T he design of artificial receptors for biomimetic complexation of carbohydrates remains an important challenge of supramolecular chemistry. Sugars are generally bound in cavities or clefts of proteins via multiple hydrogen-bonding interactions involving saccharide OH groups (1-3). X-ray structures show the exclusion of most water molecules, which can otherwise hamper proteinsaccharide interaction by competing for hydrogen bonds. In efforts to bind sugars with high affinity and selectivity, chemists have attempted to mimic biotic saccharide recognition with artificial receptors, using multiple hydrogen-bonding interactions in aprotic solvents (4). The detection of saccharide binding via noncovalent interactions has been attained mainly by means of 1 H-NMR spectroscopy, because most saccharides have neither a chromophore nor functional groups capable of protonation or deprotonation. Thus, it is very difficult or nearly impossible to design convenient saccharide-sensing systems. To the best of our knowledge, there are only a few examples in which circular dichroism (CD) or fluorescence spectroscopic methods, which are more expeditious for developing practical saccharide-sensing systems, have been adopted (12,(18)(19)(20).Several fused-pyridine receptors have been developed for recognition of creatinine, guanidinium, urea, and related molecules (refs. 21-24 and references in refs. 23 and 24). Among them, host 1 (Fig. 1) has two carboxylate anions and four basic nitrogens capable of forming both ionic and neutral hydrogenbonding interactions, making 1 an effective host for guanidines (e.g., arginine) in water (24). As discussed (25), the naphthyridine moiety and adjacent pyridine rings of 1 are twisted with respect to each other to avoid an energetically unfavorable eclipsed conformation in the CH 2 CH 2 linkages, and the cleft composed of four basic nitrogens is comparable in size to a monosaccharide (Fig. 1). We found consequently that 1 binds cationic monosaccharides with high affinity in methanol and that the binding event could be monitored by induced CD (ICD) arising from the twisting direction of the naphthyridine-pyridine NCCN torsional angle (25). We reasoned that a neutral analog of 1 might bind neutral saccharides in a less polar solvent and that complexation could be detected by ICD, as well as by fluorescence spectroscopy. We converted 1 to diamide 2 and now found that both CD and fluorescence spectra were sensitive to the binding of saccharides 3-9 (Scheme 1). (For methods used to obtain saccharide guests and to calculate association constants, see supporting information, which is published on the PNAS web site, www.pnas.org.) In particular, the CD sign clearly reflects the absolute configuration of the saccharide guest.The absorption spectra of diamide 2 in CHCl 3 obeyed the Lambert-Beer law over concentrations of 0 to 1.0 ϫ 10 Ϫ4 M. This result implies that 2 does not aggregate in CHCl 3 in this concentration range. As shown in Fig. 2, an ICD band and an excitoncoupling-type band (25) appear at 31...