 short and 8 long bonds), and the remaining seven display other types of distortions with different combinations of long, short, and intermediate bond lengths. These results further emphasize the facile deformabililty of this core oxidation state previously observed in [Fe(4)S(4)(SR)(4)](3-) clusters. The Fe(2.25+) mean oxidation state was demonstrated from (57)Fe isomer shifts, and the appearance of two quadrupole doublets arises from the spin-coupled |9/2,4,1/2> state. The S = 1/2 ground state was further supported by electron paramagnetic resonance spectra and magnetic susceptibility data.

Investigation of the Unusual Electronic Structure of Pyrococcus furiosus 4Fe Ferredoxin by EPR Spectroscopy of Protein Reduced at Ambient and Cryogenic Temperatures

The hyperthermophilic archaeon Pyrococcus furiosus contains a novel ferredoxin (Pf-Fd) in which, in the native 4Fe form, three of the Fe ions are coordinated to the protein by cysteinyl thiolato ligands, but the fourth Fe is coordinated by an aspartyl carboxylato ligand ([Fe(4)S(4)(cys)(3)(asp)](2)(-)(,3)(-)). Chemical reduction at ambient temperature of the oxidized 4Fe form (Pf-Fd 4Fe-ox, S = 0 ground state, with the cluster core indicated by [Fe(4)S(4)](2+)(ox)) produces a reduced 4Fe form (Pf-Fd 4Fe-red, with the cluster core indicated by [Fe(4)S(4)](+)(red)). Pf-Fd 4Fe-red, [Fe(4)S(4)](+)(red) core, in frozen solution exhibits S = (1)/(2) and (3)/(2) electronic states that are not in thermal equilibrium. The two spin states thus represent alternate ground states of the reduced cluster (cluster cores indicated by [Fe(4)S(4)](+)(red1) and [Fe(4)S(4)](+)(red2), respectively), rather than a ground and excited spin state. Low-temperature (77 K) reduction of 4Fe-ox in frozen solution by gamma-irradiation produces in high yield the reduced state of the cluster that is trapped in the structure of the oxidized parent cluster, and thus has a cluster core denoted by [Fe(4)S(4)](+)(ox). The [Fe(4)S(4)](+)(ox) form also exhibits non thermally converting S = (3)/(2) and (1)/(2) components in the same proportion as seen for [Fe(4)S(4)](+)(red). The EPR signal of the S = (3)/(2) component that results from cryoreduction ([Fe(4)S(4)](+)(ox2)) is indistinguishable, within experimental variability, from that seen in the ambient-temperature, chemically reduced protein ([Fe(4)S(4)](+)(red2)), and the signals of the two S = (1)/(2) components ([Fe(4)S(4)](+)(ox1) and [Fe(4)S(4)](+)(red1), respectively) closely resemble each other, although they are not identical. Previous NMR studies at ambient temperature showed evidence for only one species in fluid solution for both Pf-Fd 4Fe-ox and 4Fe-red. Taken together, the NMR and EPR results indicate that fluid solutions of either oxidized or reduced Pf-Fd contain only one conformer, but that frozen solutions of each contain two distinct conformers, with each one of the pair of oxidized protein forms having a corresponding reduced form. A shift in the coordination mode of the aspartyl carboxylato ligand is proposed to account for this conformational flexibility.

Variable temperature magnetic circular dichroism studies of reduced nitrogenase iron proteins and [4Fe-4S]+ synthetic analog clusters

Variable temperature magnetic circular dichroism (VTMCD) and EPR spectroscopies have been used to investigate the ground and excited-state properties of [4Fe-4S]+ clusters in Mo- and V-nitrogenase Fe-proteins from Azotobacter vinelandii and two synthetic analog clusters, [Fe4S4(SEt)4]3- and [Fe4S4(SC6H11)4]3-. The results indicate similar [4Fe-4S]+ clusters with analogous S = 1/2 and S = 3/2 ground states in both Fe-proteins. However, the Fe-proteins do differ in terms of the medium effects on the S = 1/2 and S = 3/2 spin mixtures in frozen solution. By utilizing medium effects in both Fe-proteins, the VTMCD characteristics of both the S = 1/2 and S = 3/2 forms of the [4Fe-4S]+ have been determined. Together with the VTMCD studies of [Fe4S4(SEt)4]3- and [Fe4S4(SC6H11)4]3-, which are shown to be predominantly S = 1/2 and 3/2, respectively, in frozen DMF/toluene solutions, the results demonstrate that the form of the VTMCD spectra provides a means of identifying and distinguishing S = 1/2 and S = 3/2 [4Fe-4S]+ clusters. Ground state zero-field splitting parameters for the S = 3/2 clusters are determined for both Fe-proteins. In addition to spin state heterogeneity, samples of the Mo-nitrogenase Fe-protein in the presence of 50% (v/v) ethylene glycol were found to exhibit heterogeneity in the S = 1/2 resonance. A rapidly relaxing axial resonance, g perpendicular = 1.94 and g parallel = 1.82, was observed in addition to the characteristic rhombic resonance, g = 2.05, 1.94 and 1.87. The origin of the heterogeneity exhibited by [4Fe-4S]+ clusters in frozen solution is discussed in light of these results.