Enantioselective recognition of aliphatic amino acids by β-cyclodextrin derivatives bearing aromatic organoselenium moieties on the primary or secondary side

Spectrophotometric titrations have been performed in order to determine the stability constants of inclusion complexation of some aliphatic amino acids with four structurally related organoselenium-modified beta-cyclodextrins: mono(6-phenylseleno-6-deoxy)-beta-cyclodextrin (1a), mono[6-(beta-methoxyphenylseleno)-6-deoxy]-beta-cyclodextrin (1b), mono (2-phenylseleno-2-deoxy)-beta-cyclodextrin (2a), and mono [2-(beta-methoxyphenylseleno)-2-deoxyl-beta-cyclodextrin (2b). Conformation analysis by circular dichroism and 2D NMR spectorscopic studies revealed that the aryl-substituted beta-cyclodextrins gave self-inclusion intramolecular complexes in aqueous solution, while the extent of penetration depended both on the positions and on the structures of substituents. Quantitative investigation on the binding ability of the hosts with amino acids showed that they were able to recognize the size and the shape of guests, affording supramolecular complexes with quite small stability constants ranging from 24 to 355 W. The molecular recognition abilities are discussed from the viewpoints of induced-fitting mechanisms, geometric complementary, and cooperative binding processes. Furthermore, these beta-cyclodextrin derivatives displayed considerable enantioselectivity towards L/D-amino acid isomers, giving the highest L-enantioselectivity (up to 8.4) for inclusion complexation between leucine and 2a. The binding modes Of L/D-leucine with 1b were elucidated from NOESY studies and the chiral recognition phenomena were interpreted accordingly. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)