Functional Models for Vanadium Haloperoxidase: Reactivity and Mechanism of Halide Oxidation
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- 作者：Gerard J. Colpas ; Brent J. Hamstra ; Jeff W. Kampf ; and Vincent L. Pecoraro
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：April 10, 1996
- 年：1996
- 卷：118
- 期：14
- 页码：3469 - 3478
- 全文大小：291K
- 年卷期：v.118,no.14(April 10, 1996)
- ISSN：1520-5126
文摘
A series of oxoperoxovanadium(V) complexes (ligands:H3nta = nitrilotriacetic acid,H3heida = N-(2-hydroxyethyl)iminodiacetic acid, H2ada =N-(2-amidomethyl)iminodiacetic acid, Hbpg =N,N-bis(2-pyridylmethyl)glycine, and tpa =N,N,N-tris(2-pyridylmethyl)amine)were characterized as functional models for the vanadiumhaloperoxidase enzymes. The crystal structures ofK[VO(O2)Hheida],K[VO(O2)ada],[VO(O2)bpg], and H[VO(O2)bpg]2(ClO4) wereobtained. These complexes all possess a distorted pentagonalbipyramidal coordination spherecontaining a side-on bound peroxide. In the presence of sufficientacid equivalents these complexes catalyze thetwo-electron oxidation of bromide or iodide by peroxide.Halogenation of an organic substrate was demonstratedby following the visible conversion of Phenol Red to Bromophenol Blue.In the absence of substrate, dioxygen canbe generated by the halide-assisted disproportionation of hydrogenperoxide. In addition, some of these complexescan efficiently catalyze the peroxidative halogenation reaction,performing multiple turnovers in minutes. The kineticanalysis of the halide oxidation reaction indicates a mechanism whichis first order in protonated peroxovanadiumcomplex and halide. The bimolecular rate constants for bothbromide and iodide oxidation were determined, withthe iodide rates being approximately 5-6 times faster than thebromide rates. The rate constants obtained forbromideoxidation range from a maximum of 280 M-1s-1 for the Hheida complex to a minimum of 21M-1 s-1 for theHbpgcomplex. The pKa of activation for eachcomplex in acetonitrile was determined to range from 5.4 to 6.0.On thebasis of the chemistry observed for these model compounds, a mechanismof halide oxidation and a detailed catalyticcycle are proposed for the vanadium haloperoxidase enzyme.