Asymmetric sulfoxidation of a β-carbonyl sulfide series by chloroperoxidase

PhI(OAc)2-Promoted 1,2-Transfer Reaction between 1,1-Disubstituted Allylic Alcohols and Thiophenols

The PhI(OAc)2-promoted 1,2-transfer reaction between allylic alcohols and thiophenols, conducted in an argon atmosphere, has proven to be effective in producing β-carbonyl sulfides from 1,1-disubstituted allylic alcohols in high yields. This method offers a fast and efficient way to synthesize β-carbonyl sulfides, which are valuable intermediates in organic synthesis. This discussion focuses on the effects of the oxidizer, temperature, and solvent on the reaction. A proposed tentative mechanism for this reaction is also discussed.

Recent developments in the use of peroxygenases - Exploring their high potential in selective oxyfunctionalisations

Peroxygenases are an emerging new class of enzymes allowing selective oxyfunctionalisation reactions in a cofactor-independent way different from well-known P450 monooxygenases. Herein, we focused on recent developments from organic synthesis, molecular biotechnology and reaction engineering viewpoints that are devoted to bring these enzymes in industrial applications. This covers natural diversity from different sources, protein engineering strategies for expression, substrate scope, activity and selectivity, stabilisation of enzymes via immobilisation, and the use of peroxygenases in low water media. We believe that peroxygenases have much to offer for selective oxyfunctionalisations and we have much to study to explore the full potential of these versatile biocatalysts in organic synthesis.

Specific oxyfunctionalisations catalysed by peroxygenases: opportunities, challenges and solutions

Peroxygenases are promising oxyfunctionalisation catalysts for organic synthesis.

Protein engineering of nirobenzene dioxygenase for enantioselective synthesis of chiral sulfoxides

Nitrobenzene dioxygenase (NBDO) from Comamonas sp. is shown here to perform enantioselective oxidation of aromatic sulfides. Several para-substituted alkyl aryl sulfides were examined and it was found that the activity of the enzyme is dependent on the size of the substrate. Saturation mutagenesis was performed on different residues in the active site in order to improve activity and selectivity. Mutagenesis at position 258 in the α-hydroxylase subunit of NBDO improved both activity and enantioselectivity. Substitutions in position 293 improved the activity on all substrates and had diverse influence on enantioselectivity. Mutagenesis in position 207 provided two interesting variants, V207I and V207A, with opposite enantioselectivities. Furthermore, combining two favorable mutations, N258A and F293H, provided an improved variant with both higher activity (5.20 ± 0.01, 2.12 ± 0.21, 2.64 ± 0.14 and 4.01 ± 0.34 nmol min(-1) mg protein(-1) on thioanisole, ptolyl, Cl-thioanisole and Br-thioanisole, respectively, which is 1.7, 4.6, 7.1 and 26.7-fold compared with wild type) and improved enantioselectivity (e.g. 67% enantiomeric excess for Cl-thioanisole vs. 5% for wild type). Molecular docking and active site volume calculations were used to correlate between the structure of the substrates and the function of the enzymes. The results from this work suggest that the location of pro-chiral sulfides in the active site is coordinated by hydrophobic interactions and by steric considerations, which in turn influences the activity and enantioselectivity of NBDO.

Rapid methods for high-throughput detection of sulfoxides

Enantiopure sulfoxides are prevalent in drugs and are useful chiral auxiliaries in organic synthesis. The biocatalytic enantioselective oxidation of prochiral sulfides is a direct and economical approach for the synthesis of optically pure sulfoxides. The selection of suitable biocatalysts requires rapid and reliable high-throughput screening methods. Here we present four different methods for detecting sulfoxides produced via whole-cell biocatalysis, three of which were exploited for high-throughput screening. Fluorescence detection based on the acid activation of omeprazole was utilized for high-throughput screening of mutant libraries of toluene monooxygenases, but no active variants have been discovered yet. The second method is based on the reduction of sulfoxides to sulfides, with the coupled release and measurement of iodine. The availability of solvent-resistant microtiter plates enabled us to modify the method to a high-throughput format. The third method, selective inhibition of horse liver alcohol dehydrogenase, was used to rapidly screen highly active and/or enantioselective variants at position V106 of toluene ortho-monooxygenase in a saturation mutagenesis library, using methyl-p-tolyl sulfide as the substrate. A success rate of 89% (i.e., 11% false positives) was obtained, and two new mutants were selected. The fourth method is based on the colorimetric detection of adrenochrome, a back-titration procedure which measures the concentration of the periodate-sensitive sulfide. Due to low sensitivity during whole-cell screening, this method was found to be useful only for determining the presence or absence of sulfoxide in the reaction. The methods described in the present work are simple and inexpensive and do not require special equipment.

