Several anaerobic bacteria can couple the reduction of aromatic halides to energy conservation. This organohalide respiration is catalyzed by enzymes containing cob(I)alamin, an activated supernucleophilic form of the coenzyme vitamin B₁₂. However, the mechanism underlying the electron transfer (inner-sphere vs outer-sphere ET) still remains elusive. To clarify this issue, we selected 36 fluoro-, chloro-, and bromobenzenes as representative substrates and calculated their free-energy barriers at the quantum chemical density functional theory level, considering a wide range of theoretically possible outer-sphere ET mechanisms. Across all 336 reaction routes addressed, 334 routes involve free-energy barriers larger than 20 kcal/mol. For two reaction routes with highly brominated benzenes, free-energy barriers below 20 kcal/mol imply abiotic reduction as observed in experiments. Thus, microbial B₁₂-dependent aromatic reductive dehalogenation does not proceed through an outer-sphere ET mechanism. Instead, the present study strongly suggests that microbe-catalyzed reductive dehalogenation of aromatic halides is governed by inner-sphere ET.

中文翻译：

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Dehalococcoides-介导的B _12-依赖性芳香族化合物的还原脱卤作用不会通过外层电子转移进行

几种厌氧细菌可以将芳香族卤化物的还原与节能结合起来。这种有机卤化物的呼吸作用是通过含有辅酶维生素B ₁₂的活化超亲核形式的钴（I）阿拉明酶来催化的。但是，电子传递的基础机理（内层球与外层球ET）仍然难以捉摸。为了澄清这个问题，我们选择了36种氟代，氯代和溴代苯作为代表性底物，并在量子化学密度泛函理论水平上计算了它们的自由能势垒，同时考虑了多种理论上可能的外球ET机制。在所有涉及的336条反应路线中，有334条路线涉及大于20 kcal / mol的自由能势垒。对于两条具有高溴化苯的反应路线，低于20 kcal / mol的自由能势垒意味着非生物还原，如在实验中观察到的。因此，微生物B ₁₂依赖的芳族还原性脱卤不通过外球ET机制进行。相反，本研究强烈表明，微生物催化的芳族卤化物的还原脱卤作用受内球ET的支配。