Department of Chemistry and Biochemistry, University of Delaware, Delaware, USA.
J Phys Chem A. 2011 Oct 13;115(40):11087-100. doi: 10.1021/jp208087u. Epub 2011 Sep 20.
Model quantum mechanical calculations presented for C-4a-flavin hydroperoxide (FlHOOH) at the B3LYP/6-311+G(d,p) level suggest a new mechanism for flavoprotein monooxygenase (FMO) oxidation involving a concerted homolytic O-O bond cleavage in concert with hydroxyl radical transfer from the flavin hydroperoxide rather than an S(N)2-like displacement by the substrate on the C-4a-hydroperoxide OOH group. Homolytic O-O bond cleavage in a somersault-like rearrangement of hydroperoxide C-4a-flavinhydroperoxide (1) (FLHO-OH → FLHO···HO) produces an internally hydrogen-bonded HO(•) radical intermediate with a classical activation barrier of 27.0 kcal/mol. Model hydroperoxide 1 is used to describe the transition state for the key oxidation step in the paradigm aromatic hydroxylase, p-hydroxybenzoate hydroxylase (PHBH). A comparison of the electron distribution in the transition structures for the PHBH hydroxylation of p-hydroxybenzoic acid (ΔE(‡) = 23.0 kcal/mol) with that of oxidation of trimethylamine (ΔE(‡) = 22.3 kcal/mol) and dimethyl sulfide (ΔE‡ = 14.1 kcal/mol) also suggests a mechanism involving a somersault mechanism in concert with transfer of an HO(•) radical to the nucleophilic heteroatom center with a hydrogen transfer back to the FLH-O residue after the barrier is crossed to produce the final product, FLH-OH. In each case the hydroxylation barrier was less than that of the O-O rearrangement barrier in the absence of a substrate supporting an overall concerted process. All three transition structures bear a resemblance to the TS for the comparable hydroxylation of isobutane (ΔE(‡) = 29.2 kcal/mol) and for simple Fenton oxidation by aqueous iron(III) hydroperoxides. To our surprise the oxidation of N- and S-nucleophiles with conventional oxidants such as alkyl hydroperoxides and peracids also proceeds by HO(•) radical transfer in a manner quite similar to that for tricyclic hydroperoxide 1. Stabilization of the developing oxyradical produced by somersault rearrangement for concerted enzymatic oxidation with tricyclic hydroperoxide 1 results in a reduced overall activation barrier.
模型量子力学计算表明，C-4a-黄素过氧化物（FlHOOH）在 B3LYP/6-311+G（d，p）水平上涉及黄素蛋白单加氧酶（FMO）氧化的新机制，涉及协同的均裂 O-O 键断裂以及羟基自由基从黄素过氧化物转移到过氧化物 C-4a-氢过氧化物 OOH 基团而不是底物的 S（N）2 样置换。过氧化物 C-4a-黄素氢过氧化物（1）（FLHO-OH → FLHO···HO）中的均裂 O-O 键断裂以类似翻筋斗的方式重排产生内部氢键的 HO（•）自由基中间体，其经典活化能为 27.0 kcal/mol。模型氢过氧化物 1 用于描述典范芳香羟化酶对羟基苯甲酸羟化酶（PHBH）中关键氧化步骤的过渡态。对 PHBH 对对羟基苯甲酸（ΔE（‡）= 23.0 kcal/mol）和二甲硫醚（ΔE‡= 14.1 kcal/mol）的氧化的过渡态中电子分布的比较也表明，涉及一种协同翻筋斗机制以及将 HO（•）自由基转移到亲核杂原子中心的机制，并且在越过障碍后，氢转移回 FLH-O 残基以产生最终产物，FLH-OH。在每种情况下，羟化障碍均小于无底物支持整体协同过程时 O-O 重排障碍。所有三个过渡态结构都与异丙烷（ΔE（‡）= 29.2 kcal/mol）的可比羟化和水相铁（III）过氧化物的简单芬顿氧化的 TS 相似。令我们惊讶的是，与常规氧化剂如烷基过氧化物和过氧酸的 N-和 S-亲核试剂的氧化也通过 HO（•）自由基转移以与三环过氧化物 1 非常相似的方式进行。通过协同酶促氧化与三环氢过氧化物 1 进行的翻筋斗重排产生的发展氧自由基的稳定导致总活化能垒降低。
