The sulphoxidation of thioanisole catalysed by lactoperoxidase and Coprinus cinereus peroxidase: Evidence for an oxygen-rebound mechanism

Using both stopped-flow and conventional spectroscopy, the oxygenation of m ethyl phenyl sulphide by both lactoperoxidase (LPO) and Coprinus cinereus p eroxidase (CIP) was monitored. Controlled continuous addition of H2O2 durin g turnover and monitoring the presence of native enzymes, compounds I, II a nd III, led to formation of the sulphoxide in high yield and enantioselecti vity. Under those conditions, LPO catalysed the formation of (R) methyl phe nyl sulphoxide with a yield of 85% and an enantiomeric excess (e.e.) of 80% . CiP catalysed the formation of (S) methyl phenyl sulphoxide with a yield of 84% and an e.e. of 73%. The enantioselective performance was markedly in fluenced by the purity of the enzymes used. Presence of compound III during turnover led to rapid inactivation of the peroxidases and, therefore, to b oth a lower yield of the sulphoxides and a lower enantioselectivity. Stoppe d-flow kinetic data show that, for both LPO and CiP, the transition of comp ound I to compound II depends on the concentration of the methyl phenyl sul phide, suggesting an oxygen-rebound mechanism. In line with this mechanism, a methyl phenyl sulphide radical cation was detected by EPR during turnove r for LPO.