Reactivity of the Binuclear Non-Heme Iron Active Site of 螖9 Desaturase Studied by Large-Scale Multireference Ab Initio Calculations

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• 作者：Jakub Chalupsk媒 ; Tibor Andr谩s Rokob ; Yuki Kurashige ; Takeshi Yanai ; Edward I. Solomon ; Lubom铆r Rul铆拧ek ; Martin Srnec
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：November 12, 2014
• 年：2014
• 卷：136
• 期：45
• 页码：15977-15991
• 全文大小：773K
• ISSN：1520-5126

文摘

The results of density matrix renormalization group complete active space self-consistent field (DMRG-CASSCF) and second-order perturbation theory (DMRG-CASPT2) calculations are presented on various structural alternatives for the O鈥揙 and first C鈥揌 activating step of the catalytic cycle of the binuclear nonheme iron enzyme 螖⁹ desaturase. This enzyme is capable of inserting a double bond into an alkyl chain by double hydrogen (H) atom abstraction using molecular O₂. The reaction step studied here is presumably associated with the highest activation barrier along the full pathway; therefore, its quantitative assessment is of key importance to the understanding of the catalysis. The DMRG approach allows unprecedentedly large active spaces for the explicit correlation of electrons in the large part of the chemically important valence space, which is apparently conditio sine qua non for obtaining well-converged reaction energetics. The derived reaction mechanism involves protonation of the previously characterized 1,2-渭 peroxy Fe^IIIFe^III (P) intermediate to a 1,1-渭 hydroperoxy species, which abstracts an H atom from the C₁₀ site of the substrate. An Fe^IV-oxo unit is generated concomitantly, supposedly capable of the second H atom abstraction from C₉. In addition, several popular DFT functionals were compared to the computed DMRG-CASPT2 data. Notably, many of these show a preference for heterolytic C鈥揌 cleavage, erroneously predicting substrate hydroxylation. This study shows that, despite its limitations, DMRG-CASPT2 is a significant methodological advancement toward the accurate computational treatment of complex bioinorganic systems, such as those with the highly open-shell diiron active sites.