Progress On Methane Monooxygenase And Its Model Complexes

Progress on Methane Monooxygenase and Its Model ComplexesLiuYiAbstract: Methane monooxygenase(MMO) is one of the most important enzymes during the metabolism of methantrophs. It is the only enzyme that can realize the selective oxidation of methane at the ordinary temperature and pressure and supply the ideal model for selective, partial oxidation of methane. Meanwhile, as a non-special biocatalyst, MMO can catalyze hydroxylation of all kinds of alkanes from Cj to €20 and epoxidation of alkenes from €2 to Qoas well. In addition, it has wide potential application in medicine and industry. It is just its advantage that makes it a hot area in research works.There are two forms of MMO exist natrually. One is the particulate, membrane-bound (pMMO) form and the other is the soluble (sMMO) form. The later consists of three components such as MMOH, MMOB, and MMOR. MMOH is the most important component, which consists of three subunits a, P, and 7. The active site, located at the a-subunit, contains a carboxylate-bridged diiron core where the activition of O2 and hydroxylation of C-H bond take place.Based on the structure of active site of MMO, a lot of small molecule model complexes analog with different diiron cores have been synthesized for the purpose of understanding the structure and catalytic properties of MMO in biological oxidation, and further, the biological phenomena of life. The structural and functional mimicing of MMO with binuclear iron complexes have been an active frontier and a challenge in biology and chemistry. In this aspect, many creative research works have been done in the passed decade, especially those in the laboratories leaded by S. J. Lippard, L. Que, Jr., and M. Suzuki, and elsewhere.The researches on mimicing of MMO can be divided into structural and functional mimic. A great many dinuclear iron complexes have been synthesized for the structural mimics of MMOH with the ligands including F^Hbab, BiphMe, XDK, tmen, and so on. The mimic of the active intermediate, MMOH-P and -Q, during the catalytic process is always an important topic. MMOH-P is considered as an active intermediate with an Fei'^-Oi) core and Q with an Fe2IV(/*-O)2 core. The firstMMOH-P intermediate analog, Fe2IH(a-l,2-O2), was obtained from the reaction of a mononuclear iron complex [FeI1{HB(3,5-iPr2Pz)3(O2CR)}(CH3CN) and O2 by N. Kitajima in 1990. MMOH-P mimics via [Fe2lIL(O2CC6H5)X2](L=HPTB, A^-Et-HPTB, or HPTP) with O2 and [Fe2"(RCO2)L](BF4)2 [L = tpdp, Me2-tpdp, or Me4-tpdp] with O2 have been also reported. Many useful conclusions can be drawn from these reactions. Firstly, the unsaturated coordination of metal center is favorable to the bonding of O2. Secondly, the steric and electronic factors also affect the stability of O2-adducts.A new ligand, H2XDK, has more advantages in synthesizing the Fe2H complexes due to it has more carboxylic groups than the ligands with N atoms. The diiron complexes with this ligand can reproduce nicely the active site of MMO and RNR. Recently, various diferric complexes with XDK, HPXDK, and HBXDK have been successfully applied to the reaction with O2 for mimicing MMOH-P. Many complexes with an Fe2C"-O)2, Fe2(/*-O)(//-OH), or Fe2(u-O)(^-Hs02) core have been reported with ligands of TPA and its derivatives. The first complex containing Fe2(?O)2 core is [Fe21'ICu-O)2(6-Me3-TPA)](ClO4)2 and the complex containing Fe2(yU-O)C?OH) is [Fe2Gu-O)Gu-OH)(6-Me3-TPA)](ClO4)3. The most close mimics of MMOH-Q and RNR R2-X maybe [FemFelv(/z-O)2(5-Me3-TPA)](ClO4)3 and [Fe10Felv(^-O)2(6-Me-TPA)]2 (C104)3 ?In the aspect of functional mimic of MMO, many diiron complexes have been used in the reaction of catalytic hydroxylation of alkanes. Among which the Fe(TPA) systems have been thoroughly investigated. There are many factors affect the catalytic activity of the hydroxylation of alkanes. This makes the catalyic system more complex and, in turn, more plenty. The research is focus on the reaction mechanism to find out the true oxidant. A free radical BuO- derived from BuOOH (TBHP) an...