Bromoperoxidase kinetics

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under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations).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. 

Another class of peroxidases which can perform asymmetric sulfoxidations, and which have the advantage of inherently higher stabilities because of their non-heme nature, are the vanadium peroxidases. It was shown that vanadium bromoperoxidase from Ascophyllum nodosum mediates the production of (R)-methyl phenyl sulfoxide with a high 91% enantiomeric excess from the corresponding sulfide with H202 [38]. The turnover frequency of the reaction was found to be around 1 min-1. In addition this enzyme was found to catalyse the sulfoxidation of racemic, non-aromatic cyclic thioethers with high kinetic resolution [309]. 

Steady-state kinetic studies showed that the kinetics of the enzyme resemble those of the vanadium bromoperoxidases. The chloroperoxidase exhibits a pH profile similar to vanadium bromoperoxidases although the optimal pH of 4.5-5.0 is at a lower value. At low pH the enzyme is inhibited by chloride in a competitive way whereas at higher pH values the activity displays normal Michaelis-Menten type of behavior (see Michaelis Constant). The log Km for chloride increases linearly with pH whereas that for hydrogen peroxide decreases with pH demonstrating that in the catalytic mechanism protons are involved. These observations have led to a simplified ping-pong type of mechanism for the chloroperoxidase similar to that shown in (Figure 1). 

It would be of considerable interest to see whether vanadium-peroxo complexes are also able to oxidize bromide and display kinetic behavior similar to that of the vanadium-containing bromoperoxidases. In this respect the complexes reported by Li et al. (80) may provide a useful contribution. Conversely, some attention should be paid to whether bromoperoxidases show specificity only toward bromide or iodide. These enzymes may perhaps be tuned to catalyze the oxidation and oxygenation of other nucleophiles. 

Complexes of ligands with O-donors and two or three N-donors are known. The kinetics of the outer-sphere oxidation of cis-aq uaoxovanad i u m (IV) complexes of [2-(pyridylmethyl)imino]diacet-ate and its derivatives were determined.675 Complexes with Schiff bases have been used to mimic the structure and chemistry of vanadium bromoperoxidase.275 The ligation of an imidazole functionality in the ligand has been found to readily dissociate, and is important to the functional aspects of this complex.275 A variety of five-coordinate complexes with tridentate Schiff base complexes have been prepared, several of which have been found to form supramolecular polymeric structures through association between the V=0 groups in a V=0 V=0 V=0 pattern (140).627... 

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