Molecular Recognition Study of a Supramolecular System

The competitive inclusion method was used to determine the stability constants (logKa) for complexation of natural α-cyclodextrin (1) with aliphatic amino acids in acidic solution. The stability constants (logKa) for the complexation of modified α-cyclodextrin, mono- [6-(1-pyridinio)-6-deoxy]-α-cyclodextrin (2), with these biological molecules were measured using the differential spectra method. Both natural α-cyclodextrin (1) and chemically modified α-cyclodextrin (2) can recognize not only the size of guest amino acid molecules and the length of their hydrophobic side chains, but also the chirality of enantiotopicL/D-amino acids. The longer the hydrophobic side chain borne on an amino acid, the more stable the complex formed by α-cyclodextrin hosts (1) and (2). Host compounds (1) and (2) preferably includeL-amino acids, which would benefit from discrimination ofL/D-amino acids. Introduction of a positively charged 1-pyridinio moiety to C-6 of α-cyclodextrin apparently enlarges the inclusion ability and enantioselectivity by electrostatic interaction. Comparing the data obtained in acidic medium with those formerly measured in pH 7.20 phosphate buffer solution, a result can be given: when the pH of the medium increases, the complexation ability and selectivity of modified α-cyclodextrin (2) for most amino acids examined here are slightly enhanced, showing the highest enantioselectivity up to 10.3 forL/D-serine.