Oxidation of aromatic sulfides by lignin peroxidase from Phanerochaete chrysosporium

The reaction of H2O2 With 4-substituted aryl alkyl sulfides (4-XC6H4SR), ca talysed by lignin peroxidase (LiP) from Phanerochaete chrysosporium, leads to the formation of sulfoxides, accompanied by diaryl disulfides. The yield s of sulfoxide are greater than 95% when X = OMe, but decrease significantl y as the electron donating power of the substituent decreases. No reaction is observed for X = CN. The bulkiness of the R group has very little influe nce on the efficiency of the reaction, except for R = tBu. The reaction exh ibits enantioselectivity (up to 62% enantiomeric excess with X = Br, with p referential formation of the sulfoxide with S configuration). ;Enantioselec tivity decreases with increasing electron density of the sulfide. Experimen ts in (H2O)-O-18 show partial or no incorporation of the labelled oxygen in to the sulfoxide, with the extent of incorporation decreasing as the ring s ubstituents become more electron-withdrawing. On the basis of these results , it is suggested that Lip compound I (formed by reaction between the nativ e enzyme and H2O2), reacts with the sulfide to form a sulfide radical catio n and LiP compound II. The radical cation is then converted to sulfoxide ei ther by reaction with the medium or by a reaction with compound II, the com petition between these two pathways depending on the stability of the radic al cation.