Functional Models for Vanadium Haloperoxidase: Reactivity and Mechanism of Halide Oxidation Gerard J. Colpas, Brent J. Hamstra, Jeff W. Kampf, and Vincent L. Pecoraro* Contribution from the Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055 ReceiVed NoVember 13, 1995X
Abstract: A series of oxoperoxovanadium(V) complexes (ligands: H3nta ) nitrilotriacetic acid, H3heida ) N-(2hydroxyethyl)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 vanadium haloperoxidase enzymes. The crystal structures of K[VO(O2)Hheida], K[VO(O2)ada], [VO(O2)bpg], and H[VO(O2)bpg]2(ClO4) were obtained. These complexes all possess a distorted pentagonal bipyramidal coordination sphere containing a side-on bound peroxide. In the presence of sufficient acid equivalents these complexes catalyze the two-electron oxidation of bromide or iodide by peroxide. Halogenation of an organic substrate was demonstrated by following the visible conversion of Phenol Red to Bromophenol Blue. In the absence of substrate, dioxygen can be generated by the halide-assisted disproportionation of hydrogen peroxide. In addition, some of these complexes can efficiently catalyze the peroxidative halogenation reaction, performing multiple turnovers in minutes. The kinetic analysis of the halide oxidation reaction indicates a mechanism which is first order in protonated peroxovanadium complex and halide. The bimolecular rate constants for both bromide and iodide oxidation were determined, with the iodide rates being approximately 5-6 times faster than the bromide rates. The rate constants obtained for bromide oxidation range from a maximum of 280 M-1 s-1 for the Hheida complex to a minimum of 21 M-1 s-1 for the Hbpg complex. The pKa of activation for each complex in acetonitrile was determined to range from 5.4 to 6.0. On the basis of the chemistry observed for these model compounds, a mechanism of halide oxidation and a detailed catalytic cycle are proposed for the vanadium haloperoxidase enzyme.
Introduction While isolation of the vanadium haloperoxidase (VHPO) from the marine algae Ascophyllum nodosum in 1984 provided the first example of a vanadium-dependent enzyme,1,2 it is now apparent that these vanadoproteins are found in most marine algae, seaweed, and some lichens.3 Haloperoxidases isolated from different sources are characterized as iodoperoxidases, bromoperoxidases, or chloroperoxidases,4 depending on the most electronegative halogen the enzyme is capable of oxidizing. VHPOs are thought to be involved in the production of a large quantity of halogenated organics in ViVo.5 Recent work has shown that VHPO has the ability to selectively halogenate particular substrates at specific locations on the phenyl ring.6 These compounds include a number with potent antifungal, antibacterial, and antitumor properties.7 In the absence of an organic substrate, oxygen is generated by a halide-assisted disproportionation of hydrogen peroxide.8 In the presence of bromide, this was shown to be singlet oxygen.9 A slight pH dependence has been observed for these X Abstract published in AdVance ACS Abstracts, March 15, 1996. (1) Vilter, H. Bot. Mar. 1983, 26, 451-455. (2) Vilter, H. Phytochemistry 1984, 23, 1387-1390. (3) Wever, R.; Krenn, B. E. Vanadium Haloperoxidases In Vanadium in Biological Systems; Chasteen, N. D., Ed.; Kluwer Academic Publishers: Dordrecht, 1990; pp 81-97. (4) Soedjak, H. S.; Butler, A. Inorg. Chem. 1990, 29, 5015-5017. (5) Neidleman, S. L.; Geigert, J. L. Biohalogenation; Ellis Horwood Ltd. Press: New York, 1986. (6) Tschirret-Guth, R. A.; Butler, A. J. Am. Chem. Soc. 1994, 116, 411412. (7) Butler, A.; Walker, J. V. Chem. ReV. 1993, 93, 1937-1944. (8) Everett, R. R.; Butler, A. Inorg. Chem. 1989, 28, 393-395. (9) Everett, R. R.; Soedjak, H....