MOLECULAR RECOGNITION STUDY OF A SUPRAMOLECULAR SYSTEM .11. CHIRAL RECOGNITION OF ALIPHATIC AMINO-ACIDS BY NATURAL AND MODIFIED ALPHA-CYCLODEXTRINS IN ACIDIC AQUEOUS-SOLUTION

The competitive inclusion method was used to determine the stability c onstants (log K-a) for complexation of natural alpha-cyclodextrin (1) with aliphatic amino acids in acidic solution. The stability constants (log K-a) for the complexation of modified alpha-cyclodextrin, mono-[ 6-(1-pyridinio)-6-deoxy]-alpha-cydodextrin (2), with these biological molecules were measured using the differential spectra method. Both na tural alpha-cyclodextrin (1) and chemically modified alpha-cyclodextri n (2) can recognize not only the size of guest amino acid molecules an d the length of their hydrophobic side chains, but also the chirality of enantiotropic L/D-amino acids. The longer the hydrophobic side chai n borne on an amino acid, the more stable the complex formed by alpha- cyclodextrin hosts (1) and (2). Host compounds (1) and (2) preferably include L-amino acids, which would benefit from discrimination of L/D- amino acids. Introduction of a positively charged 1-pyridinio moiety t o C-6 of alpha-cyclodextrin apparently enlarges the inclusion ability and enantioselectivity by electrostatic interaction. Comparing the dat a obtained in acidic medium with those formerly measured in pH 7.20 ph osphate buffer solution, a result can be given: when the pH of the med ium increases, the complexation ability and selectivity of modified al pha-cyclodextrin (2) for most amino acids examined here are slightly e nhanced, showing the highest enantioselectivity up to 10.3 for L/D-ser ine. (C) 1997 Academic Press.