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

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• 作者：Antonia Albers ; Serhiy Demeshko ; Sebastian Dechert ; Caroline T. Saouma ; James M. Mayer ; Franc Meyer
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：March 12, 2014
• 年：2014
• 卷：136
• 期：10
• 页码：3946-3954
• 全文大小：411K
• 年卷期：v.136,no.10(March 12, 2014)
• ISSN：1520-5126

文摘

Rieske cofactors have a [2Fe鈥?S] 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鈥?S] cluster reported so far. This synthetic analogue 5^{x鈥?/sup> emulates the heteroleptic {His₂Cys₂} ligation of the [2Fe鈥?S] 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鈥?/sup> with TEMPO. The thermodynamics of the PCET square scheme for 5x鈥?/sup> have been determined, and three species (diferric 52鈥?/sup>, protonated diferric 5H鈥?/sup>, and mixed-valent 53鈥?/sup>) have been characterized by X-ray diffraction. pK_a values for 5H鈥?/sup> and 5H2鈥?/sup> differ by about 4 units, and the reduction potential of 5H鈥?/sup> is shifted anodically by about +230 mV compared to that of 52鈥?/sup>. While the N鈥揌 bond dissociation free energy of 5H2鈥?/sup> (60.2 卤 0.5 kcal mol鈥?) and the free energy, 螖G掳_CPET, of its reaction with TEMPO (鈭?.3 kcal mol鈥?) are similar to values recently reported for a homoleptic {N₂/N₂}-coordinated [2Fe鈥?S] cluster, CPET is significantly faster for 5H2鈥?/sup> with biomimetic {N₂/S₂} ligation (k = (9.5 卤 1.2) 脳 104 M鈥? s鈥?, 螖H鈥?/sup> = 8.7 卤 1.0 kJ mol鈥?, 螖S鈥?/sup> = 鈭?20 卤 40 J mol鈥? K鈥?, and 螖G鈥?/sup> = 43.8 卤 0.3 kJ mol鈥? 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.}