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

Rieske cofactors have a [2Fe-2S] cluster with unique {His(2)Cys(2)} 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 {His(2)Cys(2)} 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. pK(a) 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, Delta G(CPET)(0), of its reaction with TEMPO (-6.3 kcal mol(-1)) are similar to values recently reported for a homoleptic {N-2/N-2}-coordinated [2Fe-2S] cluster, CPET is significantly faster for 5H2- with biomimetic {N-2/S-2} ligation (k = (9.5 +/- 1.2) X 10(4) M-1 s(-1), Delta H-double dagger = 8.7 +/- 1.0 kJ mol(-1), Delta S-double dagger = -120 +/- 40 J mol(-1) K-1, and Delta G(double dagger) = 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.