Separation of amino acid enantiomers by micelle-enhanced ultrafiltration.
A Micelle-enhanced ultrafiltration (MEUF) separation process was investigated that can potentially be used for large-scale enantioseparations. Copper(II)-amino acid derivatives dissolved in nonionic surfactant micelles were used as chiral selectors for the separation of dilute racemic amino acids solutions. For the alpha-amino acids phenylalanine, phenylglycine, O-methyltyrosine, isoleucine, and leucine good separation was obtained using cholesteryl L-glutamate and Cu(II) ions as chiral selector with an operational enantioselectivity (alpha(op)) up to 14.5 for phenylglycine. From a wide set of substrates, including four beta-amino acids, it was concluded that the performance of this system is determined by two factors: the hydrophobicity of the racemic amino acid, which results in a partitioning of the racemic amino acid over micelle and aqueous solution, and the stability of the diastereomeric complex formed upon binding of the amino acid with the chiral selector. The chiral hydrophobic cholesteryl anchor of the chiral selector also plays an active role in the recognition process, since inversion of the chirality of the glutamate does not yield the reciprocal enantioselectivities. However, if the cholesteryl group is replaced by a nonchiral alkyl chain, reciprocal operational enantioselectivities are found with enantiomeric glutamate selectors.