Molecular recognition of amino acids by copper(II) complexes of 6(A),6(X)-diamino-6(A),6(X)-dideoxy-beta-cyclodextrin (X=B,C,D)

The three regioisomers of beta-cyclodextrin 6-difunctionalized with NH2 groups (6(A),6(X)-diamino-6(A),6(X)-dideoxy-beta-cyclodextrin, A,X-CDNH2, X = B, C, or D) were synthesized. Their binary and ternary copper(II) complexes with amino acids were characterized by ESR and electronic spectroscopy. Furthermore, the binary copper(II) complexes were used as eluent in ligand exchange chromatography (LEC), to resolve racemates of unmodified amino acids. HPLC separation of enantiomers of aromatic amino acids was obtained only when the complex [Cu(A,B-CDNH2)](2+) was used as eluent. The two complexes with the other two regioisomers did not show chiral recognition ability. Circular dichroism (c.d.) spectroscopy studies of the ternary complexes with D- and L-amino acids carried out in the presence and in the absence of 1-adamantanol, suggested a recognition mechanism that involves the cyclodextrin cavity, only in the case of ternary A,B-CDNH2 complexes.