Peroxide oxidation of indole to oxindole by chloroperoxidase catalysis

Abstract

In the presence of chloroperoxidase, indole was oxidized by H2O2 to give oxindole as the major product. Under most conditions oxindole was the only product formed, and under optimal conditions the conversion was quantitative. This reaction displayed maximal activity at pH 4.6, although appreciable activity was observed throughout the entire pH range investigated, namely pH 2.5-6.0. Enzyme saturation by indole could not be demonstrated, up to the limit of indole solubility in the buffer. The oxidation kinetics were first-order with respect to indole up to 8 mM, which was the highest concentration of indole that could be investigated. On the other hand, 2-methylindole was not affected by H2O2 and chloroperoxidase, but was a strong inhibitor of indole oxidation. The isomer 1-methylindole was a poor substrate for chloroperoxidase oxidation, and a weak inhibitor of indole oxidation. These results suggest the possibility that chloroperoxidase oxidation of the carbon atom adjacent to the nitrogen atom in part results from hydrogen-bonding of the substrate N-H group to the enzyme active site.

Nuclear Magnetic Resonance Spectroscopic and Computational Investigations of Chloroperoxidase Catalyzed Regio- and Enantio-Selective Transformations

“…Additionally, the N-H of indole is 1.7 Å from the γ-carboxyl group of Glu 183, indicating that, potentially, hydrogen bond can be formed between the N-H of indole and Glu 183. This hydrogen bond, even if it does form, is not sufficient to be the major factor in keeping indole in its binding site in CPO [33]. However, it is reported that the hydrogen bond between Glu 183 and alcoholic OH group directs benzyl alcohol or 2-phenylethanol to a specific orientation [171].…”

“…CPO is able to catalyze a wide variety of reactions including halogenation [28,29,30], oxidation [31,32,33,34,35,36], N-demethylation [37,38], dehalogenation [39,40], and dismutation reactions [31] (see Figure 1.6). Significantly, the epoxidation and sulfoxidation reactions catalyzed by CPO are highly enantioselective [41,42,43,44,45,46,47], which suggests a potential application in pharmaceutical industry.…”

“…The substrates of CPO oxidation include a wide variety of organic compounds such as indole [33,55], monoterpenes [56], 4-chloroaniline (to 4-chloronitrosobenzene) [32], and the classical peroxidase substrates such as guaiacol, pyrogallol, and leucomalachite green [31]. In particular, the primary alcohols can be selectively oxidized to aldehyde in the presence of CPO and hydrogen peroxide (H 2 O 2 ) [34].…”

“…It is obvious to suspect the indolic N-H group as a structural feature controlling the position of oxidation, even though indole is an extremely weak base. However, the presence of a hydrogen-bond between the indole N-H group and an unknown group at the CPO active site has proven to be insufficient to explain the regiospecificity of indole oxidation [33]. Therefore, a detailed structural characterization of the CPO-indole complex would offer great insight into the mechanisms of the CPO-catalyzed oxidations and solve the long lasting puzzle of the unusual product from indole oxidation.…”

“…However, to the best of my knowledge, the structural basis for the unusual regioselectivity of indole oxidation of indole catalyzed by CPO remains undefined. Based on the structure of indole, the oxidation would be expected to occur at the 3-position of indole, which possesses the highest electron density and is thus most susceptible to attack by oxidizing agents and by other electrophilic reagents [33]. It is obvious to suspect the indolic N-H group as a structural feature controlling the position of oxidation, even though indole is an extremely weak base.…”