Department of Chemistry , Johns Hopkins University , Baltimore , Maryland 21218 , United States.
J Am Chem Soc. 2019 Aug 14;141(32):12558-12569. doi: 10.1021/jacs.9b01253. Epub 2019 Aug 2.
Secondary coordination sphere interactions are critical in facilitating the formation, stabilization, and enhanced reactivity of high-valent oxidants required for essential biochemical processes. Herein, we compare the C-H bond oxidizing capabilities of spectroscopically characterized synthetic heme iron(IV) oxo complexes, FCmpd-II (F = tetrakis(2,6-difluorophenyl)porphyrinate), and a 2,6-lutidinium triflate (LutH) Lewis acid adduct involving ferryl O-atom hydrogen-bonding, FCmpd-II(LutH). Second-order rate constants utilizing C-H and C-D substrates were obtained by UV-vis spectroscopic monitoring, while products were characterized and quantified by EPR spectroscopy and gas chromatography (GC). With xanthene, FCmpd-II(LutH) reacts 40 times faster ( = 14.2 M s; -90 °C) than does FCmpd-II, giving bixanthene plus xanthone and the heme product [FFeOH]. For substrates with greater C-H bond dissociation energies (BDEs) FCmpd-II(LutH) reacts with the second order rate constants (9,10-dihydroanthracene; DHA) = 0.485 M s and (fluorene) = 0.102 M s (-90 °C); by contrast, FCmpd-II is unreactive toward these substrates. For xanthene vs xanthene-(), large, nonclassical deuterium kinetic isotope effects are roughly estimated for both FCmpd-II and FCmpd-II(LutH). The deuterated H-bonded analog, FCmpd-II(LutD), was also prepared; for the reaction with DHA, an inverse KIE (compared to FCmpd-II(LutH)) was observed. This work originates/inaugurates experimental investigation of the reactivity of authentic H-bonded heme-based Fe═O compounds, critically establishing the importance of oxo H-bonding (or protonation) in heme complexes and enzyme active sites.
次级配位球相互作用对于促进必需生化过程所需的高价氧化物种的形成、稳定和增强反应性至关重要。在此，我们比较了光谱表征的合成血红素铁(IV)氧合配合物 FCmpd-II（F = 四(2,6-二氟苯基)卟啉）和涉及铁氧原子氢键合的 2,6- 啶三氟甲磺酸（LutH）路易斯酸加合物的 C-H 键氧化能力，FCmpd-II（LutH）。通过紫外-可见光谱监测获得了利用 C-H 和 C-D 底物的二级速率常数，而通过电子顺磁共振（EPR）光谱和气相色谱（GC）对产物进行了表征和定量。对于香豆素，FCmpd-II（LutH）的反应速度比 FCmpd-II 快 40 倍（= 14.2 M s；-90°C），生成比沙汀、氧杂比沙汀和血红素产物[FFeOH]。对于具有更高 C-H 键离解能（BDE）的底物，FCmpd-II（LutH）的反应二级速率常数（9,10-二氢蒽；DHA）= 0.485 M s 和（芴）= 0.102 M s（-90°C）；相比之下，FCmpd-II 对这些底物没有反应。对于香豆素与香豆素（），对于 FCmpd-II 和 FCmpd-II（LutH），大致估计了大的、非经典的氘动力学同位素效应。还制备了氘代氢键合类似物 FCmpd-II（LutD）；对于与 DHA 的反应，观察到与 FCmpd-II（LutH）相反的 KIE。这项工作开创了对真实氢键合血红素基 Fe═O 化合物反应性的实验研究，批判性地确立了在血红素配合物和酶活性位点中氧合氢键（或质子化）的重要性。
