Enantioselective recognition for N-octanoyltryptophan by modified α-cyclodextrin possessing convergent recognition sites

A novel fluorescent derivative of α-cyclodextrin (α-CD), possessing a pyrenylmethylamino moiety (1), was synthesized to investigate its chiral discrimination ability for N-octanoyltrptophan (D-Trp-C₈ and L-Trp-C₈). From the pH-dependent fluorescence variations, the pK _a of 1 was determined to be 6.95. Under a weakly acidic condition (pH 5.9), although an ammonium form prevailed for 1 and, as a result, photo-induced electron transfer should be absent, the fluorescence of 1 was markedly quenched, probably due to electron or energy transfer between the pyrene residue of 1 and the indole moiety of the guests. A simple 1:1 host-guest complexation model was successfully applied to the guest concentration-dependent fluorescence variations, yielding 1:1 host-guest association constants for D-Trp-C ₈ and L-Trp-C₈ as 2160 and 1200 M^-1, respectively, while the values were decreased to 790 and 557 M^-1, respectively, at pH 4.0, where the carboxy groups of the guests protonated. In ¹H-NMR spectra measured with a mixture of D-Trp-C₈ and 1, significant upfield shifts were observed for the aromatic signals of both D-Trp-C₈ and 1, indicating that the two aromatic groups existed close to each other. On the other hand, no such significant upfield shifts were observed for a mixture of L-Trp-C₈ and 1. These results indicate that the pyrene residue and the ammonium group of 1 provided effective recognition sites to 1 in addition to the α-CD cavity when 1 complexed with D-Trp-C₈, but not with D-Trp-C₈.