Steric Requirements for the Active Site of a Lipase from Candida Rugosa Studied by the use of a Sulfinyl Group as a Chiral Probe

Abstract

This work presents an investigation of the Candida rugosa lipase-catalyzed ester hydrolysis of a number of carboxy-functionalized sulfoxides. In the substrates studied, the sulfinyl group is preferentially located in the β-position of the acid part of the molecule. The excellent chiral recognition obtained for hydrolysis of methy1 2-(octylsulfinyl)benzoate, 1, (E > 100 in favour of the (S)-enantiomer) was found to fall off with decreasing steric constraints in the molecular structure, and from the results obtained from the thio and sulfonyl analogs of 1, the steric and electronic effects of the oxygen substituents at the sulfur atom have been rationalized in terms of an active site binding model. Of two proteins isolated from a heterogenous lipase fraction of Candida rugosa, the one with the higher pi displayed the highest hydrolytic activity as well as enantioselectivity towards 1. A desymmetrization of the prochiral compound bis(2-carbomethoxyphenyl)sulfoxide was obtained with an ee_p of 93.5% of the favoured mono methyl ester. Comparative experiments including lipases from Aspergillus niger, Candida antarctica B and Pseudomonas sp. AK showed that the Candida rugosa lipase gave the highest enantioselectivity.

Key Words:

• Sulfoxides
• Sulfinyl group
• Ester hydrolysis
• Kinetic resolution
• Lipases
• Candida rugosa