Probing the Intrinsic Electronic Structure of the Cubane [4Fe−4S] Cluster:  Nature's Favorite Cluster for Electron Transfer and Storage

Abstract

The cubane [4Fe-4S] is the most common multinuclear metal center in nature for electron transfer and storage. Using electrospray, we produced a series of gaseous doubly charged cubane-type complexes, [Fe4S4L4]2- (L = -SC2H5, -SH, -Cl, -Br, -I) and the Se-analogues [Fe4Se4L4]2- (L = -SC2H5, -Cl), and probed their electronic structures with photoelectron spectroscopy and density functional calculations. The photoelectron spectral features are similar among all the seven species investigated, revealing a weak threshold feature due to the minority spins on the Fe centers and confirming the low-spin two-layer model for the [4Fe-4S](2+) core and its "inverted level scheme". The measured adiabatic detachment energies, which are sensitive to the terminal ligand substitution, provide the intrinsic oxidation potentials of the [Fe4S4L4]2- complexes. The calculations revealed a simple correlation between the electron donor property of the terminal thiolate as well as the bridging sulfide with the variation of the intrinsic redox potentials. Our data provide intrinsic electronic structure information of the [4Fe-4S] cluster and the molecular basis for understanding the protein and solvent effects on the redox properties of the [4Fe-4S] active sites.

Electronic structure and fragmentation properties of [Fe4S4(SEt)4−x(SSEt)x]2−

“…Since these species are all doubly charged anions, the mass difference for the peaks is 32 u, which could be due to either addition of an O 2 molecule or addition of an S atom to the precursor ion. , which has been reported previously [16]. The PES spectra of the new species exhibited similar spectral patterns as the parent cubane complex.…”

“…2) suggests that the coordination of the -SSEt ligand does not have a significant effect on the electronic structure of the [4Fe-4S] cluster core. As reported previously for the PES spectrum of [Fe 4 S 4 (SEt) 4 ] 2− [16], feature X is assigned to the highest occupied molecular orbital (HOMO) of the cubane with two degenerate minority 3d electrons and feature A is assigned to HOMO-1, which is mainly from the bridging sulfur ligand [16,17]. Features B and C are assigned to the -SEt and -SSEt ligand orbitals, consistent with the fact that their binding energies do not change with x.…”

“…Details of the experimental method have been given elsewhere [15][16][17]. Briefly, solution samples were sprayed from a syringe and the produced anions were guided into the QIT, where they were accumulated for 0.1 s before being extracted into the TOF tube for mass analyses.…”

“…The technique developed in our laboratory that combines ESI and photoelectron spectroscopy (PES) provides an excellent platform for investigation of the electronic structures of molecules in the gas phase [15]. Using this technique, we have been able to transport a variety of synthetic Fe-S analogue complexes from solution into the gas phase and systematically investigated their electronic properties [16][17][18][19][20]. The PES data provided intrinsic oxidation potentials of the complexes revealing the electrostatic nature of the interactions between the cubane core [4Fe-4S] and the terminal ligands [16,17].…”

“…Using this technique, we have been able to transport a variety of synthetic Fe-S analogue complexes from solution into the gas phase and systematically investigated their electronic properties [16][17][18][19][20]. The PES data provided intrinsic oxidation potentials of the complexes revealing the electrostatic nature of the interactions between the cubane core [4Fe-4S] and the terminal ligands [16,17]. Furthermore, we used collision induced dissociation (CID) to probe the fragmentation properties of the [4Fe-4S] cubane core.…”