CHIRAL RECOGNITION AND SEPARATION OF AMINO-ACIDS BY MEANS OF A COPPER(II) COMPLEX OF HISTAMINE MONOFUNCTIONALIZED BETA-CYCLODEXTRIN

The copper(II) complex of a histamine-modified cyclodextrin (6-deoxy-6 -N-histamine-beta-cyclodextrin, CDhm) was used for the chiral recognit ion of amino acids. HPLC separation of the enantiomers of unmodified a romatic amino acids (Phe, Trp, and Tyr) was obtained by using the comp lex [Cu(CDhm)](2+) as additive to the eluent and an achiral column C-1 8. Evidence for enantioselectivity was provided by thermodynamic and s pectroscopic measurements. Potentiometric studies of the ternary compl exes formed by [Cu(CDhm)](2+) and D- or L-amino acids showed that enan tioselectivity in the complexation of aromatic amino acids occurs also in aqueous solution, the stability constants of the complexes contain ing the D-enantiomers of Trp, Phe, and Tyr being larger than those of the corresponding L-ones. In contrast, aliphatic amino acids showed sm all, if any, differences in the stability of diastereomeric ternary co mplexes and were not separated by HPLC. Calorimetric studies were carr ied 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 a romatic amino acids, however, the enthalpy contribution was found to b e more favorable for the D-enantiomers, while entropy was less favorab le. These results are consistent with a model in which the complexatio n of the D-enantiomers is favored by the preferential inclusion of the aromatic side chain in the cyclodextrin cavity. Accordingly, the CD s pectra of the complexes containing aromatic D-amino acids showed much higher intensity (Delta epsilon) than those of the corresponding L-one s, the difference Delta(Delta epsilon) increasing as the size of the s ide chain increased. Furthermore, the fluorescence of D-Trp in the ter nary complex was found to be smaller than that of L-Trp. Fluorescence lifetime measurements suggested that enantioselectivity in fluorescenc e 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.