Chiral separation of several naphthalenesulfonylamino acids by capillary zone electrophoresis with selectively methylated cyclodextrin derivatives

The chiral separation of six pairs of α- and β-naphthalenesulfonylamino acid enantiomers was examined by capillary zone electrophoresis using selectively methylated β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD) derivatives as chiral selectors. On the whole, the γ-CD derivatives exhibited higher enantioselectivities than the β-CD derivatives. Unmodified γ-CD and the γ-CD derivative whose 6-hydroxyl groups were methylated (6-monomethylated γ-CD) exhibited similar high enantioselectivities. After methylation of the hydroxyl groups at the 3-position of the unmodified and 6-monomethylated γ-CDs, their chiral recognition ability for β-naphthalenesulfonylamino acids was drastically enhanced. Methylation of both the 2- and 3-positions of γ-CD resulted in the complete disappearance of the enantioselectivity regardless of 6-monomethylation. In addition, both β- and γ-CD derivatives showed different enantioselectivities for α-naphthalenesulfonylamino acids from those for β-naphthalenesulfonylamino acids, especially remarkable in the case of unmodified and 6-monomethylated CDs. Using 1H-NMR measurements, it was confirmed that the naphthyl group of α-naphthalenesulfonyl-DL-leucine penetrated more deeply into the γ-CD cavity than into that of β-CD and that the naphthyl group of β-naphthalenesulfonyl-DL-leucine penetrated more deeply into the β-CD cavity than that of α-naphthalenesulfonyl-DL-leucine.