Binding Behavior of Aliphatic Oligopeptides by Bridged and Metallobridged Bis((beta)-cyclodextrin)s Bearing an Oxamido Bis (2-benzoic) Carboxyl Linker

(beta)-Cyclodextrin dimers bearing an oxamido bis(2-benzoic) carboxyl linker (1) or its metal complexes (2 and 3) were newly synthesized, and their inclusion complexation behavior with a series of representative aliphatic oligopeptides, i.e., LeuGly, Gly-Leu, Gly-Pro, Glu-Glu, Gly-Gly, Gly-Gly-Gly, and Glu(Cys-Gly), was elucidated by means of UV/vis, circular dichroism, fluorescence, and 2D NMR spectroscopy in Tris-HCl buffer solution (pH 7.4) at 25 (degree)C. The results obtained indicated that metallobridged bis((beta)-cyclodextrin)s 2 or 3 could significantly enhance the original molecular binding abilities of parent bis((beta)-cyclodextrin) 1 toward model substrates through the cooperative binding of two cyclodextrin moieties and the additional chelation effect supplied by the coordinated metal centers. It is interesting that hosts 2 and 3 displayed an entirely different fluorescence behavior upon complexation with guest oligopeptides. Among the guest peptides examined, 3 showed the highest complex formation constant of 68 200 M^-1 for Glu-Glu, up to 510-fold as compared with 1 (135 M-1), while 1 gave excellent molecular selectivity for Glu(Cys-Gly)/Glu-Glu pair, up to 51-fold. The molecular binding ability and selectivity were discussed from the viewpoints of the induced-fit and multiple recognition mechanism between host and guest.