Circular dichroism readout of sugar recognition in the cleft of a fused-pyridine receptor

Dicarboxamide host 2 forms 1:1 complexes with n-octyl pyranosides derived from d-glucose, d-mannose, d-galactose, d-fucose, d-lyxose, and d-arabinose. Association constants (K(a)) in the range of 77–940 M(−1) were measured in chloroform by means of induced circular dichroism and fluorescence spectroscopy. Variations in K(a) values correspond qualitatively to expected differences in hydrogen-bonding abilities of guest hydroxyl groups. Induced circular dichroism effects for complexes of saccharides bearing equatorial 3-OH, 4-OH, and 6-OH groups show that the host chromophore is twisted in a P-helical conformation. A structural model is proposed that is also consistent with the results of previous studies involving complexation of dicarboxylic acid 1 with cationic saccharides in methanol.