title: Fast proton-coupled electron transfer observed for a high-fidelity structural and functional [2Fe-2S] Rieske model.
abstract: 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 5(x-) 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 5H(2-) with TEMPO. The thermodynamics of the PCET square scheme for 5(x-) have been determined, and three species (diferric 5(2-), protonated diferric 5H(-), and mixed-valent 5(3-)) have been characterized by X-ray diffraction. pKa values for 5H(-) and 5H(2-) differ by about 4 units, and the reduction potential of 5H(-) is shifted anodically by about +230 mV compared to that of 5(2-). While the N-H bond dissociation free energy of 5H(2-) (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 5H(2-) with biomimetic {N2/S2} ligation (k = (9.5 ± 1.2) × 10(4) 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.
authors: Institute of Inorganic Chemistry, Georg-August-University Göttingen , Tammannstrasse 4, D-37077 Göttingen, Germany.
authors: Antonia
authors: A
authors: Albers
authors: Serhiy
authors: S
authors: Demeshko
authors: Sebastian
authors: S
authors: Dechert
authors: Caroline T
authors: CT
authors: Saouma
authors: James M
authors: JM
authors: Mayer
authors: Franc
authors: F
authors: Meyer
mesh_terms: D040761:Biomimetic Materials / Q000737:chemistry*; D003497:Cyclic N-Oxides / Q000737:chemistry; D004579:Electron Transport; D004583:Electrons; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011522:Protons; D013816:Thermodynamics; D014961:X-Ray Diffraction
chemical_list: D003497:Cyclic N-Oxides; D005290:Ferric Compounds; D005296:Ferrous Compounds; D007506:Iron-Sulfur Proteins; D011522:Protons; D006639:Histidine; C003959:TEMPO