Reacting to chiral separation

Surfactant-enzyme complexes successfully separate enantiomers.

Novel chemical techniques are being used to try to solve a problem that has troubled the chemical industry for many years: how to separate different enantiomers from racemic mixtures. A team of chemists from Kyushu University, Fukuoka, Japan, has combined supported liquid membranes (SLMs) and surfactant-enzyme complexes to develop an efficient method for separating different organic enantiomers.

SLMs consist of an organic liquid entrapped in a porous membrane and previously have been used to separate metal ions and amines. Several scientific teams have had limited success using SLMs to separate chiral organic compounds by utilising the enzyme lipase, which can selectively catalyse esterification and hydrolysis reactions with specific enantiomers.

To increase the efficacy of the lipase, the Japanese team combined it with a surfactant to produce a surfactant-lipase complex, which can be trapped within the SLM. The idea is that a racemic mixture is applied to one side of the SLM, where the surfactant-lipase complex catalyses the formation of ethyl esters of one of the enantiomers. These ethyl esters dissolve into the organic phase of the SLM and diffuse across the membrane, but the unmodified enantiomer is blocked. To test this idea, the team combined a SLM consisting of hydrophobic polypropylene film with a complex of the surfactant dioleyl- L-glutamate ribitol and a lipase from the microbe Candida rugosa. The researchers tested the method by attempting to separate enantiomers of ibuprofen from a racemic mixture, and achieved an enantiomeric excess of 91 per cent for ( S)-ibuprofen.

The method is highly adaptable and can be used to separate a range of organic enantiomers by simply changing the enzyme within the complex. For instance, the researchers successfully used a surfactant-α-chymotrypsin complex to separate enantiomers of phenylalanine.

Jon Evans