Bromoperoxidase selectivity

Figure 1 The microbial fouling process on surfaces of certain macroalgae in aquatic environments is controlled by the selective oxidation of bromide with hydrogen peroxide and bromoperoxidase. Although chloride is many orders of magnitude more abundant in the sea, bromide is oxidized to hypobromous acid in situ.

Figure 4 Stabilized bromine antimicrobials are produced by eosinophils, a type of mammalian white blood cell. Bacteria are captured by phagocytosis and contained intracellularly within vesicles called phagosomes. Granules release cationic surfactants, lytic enzymes, and eosinophil peroxidase into the phagosome in a process known as degranulation. Eosinophil peroxidase, an enzyme that is structurally similar to the bromoperoxidases found in seaweed (Figure I), selectively catalyzes oxidation of bromide to hypobromite by reducing hydrogen peroxide to water. The hypobromite immediately reacts with nitrogenous stabilizers such as aminoethanesulfonic acid (taurine) to form more effective and less toxic antimicrobial agents.

Alkyl hydroperoxides, including ethyl hydroperoxide, cuminyl hydroperoxide, and tert-butyl hydroperoxide, are not used by V-BrPO to catalyze bromination reactions [29], These alkyl hydroperoxides have the thermodynamic driving force to oxidize bromide however, they are kinetically slow. Several examples of vanadium(V) alkyl peroxide complexes have been well characterized [63], including [V(v)0(OOR)(oxo-2-oxidophenyl) salicylidenaminato] (R = i-Bu, CMe2Ph), which has been used in the selective oxidation of olefins to epoxides. The synthesis of these compounds seems to require elevated temperatures, and their oxidation under catalytic conditions has not been reported. We have found that alkyl hydroperoxides do not coordinate to vanadate in aqueous solution at neutral pH, conditions under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations). Thus, the lack of bromoperoxidase reactivity with the alkyl hydroperoxides may arise from slow binding of the alkyl hydroperoxides to V-BrPO. 

Few redox studies with cubic mesoporous materials have been reported [52]. The large, complex, three-dimensional pore system offers a unique environment. Ti- and Cr-substituted MCM-48 have been studied for the selective oxidation of methyl methacrylate and styrene to methyl pyruvate and benzaldehyde, respectively, using peroxides as oxidants and were found to outperform TS-1. Ti-MCM-48 has also been found to be better than Ti-MCM-41, TS-1 and Ti02 for the photocatalytic reduction of CO2 and H2O to methane and methanol. Ti-grafted MCM-48 has also been reported as the first functional biomimic of vanadium bromoperoxidase, active at neutral pH and used in the peroxidative halogenation of bulky organic dyes. 

As mentioned above, sulfides are also substrates for the VBrPOs. Scheme 4.5 summarises selected results obtained by oxidation of prochiral sulfides with H2O2 in mixtures of water and alcohol. Yields and enantiomeric excess (e.e.) are subject to large variations, depending on the source of the bromoperoxidase Cor. officinalis vs A. nodosum) and... 

Other peroxidases such as HRP or CPO were also able to perform such reactions. Another approach to the production of nitroarenes with peroxidases is based on the CPO (or bromoperoxidase)-catalyzed oxidation of arylamines. Table 16.3-10 gives a selection of peroxidase-catalyzed conversions of aniline derivatives to corresponding nitroarenes. 

When investigating the substrate selectivity using a series of aryl, alkyl, dialkyl, and heterocyclic sulfides, it was found that p-substitution led to higher enatioselectiv-ity and higher chemical yields with respect to o-substitution [20l A similar influence of the p-substitution was found for sulfoxidation catalyzed by bromoperoxidase from the marine alga Ascophyllum nodosum1271. 

Butler, A. and Tschirret-Guth, R.A. (1996) On the selectivity of vanadium bromoperoxidase, in Mechanisms of Biohalogenation and Dehalogenation, vol. 25F (eds D.B. Janssen, K. Soda, and R. Wever), Royal Netherlands Academy of Arts and Sciences, Amsterdam, pp. 55-68. 

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