Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin
Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin
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DOI：
10.1016/j.jinorgbio.2007.07.017
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发表时间：
2007-11-01
影响因子：
3.9
通讯作者：
Groves, John T.
中科院分区：
文献类型：
--
作者：
Lahaye, Dorothee;Groves, John T.
锰的meso-二甲基咪唑卟啉配合物Mn（III）[TDMImP]与HOBr/OBr-反应生成相应的氧代Mn（V）[TDMImP]物种。该过程的速率随着pH值的增加而加速。在pH 8下测定的正向速率常数kappa（for）为1.65 x 10（6）M-1 S-1。在这些条件下，氧代-Mn（V）物种是短暂的，并转化为相应的氧代-Mn（IV）络合物。一级速率常数，卡帕（obs），为0.66秒（-1），发现在pH值为8的还原过程。该还原过程的机理依赖于溴离子，似乎是通过中间体Mn（III）-O-Br络合物进行的。因此，一个快速的，可逆的Mn（III）-O-Br键异裂和一个较慢的均裂途径平行发生在这个系统中。研究了氧代Mn（V）[TDMImP]和溴化物之间的逆氧化反应作为pH的函数。该氧代转移反应的速率（kappa（rev）= 1.4 x 10（3）M-1 s（-1）at pH 8）随着pH的降低而显著加速。所观察到的速率对[H+]的一级依赖性表明，负责溴化物氧化的反应性物种是质子化的氧代-羟基络合物，并且在高pH下溶液中存在的稳定物种是二氧代-Mn（V）[TDMImP]，[O=Mn（V）=O]（-）。氧代锰（V）物种几乎保留了次卤酸盐的所有氧化驱动力。在三种不同的pH值下测定了可逆过程的平衡常数K-equi = kappa（for）/kappa（rev）（K-equi = 1. 15 X 10（3），pH 8），允许测量氧代-Mn（V）/Mn（III）的氧化还原电位E（E = 1.01 V，pH 8）。使用能斯特关系在整个pH范围内外推该对的氧化还原电位，并与锰2-和4-内消旋-N-甲基吡啶鎓卟啉对氧代-Mn（V）[2-TMPyP]/Mn（III）[2-TMPyP]、氧代-Mn（V）[4-TMPyP]/Mn（III）[4-TMPyP]、OBr-/Br-和H2 O2/H2O的氧化还原电位进行比较。值得注意的是，咪唑卟啉的氧代-Mn（V）/Mn（III）的氧化还原电位接近低pH下H2 O2/H2O的氧化还原电位。All rights reserved.
The manganese meso-dimethylimidazolium porphyrin complex Mn(III)[TDMImP] reacted with HOBr/OBr- to generate the corresponding oxo-Mn(V)[TDMImP] species. The rate of this process accelerated with increasing pH. A forward rate constant, kappa(for), of 1.65 x 10(6) M-1 S-1 was determined at pH 8. Under these conditions, the oxo-Mn(V) species is short-lived and is transformed into the corresponding oxo-Mn(IV) complex. A first-order rate constant, kappa(obs), of 0.66 s(-1) was found for this reduction process at pH 8. The mechanism of this reduction process, which was dependent on bromide ion, appeared to proceed via an intermediate Mn(III)-O-Br complex. Thus, both a fast, reversible Mn(III)-O-Br bond heterolysis and a slower homolytic pathway occur in parallel in this system. The reverse oxidation reaction between oxo-Mn(V)[TDMImP] and bromide was investigated as a function of pH. The rate of this oxo-transfer reaction (kappa(rev) = 1.4 x 10(3) M-1 s(-1) at pH 8) markedly accelerated as the pH was lowered. The observed first-order dependence of the rate on [H+] indicates that the reactive species responsible for bromide oxidation is a protonated oxo-hydroxo complex and the stable species present in solution at high pH is dioxo-Mn(V)[TDMImP], [O=Mn(V)=O](-). The oxo-Mn(V) species retains nearly all of the oxidative driving force of the hypohalite. The equilibrium constant K-equi = kappa(for)/kappa(rev) for the reversible process was determined at three different pH values (K-equi = 1. 15 x 10(3) at pH 8) allowing the measurement of the redox potentials E of oxo-Mn(V)/Mn(III) (E = 1.01 V at pH 8). The redox potential for this couple was extrapolated over the entire pH scale using the Nernst relationship and compared to those of the manganese 2- and 4-meso-N-methylpyridinium porphyrin couples oxo-Mn(V)[2-TMPyP]/Mn(Ill)[2-TMPyP], oxo-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP], OBr-/Br- and H2O2/H2O. Notably, the redox potential of oxo-Mn(V)/Mn(III) for the imidazolium porphyrin approaches that of H2O2/H2O at low pH. (c) 2007 Elsevier Inc. All rights reserved.