A novel bis(β-cyclodextrin) was synthesized, and its binding behavior with steroids was investigated to demonstrate that the cooperative co-inclusion of guest and tether by two cyclodextrin moieties is operative to afford the highest molecular selectivity of up to 3.6 for deoxycholate over taurocholate.

A series of bridged β-cyclo- dextrin(β-CyD) dimers possessing functional tethers of various lengths was synthesized in moderate yield by the treatment of 2,2’-biquinoline- 4,4’-dicarboxylic dichloride with β-CyD or mono[6-oligo(ethylenediamino)-6- deoxy]-β-CyDs. The products were 2,2’-biquinoline-4,4’-dicarboxy-bridged bis(6-O-β-CyD)(8), N,N’-bis(2-amino- ethyl)-2,2’-biquinoline-4,4’-dicarbox- amide-bridged bis(6-amino-6-deoxy-β- CyD)(9), and N,N’-bis(5-amino-3-aza- pentyl)-2,2’-biquinoline-4,4’-dicarbox- amide-bridged bis(6-amino-6-deoxy-β- CyD)(10). The reaction of 8-10 with copper perchlorate give their copper(II) complexes 11-13 in satisfactory yields of over 77%. All the bis(β-CyD)s 8-13 act as efficient fluorescent sensors and display remarkable fluorescence enhancement upon addition of optically inert steroids. The inclusion complexa- tion behaviors of 8-13 when treated with the representative steroids cholate(14), deoxycholate(15), and glycocho- late(16) in aqueous solution at 25℃ were investigated by means of UV/Vis spectroscopy, conductivity and fluorescence measurements, circular dichroism spectroscopy, and 2D NMR spectroscopy. The tether length of bis(β-CyD) 9 allows it to adopt a cooperative host-tether-guest binding mode in which the spacer and guest are co-included in the two CyD cavities. As a result of this cooperation, 9 has a stability constant[Ks] about 2×102 times higher than that of monomodified β-CyD 4 for inclusion complexation with cholate. Metallooligo(β-CyD)s with four β- CyD units have enhanced binding abilities compared with monomodified β-CyDs. These metallo compounds have binding affinities for guest steroids that are up to 50-4.1 ×103 times higher than those of CyDs 2-4. The guest-induced fluorescence enhancement of bis(CyD)s opens a new channel for the design of sensor materials. The complex stability constants of these compounds are discussed from the viewpoint of induced-fit interaction and cooperative multiple binding between host and guest.

Bis(polypseudorotaxane)s possessing the bridged bis(β-cyclodextrin)s with multi coordinated metal centers are prepared, in which β-cyclodextrin (β-CD) acts as the host structure, amino-terminated polypropylene glycol(PPG; MW=2000) as the guest, and nickel(II) complexes as templating moieties. The assembly behavior is comprehensively studied by NMR spectroscopy, gel permeation chromatography(GPC), end group assay, static light scattering, thermogravimetric(tG), differential thermal analysis(dTA), and scanning tunneling microscopy(STM). The obtained results indicate that not only could the two PPG molecules be respectively threaded into metallo-bridged bis(β-CD)s forming the double-strand structure, but they also could play a stabilizing role in the molecular aggregation. The fluorescence titrations are performed in a mixture of acetonitrile/water to calculate the effective binding constants and thermodynamic parameters for the threading process of 2 onto PPG. The results reveal that the assembly process as being driven by favorable entropic contributions accompanying a large positive enthalpic change. The present investigations provide a simple way to prepare multicomponent nanometer-scale organic supermolecules with multiple CD dimers coordinated metal centers.

A series of β-cyclodextrin(β-CD) dimers with 4,4′-diselenobis(benzoyl) linkers, that is, 6,6′-[4,4′-diselenobis-(benzoyloxyl)]-bridged bis(β-CD)(1a), 6,6′-[4,4′-diselenobis(2-(benzoylamino)ethyleneamino)]-bridged bis(β-CD)(2a), and 6,6′-[4,4′-diselenobis(2-(benzoylamino)-3,6-diazaoctylamino)]-bridged bis(β-CD)(3a), were synthesized in moderate yields by the reaction of 4,4′-diselenobis(benzoic acid] with β-CD or oligo(ethylenediamino)-6-deoxy-β-CD. Their binding behaviors with some structure-related substrates, such as acridine red(AR), neutral red(NR), rhodamine B(RhB), ammonium 8-anilino-1-naphthalenesulfonate(ANS), and 6-p-toluidino-2-naphthalenesulfonic acid(tNS), were investigated in aqueous phosphate buffer solution(pH 7.20) at 298.15 K by means of fluorescence, NMR, as well as circular dichroism spectroscopy and compared with those of their 2,2′-diselenobis(benzoyl)-linked analogues, that is, 6,6′-[2,2′-diselenobis-(benzoyloxyl)]-bridged bis(β-CD)(1b), 6,6′-[2,3′-diselenobis(2-(benzoylamino)ethyleneamino)]-bridged bis-(β-CD)(2b), and 6,6′-[2,2′-diselenobis(2-(benzoylamino)-3,6-diazaoctylamino)]-bridged bis(β-CD)(3b). The results showed that bis(β-CD)s 1a-3a, whose Se-Se bonds were located at the para position of the carboxyl group, gave stronger binding abilities toward nonlinear guests(RhB and ANS) than their analogues 1b-3b, whose Se-Se bonds were located at the ortho position relative to the carboxyl group. The molecular binding ability and selectivity of model substrates by these ditopic hosts were sufficiently discussed to reveal not only the cooperative contributions of the linker group and CD cavities upon inclusion complexation with dye guest molecules but also the controlling factors for the molecular selective binding.