Chiral Recognition and Separation of Amino Acids by Means of a Copper(II) Complex of Histamine Monofunctionalized β-Cyclodextrin

The copper(II) complex of a histamine-modified cyclodextrin (6-deoxy-6-N-histamine-beta-cyclodextrin, CDhm) was used for the chiral recognition of amino acids. HPLC separation of the enantiomers of unmodified aromatic amino acids (Phe, Trp, and Tyr) was obtained by using the complex [Cu(CDhm)](2+) as additive to the eluent and an achiral column C-18. Evidence for enantioselectivity was provided by thermodynamic and spectroscopic measurements. Potentiometric studies of the ternary complexes formed by [Cu(CDhm)](2+) and D- or L-amino acids showed that enantioselectivity in the complexation of aromatic amino acids occurs also in aqueous solution, the stability constants of the complexes containing the D-enantiomers of Trp, Phe, and Tyr being larger than those of the corresponding L-ones. In contrast, aliphatic amino acids showed small, if any, differences in the stability of diastereomeric ternary complexes and were not separated by HPLC. Calorimetric studies were carried out in order to determine the enthalpy and entropy contribution to enantioselectivity: the overall complexation process was found to be enthalpically and entropically favored. For the complexes containing aromatic amino acids, however, the enthalpy contribution was found to be more favorable for the D-enantiomers, while entropy was less favorable. These results are consistent with a model in which the complexation of the D-enantiomers is favored by the preferential inclusion of the aromatic side chain in the cyclodextrin cavity. Accordingly, the CD spectra of the complexes containing aromatic D-amino acids showed much higher intensity (Delta epsilon) than those of the corresponding L-ones, the difference Delta(Delta epsilon) increasing as the size of the side chain increased. Furthermore, the fluorescence of D-Trp in the ternary complex was found to be smaller than that of L-Trp. Fluorescence lifetime measurements suggested that enantioselectivity in fluorescence could arise from the more efficient quenching of D-Trp by copper(II) ion, due to conformational constraints holding the indole moiety near the metal ion.