Protein engineering of toluene monooxygenases for synthesis of chiral sulfoxides

Enantiopure sulfoxides are valuable asymmetric starting materials and are important chiral auxiliaries in organic synthesis. Toluene monooxygenases (TMOs) have been shown previously to catalyze regioselective hydroxylation of substituted benzenes and phenols. Here we show that TMOs are also capable of performing enantioselective oxidation reactions of aromatic sulfides. Mutagenesis of position V106 in the alpha-hydroxylase subunit of toluene ortho-monooxygenase (TOM) of Burkholderia cepacia G4 and the analogous position I100 in toluene 4-monooxygenase (T4MO) of Pseudomonas mendocina KR1 improved both rate and enantioselectivity. Variant TomA3 V106M of TOM oxidized methyl phenyl sulfide to the corresponding sulfoxide at a rate of 3.0 nmol/min/mg protein compared with 1.6 for the wild-type enzyme, and the enantiomeric excess (pro-S) increased from 51% for the wild type to 88% for this mutant. Similarly, T4MO variant TmoA I100G increased the wild-type oxidation rate by 1.7-fold, and the enantiomeric excess rose from 86% to 98% (pro-S). Both wild-type enzymes showed lower activity with methyl para-tolyl sulfide as a substrate, but the improvement in the activity and enantioselectivity of the mutants was more dramatic. For example, T4MO variant TmoA I100G oxidized methyl para-tolyl sulfide 11 times faster than the wild type did and changed the selectivity from 41% pro-R to 77% pro-S. A correlation between regioselectivity and enantioselectivity was shown for TMOs studied in this work. Using in silico homology modeling, it is shown that residue I100 in T4MO aids in steering the substrate into the active site at the end of the long entrance channel. It is further hypothesized that the main function of V106 in TOM is the proper positioning or docking of the substrate with respect to the diiron atoms. The results from this work suggest that when the substrate is not aligned correctly in the active site, the oxidation rate is decreased and enantioselectivity is impaired, resulting in products with both chiral configurations.

Chloroperoxidase-catalyzed oxidation of methionine derivatives

Treatment of N-methoxycarbonyl C-carboxylate ester derivatives of L- and D-methionine and L-ethionine by chloroperoxidase–hydrogen peroxide resulted in oxidation at sulfur to produce the (RS) sulfoxide in moderate to high diastereomeric excess. The (RS) sulfoxide of methionine was also obtained in moderate to high diastereomeric excess from (±)SO-N-methoxycarbonyl-L-methionine methyl ester sulfoxide by ester hydrolysis using α-chymotrypsin, Aspergillus sp. protease or subtilisin Carlsberg. Key words: amino acid oxidation, biocatalysis, biotransformation, chloroperoxidase, enzyme catalysis, lipase, sulfoxidation.

Selective oxygen transfer catalysed by heme peroxidases: synthetic and mechanistic aspects

The synthetic and mechanistic aspects of the use of heme peroxidases as functional mimics of the cytochrome P450 monooxygenases in oxygen-transfer reactions have been described. The chloroperoxidase from Caldariomyces fumago (CPO) is the catalyst of choice in sulfoxidation, hydroxylation and epoxidation on account of its high activity and enantioselectivity. Other heme peroxidases were less active by orders of magnitude; protein engineering has resulted in impressive improvements but even the most active mutant was still at least an order of magnitude less active than CPO. The 'oxygen-rebound' mechanisms of oxygen transfer mediated by heme enzymes - as originally conceived - have proved to be untenable. Dual pathway mechanisms, via oxoferryl species that insert oxygen as well as iron hydroperoxide species that insert OH(+), have been proposed that accommodate all of the known experimental data.