Main content area

Fast Proton-Coupled Electron Transfer Observed for a High-Fidelity Structural and Functional [2Fe–2S] Rieske Model

Albers, Antonia, Demeshko, Serhiy, Dechert, Sebastian, Saouma, Caroline T., Mayer, James M., Meyer, Franc
Journal of the American Chemical Society 2014 v.136 no.10 pp. 3946-3954
Gibbs free energy, X-ray diffraction, biomimetics, chemical bonding, dissociation, electron transfer, histidine, iron, ligands, models, oxidation, photosynthesis
Rieske cofactors have a [2Fe–2S] cluster with unique {His₂Cys₂} ligation and distinct Fe subsites. The histidine ligands are functionally relevant, since they allow for coupling of electron and proton transfer (PCET) during quinol oxidation in respiratory and photosynthetic ET chains. Here we present the highest fidelity synthetic analogue for the Rieske [2Fe–2S] cluster reported so far. This synthetic analogue 5ˣ– emulates the heteroleptic {His₂Cys₂} ligation of the [2Fe–2S] core, and it also serves as a functional model that undergoes fast concerted proton and electron transfer (CPET) upon reaction of the mixed-valent (ferrous/ferric) protonated 5H²– with TEMPO. The thermodynamics of the PCET square scheme for 5ˣ– have been determined, and three species (diferric 5²–, protonated diferric 5H–, and mixed-valent 5³–) have been characterized by X-ray diffraction. pKₐ values for 5H– and 5H²– differ by about 4 units, and the reduction potential of 5H– is shifted anodically by about +230 mV compared to that of 5²–. While the N–H bond dissociation free energy of 5H²– (60.2 ± 0.5 kcal mol–¹) and the free energy, ΔG°CPET, of its reaction with TEMPO (−6.3 kcal mol–¹) are similar to values recently reported for a homoleptic {N₂/N₂}-coordinated [2Fe–2S] cluster, CPET is significantly faster for 5H²– with biomimetic {N₂/S₂} ligation (k = (9.5 ± 1.2) × 10⁴ M–¹ s–¹, ΔH‡ = 8.7 ± 1.0 kJ mol–¹, ΔS‡ = −120 ± 40 J mol–¹ K–¹, and ΔG‡ = 43.8 ± 0.3 kJ mol–¹ at 293 K). These parameters, and the comparison with homoleptic analogues, provide important information and new perspectives for the mechanistic understanding of the biological Rieske cofactor.