Fluorescent cyclodextrins for molecule sensing: Fluorescent properties, NMR characterization, and inclusion phenomena of N-dansylleucine-modified cyclodextrins

Fluorophore-amino acid-cyclodextrin (CD) triad systems, N-dansyl-L-leucine-modified and N-dansyl-D-leucine-modified β-CD (1, 2) and N-dansyl-L-leucine-modified and N-dansyl-D-leucine-modified γ-CD (3, 4), were synthesized and characterized as fluorescent indicators of molecular recognition. Fluorescence decay analyses of these CD derivatives indicated that there exist two lifetime components, being in equilibrium with each other in aqueous solution, with the dansyl moiety included in its own cavity (self-inclusion) for the larger lifetime component while located outside the cavity for the shorter lifetime one. The structural analyses of β-CD derivatives 1 and 2 undertaken by combined use of 1D and 2D NMR spectra indicate that the dansyl moiety of 2 is more deeply included in the CD cavity than that of 1. The leucine residue between the dansyl and the CD moieties of these hosts was more effective in enhancing the binding abilities for various guests when compared with the glycine residue of the corresponding hosts. The difference in the enantiomeric configuration of the leucine residue caused the difference in the binding constants with larger values for 1 than 2 and with the opposite trend for the γ-CD derivatives 3 and 4. Upon guest addition, the fluorescence intensities of 1 and 2 decreased, reflecting the exclusion of the dansyl moiety from inside to outside of the β-CD cavity, while the fluorescence intensities of the γ-CD derivatives 3 and 4 depended on the guest as shown by the increase induced by cyclohexanol and the decrease by (-)-borneol and other larger guests. These guest-responsive variations of the fluorescence intensity enabled these hosts to be used as effective fluorescent indicators of molecular recognition.