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/S-2} 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 Delta G degrees(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), Delta H double dagger = 17.6 +/- 3.0 kJ mol(-1), Delta S double dagger = -143 +/- 11 J mol(-1) K-1, and Delta G double dagger = 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.