Sainna Mala A, Kumar Suresh, Kumar Devesh, Fornarini Simonetta, Crestoni Maria Elisa, de Visser Sam P
Manchester Institute of Biotechnology and School of Chemical Engineering and Analytical Science , The University of Manchester , 131 Princess Street , Manchester M1 7DN , UK . Email:
Department of Applied Physics , School for Physical Sciences , Babasaheb Bhimrao Ambedkar University , Vidya Vihar, Rai Bareilly Road , Lucknow 226 025 , India . Email:
Chem Sci. 2015 Feb 1;6(2):1516-1529. doi: 10.1039/c4sc02717e. Epub 2014 Dec 8.
Cytochrome P450 enzymes are heme based monoxygenases that catalyse a range of oxygen atom transfer reactions with various substrates, including aliphatic and aromatic hydroxylation as well as epoxidation reactions. The active species is short-lived and difficult to trap and characterize experimentally, moreover, it reacts in a regioselective manner with substrates leading to aliphatic hydroxylation and epoxidation products, but the origin of this regioselectivity is poorly understood. We have synthesized a model complex and studied it with low-pressure Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry (MS). A novel approach was devised using the reaction of [Fe(TPFPP)] (TPFPP = -tetrakis(pentafluorophenyl)porphinato dianion) with iodosylbenzene as a terminal oxidant which leads to the production of ions corresponding to [Fe(O)(TPFPP˙)]. This species was isolated in the gas-phase and studied in its reactivity with a variety of olefins. Product patterns and rate constants under Ideal Gas conditions were determined by FT-ICR MS. All substrates react with [Fe(O)(TPFPP˙)] by a more or less efficient oxygen atom transfer process. In addition, substrates with low ionization energies react by a charge-transfer channel, which enabled us to determine the electron affinity of [Fe(O)(TPFPP˙)] for the first time. Interestingly, no hydrogen atom abstraction pathways are observed for the reaction of [Fe(O)(TPFPP˙)] with prototypical olefins such as propene, cyclohexene and cyclohexadiene and also no kinetic isotope effect in the reaction rate is found, which suggests that the competition between epoxidation and hydroxylation - in the gas-phase - is in favour of substrate epoxidation. This notion further implies that P450 enzymes will need to adapt their substrate binding pocket, in order to enable favourable aliphatic hydroxylation over double bond epoxidation pathways. The MS studies yield a large test-set of experimental reaction rates of iron(iv)-oxo porphyrin cation radical complexes, so far unprecedented in the gas-phase, providing a benchmark for calibration studies using computational techniques. Preliminary computational results presented here confirm the observed trends excellently and rationalize the reactivities within the framework of thermochemical considerations and valence bond schemes.
细胞色素P450酶是基于血红素的单加氧酶,可催化一系列与各种底物的氧原子转移反应,包括脂肪族和芳香族羟基化以及环氧化反应。活性物种寿命短,难以捕获和通过实验表征,此外,它与底物以区域选择性方式反应生成脂肪族羟基化和环氧化产物,但这种区域选择性的起源了解甚少。我们合成了一种模型配合物,并使用低压傅里叶变换离子回旋共振(FT-ICR)质谱(MS)对其进行了研究。设计了一种新方法,利用[Fe(TPFPP)](TPFPP = -四(五氟苯基)卟啉二价阴离子)与碘苯作为末端氧化剂的反应,该反应导致产生对应于[Fe(O)(TPFPP˙)]的离子。该物种在气相中被分离出来,并研究了其与各种烯烃的反应性。在理想气体条件下的产物模式和速率常数由FT-ICR MS测定。所有底物都通过或多或少有效的氧原子转移过程与[Fe(O)(TPFPP˙)]反应。此外,具有低电离能的底物通过电荷转移通道反应,这使我们首次能够确定[Fe(O)(TPFPP˙)]的电子亲和力。有趣的是,未观察到[Fe(O)(TPFPP˙)]与典型烯烃如丙烯、环己烯和环己二烯反应的氢原子夺取途径,并且在反应速率中也未发现动力学同位素效应,这表明在气相中环氧化和羟基化之间的竞争有利于底物环氧化。这一概念进一步意味着P450酶需要调整其底物结合口袋,以便在双键环氧化途径之上实现有利的脂肪族羟基化。MS研究产生了大量铁(IV)-氧卟啉阳离子自由基配合物的实验反应速率测试集,这在气相中是前所未有的,为使用计算技术的校准研究提供了基准。此处给出的初步计算结果出色地证实了观察到的趋势,并在热化学考虑和价键方案的框架内合理化了反应性。