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

A series of novel 6,6'-bis(beta-cyclodextrin)s linked by 2,2'-bipyridine-4,4'-dicarboxy tethers; that is, 2,2'-bipyridine-4,4'-dicarboxy-bridged bis(6-O-beta-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-beta-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 degrees 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 (deltaG degrees) for the inclusion complexation of 2-5 with the organic dyes and other thermodynamic parameters (deltaH degrees and deltaS degrees) for the inclusion complexation of 2-4 with the fluorescent dyes Acridine Red and Rhodamine B. Bis(beta-cyclodextrin)s 2-5 displayed higher binding abilities toward most of the examined dye molecules than native beta-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.