Department of Chemistry, Massachusetts Institute of Technology, Cambridge, 02139, United States.
Acc Chem Res. 2011 Apr 19;44(4):280-8. doi: 10.1021/ar1001473. Epub 2011 Mar 10.
The controlled oxidation of methane to methanol is a chemical transformation of great value, particularly in the pursuit of alternative fuels, but the reaction remains underutilized industrially because of inefficient and costly synthetic procedures. In contrast, methane monooxygenase enzymes (MMOs) from methanotrophic bacteria achieve this chemistry efficiently under ambient conditions. In this Account, we discuss the first observable step in the oxidation of methane at the carboxylate-bridged diiron active site of the soluble MMO (sMMO), namely, the reductive activation of atmospheric O(2). The results provide benchmarks against which the dioxygen activation mechanisms of other bacterial multicomponent monooxygenases can be measured. Molecular oxygen reacts rapidly with the reduced diiron(II) cen-ter of the hydroxylase component of sMMO (MMOH). The first spectroscopically characterized intermediate that results from this process is a peroxodiiron(III) species, P*, in which the iron atoms have identical environments. P* converts to a second peroxodiiron(III) unit, H(peroxo), in a process accompanied by the transfer of a proton, probably with the assistance of a residue near the active site. Proton-promoted O-O bond scission and rearrangement of the diiron core then leads to a diiron(IV) unit, termed Q, that is directly responsible for the oxidation of methane to methanol. In one section of this Account, we provide a detailed discussion of these processes, with particular emphasis on possible structures of the intermediates. The geometries of P* and H(peroxo) are currently unknown, and recent synthetic modeling chemistry has highlighted the need for further structural characterization of Q, currently assigned as a di(μ-oxo)diiron(IV) "diamond core." In another section of the Account, we discuss in detail proton transfer during the O(2) activation events. The role of protons in promoting O-O bond cleavage, thereby initiating the conversion of H(peroxo) to Q, was previously a controversial topic. Recent studies of the mechanism, covering a range of pH values and in D(2)O instead of H(2)O, confirmed conclusively that the transfer of protons, possibly at or near the active site, is necessary for both P*-to-H(peroxo) and H(peroxo)-to-Q conversions. Specific mechanistic insights into these processes are provided. In the final section of the Account, we present our view of experiments that need to be done to further define crucial aspects of sMMO chemistry. Here our goal is to detail the challenges that we and others face in this research, particularly with respect to some long-standing questions about the system, as well as approaches that might be used to solve them.
甲烷的可控氧化转化为甲醇是一种具有重要价值的化学反应，特别是在寻找替代燃料方面。但由于合成工艺效率低下且成本高昂，该反应在工业上的应用仍然受到限制。相比之下，来自甲烷氧化菌的甲烷单加氧酶（MMO）能够在环境条件下高效地实现这一化学转化。在本综述中，我们讨论了可溶性 MMO（sMMO）羧酸盐桥联二铁活性中心中甲烷氧化的第一个可观察步骤，即大气 O2 的还原活化。这些结果为衡量其他细菌多组分单加氧酶的分子氧活化机制提供了基准。
分子氧与 sMMO（MMOH）羟化酶组分的还原二价铁中心快速反应。该过程产生的第一个光谱表征中间体是过氧二铁（III）物种 P*，其中铁原子具有相同的环境。P转化为第二个过氧二铁（III）单元 H（过氧），质子转移伴随着质子转移，可能在活性位点附近的一个残基的辅助下发生。质子促进的 O-O 键断裂和二铁核心的重排导致直接负责将甲烷氧化为甲醇的二铁（IV）单元 Q。在本综述的一部分中，我们详细讨论了这些过程，特别强调了中间体可能的结构。目前，P和 H（过氧）的几何结构尚不清楚，最近的合成模拟化学强调了进一步结构表征 Q 的必要性，目前 Q 被指定为二（μ-氧）二铁（IV）“钻石核”。在本综述的另一部分中，我们详细讨论了 O2 活化过程中的质子转移。质子在促进 O-O 键断裂从而启动 H（过氧）到 Q 的转化中的作用曾是一个有争议的话题。最近对该机制的研究涵盖了一系列 pH 值和 D2O 而不是 H2O，这一研究明确证实了质子转移，可能是在活性位点或附近，对于 P*-到-H（过氧）和 H（过氧）-到-Q 的转化都是必需的。提供了对这些过程的具体机制见解。在本综述的最后一部分，我们提出了我们对进一步定义 sMMO 化学关键方面所需进行的实验的看法。在这里，我们的目标是详细说明我们和其他人在这项研究中面临的挑战，特别是针对该系统的一些长期存在的问题，以及可能用于解决这些问题的方法。
