Separation of amino acid enantiomers by Micelle-enhanced ultrafiltration

A Micelle-enhanced ultrafiltration (MEUF) separation process was investigat ed that can potentially be used for large-scale enantioseparations. Copper( II)amino acid derivatives dissolved in nonionic surfactant micelles were us ed as chiral selectors for the separation of dilute racemic amino acids sol utions. For the alpha-amino acids phenylalanine, phenylglycine, O-methyltyr osine, isoleucine, and leucine good separation was obtained using cholester yl L-glutamate and Cu(II) ions as chiral selector with an operational enant ioselectivity (alpha(op)) up to 14.5 for phenylglycine. From a wide set of substrates, including four beta-amino acids, it was concluded that the perf ormance of this system is determined by two factors: the hydrophobicity of the racemic amino acid, which results in a partitioning of the racemic amin o acid over micelle and aqueous solution, and the stability of the diastere omeric complex formed upon binding of the amino acid with the chiral select or. The chiral hydrophobic cholesteryl anchor of the chiral selector also p lays an active role in the recognition process, since inversion of the chir ality of the glutamate does not yield the reciprocal enantioselectivities. However, if the cholesteryl group is replaced by a nonchiral alkyl chain, r eciprocal operational enantioselectivities are found with enantiomeric glut amate selectors. (C) 2000 Wiley-Liss, Inc.