Enantioselective complexation of amino acids by 6^A-deoxy-6^A-hydroxyethylamino-β-cyclodextrin and its metallo-derivatives in aqueous solution

Enantioselectivity towards several amino acids by metallo-6^A-deoxy-6^A-hydroxyethyl-amino-β-cyclodextrins (metallo-βCDea's) was investigated by potentiometric titration of the various amino acid/metallo-βCDea systems with NaOH solution. It was shown that the cyclodextrin derivative is capable of distinguishing between enantiomers of amino acid species in the presence of certain metal ions (Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺). Ni²⁺-βCDea complexes show the most enantioselectivity, whereas for Cu²⁺-and Co²⁺-βCDea complexes less selectivity is observed. As expected, Zn²⁺-βCDea complexes exhibit no enantioselectivity. Stability and selectivity, however, do not go hand in hand, since the most stable complexes are formed with Cu²⁺. Several factors play a role in determining stability and selectivity in binary and ternary complexes and further study is required to gain a more comprehensive understanding of these.