Oxidation of Aromatic Sulfides by Lignin Peroxidase from Phanerochaete Chrysosporium

The reaction of H2O2 With 4-substituted aryl alkyl sulfides (4-XC6H4SR), catalysed by lignin peroxidase (LiP) from Phanerochaete chrysosporium, leads to the formation of sulfoxides, accompanied by diaryl disulfides. The yields of sulfoxide are greater than 95% when X = OMe, but decrease significantly 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 influence on the efficiency of the reaction, except for R = tBu. The reaction exhibits enantioselectivity (up to 62% enantiomeric excess with X = Br, with preferential formation of the sulfoxide with S configuration). ;Enantioselectivity decreases with increasing electron density of the sulfide. Experiments in (H2O)-O-18 show partial or no incorporation of the labelled oxygen into the sulfoxide, with the extent of incorporation decreasing as the ring substituents become more electron-withdrawing. On the basis of these results, it is suggested that Lip compound I (formed by reaction between the native enzyme and H2O2), reacts with the sulfide to form a sulfide radical cation and LiP compound II. The radical cation is then converted to sulfoxide either by reaction with the medium or by a reaction with compound II, the competition between these two pathways depending on the stability of the radical cation.