Model of the MitoNEET [2Fe-2S] Cluster Shows Proton Coupled Electron Transfer

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe-2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S₂} ligation which could be structurally characterized in its diferric form, 5^2-. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme's unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model 5^2- and a related homoleptic {SN/SN} capped [2Fe-2S] cluster 4^2- are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the N-H bond dissociation free energy (BDFE) of 5H^2- (230 ± 4 kJ mol^-1) and the free energy ΔG°_PCET for the reaction with TEMPO (-48.4 kJ mol^-1) are very similar to values for the homoleptic cluster 4H^2- (232 ± 4 kJ mol^-1, -46.3 kJ mol^-1) the latter is found to react significantly faster than the mitoNEET model (data for 5H^2-: k = 135 ± 27 M^-1 s^-1, ΔH^‡ = 17.6 ± 3.0 kJ mol^-1, ΔS^‡ = -143 ± 11 J mol^-1 K^-1, and ΔG^‡ = 59.8 kJ mol^-1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process.