Department of Applied Physics, School of Physical Sciences, Babasaheb Bhimrao Ambedkar University, Vidya Vihar, Rae Bareilly Road, Lucknow 226-025, India.
Inorg Chem. 2013 Jul 15;52(14):7968-79. doi: 10.1021/ic4005104. Epub 2013 Jul 3.
A versatile class of heme monoxygenases involved in many vital functions for human health are the cytochromes P450, which react via a high-valent iron(IV) oxo heme cation radical species called Compound I. One of the key reactions catalyzed by these enzymes is C═C epoxidation of substrates. We report here a systematic study into the intrinsic chemical properties of substrate and oxidant that affect reactivity patterns. To this end, we investigated the effect of styrene and para-substituted styrene epoxidation by Compound I models with either an anionic (chloride) or neutral (acetonitrile) axial ligand. We show, for the first time, that the activation enthalpy of the reaction is determined by the ionization potential of the substrate, the electron affinity of the oxidant, and the strength of the newly formed C-O bond (approximated by the bond dissociation energy, BDE(OH)). We have set up a new valence bond model that enables us to generalize substrate epoxidation reactions by iron(IV)-oxo porphyrin cation-radical oxidants and make predictions of rate constants and reactivities. We show here that electron-withdrawing substituents lead to early transition states, whereas electron-donating groups on the olefin substrate give late transition states. This affects the barrier heights in such a way that electron-withdrawing substituents correlate the barrier height with BDE(OH), while the electron affinity of the oxidant is proportional to the barrier height for substrates with electron-donating substituents.
涉及许多人类健康重要功能的一类多功能血红素单加氧酶是细胞色素 P450，它通过一种称为复合物 I 的高价铁(IV)氧血红素阳离子自由基物种反应。这些酶催化的关键反应之一是底物的 C═C 环氧化。在这里，我们系统地研究了影响反应性模式的底物和氧化剂的固有化学性质。为此，我们研究了复合物 I 模型中阴离子（氯化物）或中性（乙腈）轴向配体对苯乙烯和对位取代苯乙烯环氧化的影响。我们首次表明，反应的活化焓由底物的电离势、氧化剂的电子亲合势和新形成的 C-O 键的强度（由键解离能 BDE(OH)近似）决定。我们已经建立了一个新的价键模型，使我们能够概括铁(IV)-氧卟啉阳离子自由基氧化剂的底物环氧化反应，并对速率常数和反应性进行预测。我们在这里表明，吸电子取代基导致早期过渡态，而烯烃底物上的供电子基团则导致晚期过渡态。这以这样的方式影响势垒高度，即吸电子取代基将势垒高度与 BDE(OH)相关联，而氧化剂的电子亲合势与具有供电子取代基的底物的势垒高度成正比。
