Complexation of methyl orange and tropaeolin 000 no 2 by beta-cyclodextrin dimers

Spectrophotometric studies of the complexation of Methyl Orange (MO(-)) and Tropaeolin 000 No. 2 (TR(-)) anions by dimeric N,N'-bis(6(A)-deoxy-6(A)-beta-cyclodextrin)urea (beta CD)(2)ur and its oxalamide and succinamide analogues, (beta CD)(2)ox and (beta CD)(2)su, respectively, are consistent with the predominant formation of complexes of the general formulae (beta CD)(2)x . MO(-) characterized by stability constants K-1=(1.05+/-0.04) x 10(5), (1.92+/-0.04) x 10(5) and (2.50+/-0.02) x 10(4) dm(3) mol(-1) and (beta CD)(2)x . TR(-) characterized by K-1=(1.39+/-0.03) x 10(4), (7.4+/-0.1) x 10(3) and (4.60+/-0.05) x 10(3) dm(3) mol(-1), in aqueous phosphate buffer at pH 9.0 and 5.5 and 298.2 K. These values are significantly greater than K-1=2160 and 710 dm(3) mol(-1) for the beta-cyclodextrin complexes, beta CD . MO(-) and beta CD . TR(-) and are indicative of cooperative binding in (beta CD)(2)x . MO(-) and (beta CD)(2)x . TR(-). The factors affecting complex stability are discussed and comparisons are made with related systems.