Enantioseparation of amino acids by micelle-enhanced ultrafiltration : experimental and theoretical studies of copper (II) amino acid interactions

A micelle-enhanced ultrafiltration system, which can potentially be used for large scale separations, has been used to investigate the resolution of amino acid enantiomers. For this purpose amino acid derivatives were synthesized, which in combination with copper(II) ions were used as chiral selectors. In the presence of non-ionic surfactants these selectors show for several amino acids high operational enantioselectivities of up to 14.5 for phenylglycine using cholesteryl l-glutamate as a selector. The performance of the system is determined by two factors: the hydrophobicity of the racemic amino acid and the stability of the diastereomeric complex formed.The interactions that are responsible for the chiral recognition have additionally been investigated using isothermal titration calorimetry. From this study it follows that the complexation reaction of amino acid to the chiral selector is an entropically driven process and the chiral recognition is more pronounced with increasing size of the substituent of the (racemic) amino acid. Information about the geometry of the diastereomeric complexes formed was obtained from quantum mechanical B3LYP calculations. From a basis set study, performed on the model complex bis(glycinato)Cu ^II ·2 H ₂ O, it follows that the basis set C, H, N, O = 6-311+G(d,p) and Cu = LanL2DZ offers the best ratio between accuracy and computation time. This basis set was used to investigate the complexes bis(glycinato)Cu ^II · <em>n</em> H ₂ O ( <em>n</em> = 1 - 4). From this study it was concluded that the optimized geometries strongly depend on the number of hydrogen bonds formed, and independent of the number of coordinating water molecules the <em>trans</em> isomer is more stable than the corresponding <em>cis</em> isomer.