Cooperative multipoint recognition of organic dyes by bis(β-cyclodextrin)s with 2,2′-bipyridine-4,4′-dicarboxy tethers

A series of novel 6,6′-bis(β-cyclodextrin)s linked by 2,2′-bipyridine-4,4′-dicarboxy tethers; that is, 2,2′-bipyridine-4,4′-dicarboxy-bridged bis(6-O-β-cyclodextrin) (2) and N,N′-bis(2-aminoethyl)-2,2′-bipyridine-4, 4′-dicarboxamide-bridged (3), N,N′-bis(5-amino-3-azapentyl)-2,2′-bipyridine-4, 4′-dicarboxamide-bridged (4) and N,N′-bis(8-amino-3,6-diazaoctyl)-2,2′-bipyridine-4, 4′-dicarboxamide-bridged bis(6-amino-6-deoxy-β-cyclodextrin) (5), has been synthesized as cooperative multipoint-recognition receptor models. The inclusion complexation behavior of 2-5 with organic dyes; that is, ammonium 8-anilino-1-naphthalenesulfonate, Brilliant Green, Methyl Orange, Acridine Red, and Rhodamine B, has been investigated in aqueous phosphate buffer solutions (pH 7.20) at 25°C by means of ultraviolet, fluorescence, and circular dichroism spectrometry as well as by fluorescence lifetime measurements. The spectral titrations gave the complex stability constants (Ks) and Gibbs' free energy changes (ΔG°) for the inclusion complexation of 2-5 with the organic dyes and other thermodynamic parameters (ΔH° and ΔS°) for the inclusion complexation of 2-4 with the fluorescent dyes Acridine Red and Rhodamine B. Bis(β-cyclodextrin)s 2-5 displayed higher binding abilities toward most of the examined dye molecules than native β-cyclodextrin 1; this is discussed from the viewpoints of the size/shape-fit concept, the induced-fit interaction, and cooperative, multipoint recognition by the bridging chain and the dual hydrophobic cavities. Thermodynamically, the inclusion complexation of 2-4 with Acridine Red is totally enthalpy driven with a negative or minor positive entropic contribution, but the inclusion complexation with Rhodamine B is mainly entropy-driven with a mostly positive, but occasionally negative, enthalpic contribution; in some cases this determines the complex stability.