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Washed-cell preparations of recombinant Escherichia coli JM109(pDTG141), engineered to express the naphthalene dioxygenase (NDO) gene from Pseudomonas sp. NCIB 9816-4, have been used to biooxidise a range of aryl alkyl-, dialkyl- and bicyclic sulfides. A series of 16 phenyl alkyl sulfides was oxidised to equivalent sulfoxides, typically with moderate to high (>90%) yield and high enantioselectivity (>85% ee), the ( S )-enantiomer being the predominant product, with little if any further oxidation. The addition of some electron-donating or electron-withdrawing groups to the phenyl ring decreased yield and/or stereoselectivity of the NDO-catalysed biotransformation, whereas increasing the size of the alkyl chain ( n C 3 H 7 , i C 3 H 7 and n C 4 H 9 ) resulted in a notable inversion in selectivity to yield ( R )-series sulfoxides (>74% ee) as the predominant products. The addition of one or more methylene groups between the phenyl ring and sulfur atom resulted in notable reductions in both the yield and stereoselectivity of the ( S )-predominant sulfoxidations. With the exception of cyclohexyl- and n -hexyl methyl sulfide which both gave ( S )-sulfoxides with good stereoselectivity and yield, other dialkyl- and bicyclic sulfides were poor substrates for sulfoxidation by NDO. Both the close agreement with data obtained using purified NDO and the absence of stereoselective sulfoxidation in equivalent controls with the E. coli JM109 host support the contribution made by the cloned NDO carried on the pDTG141 plasmid. |
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