Alizarin Yellow-Modified β-Cyclodextrin as a Guest-Responsive Absorption Change Sensor

Alizarin yellow-modified β-cyclodextrin (ACD), in which alizarin yellow is linked to β-cyclodextrin via an ethylenediamine spacer, was synthesized as a new absorption change indicator for molecules. Alizarin yellow is a pH indicator that exhibits absorption peaks at 360 and 480 nm in the neutral region and in the alkaline region, respectively, with pK_a = 10.98 for an equilibrium between two forms. ACD has two parts to be deprotonated: one is the phenolic hydroxyl group of alizarin yellow residue and the other is the secondary amine group of the spacer. ACD exhibits pH dependency very different from that of alizarin yellow. We obtained two pK_a values, 4.88 (pK_a1) and 8.89 (pK_a2), for ACD by pH titration of its absorption intensity. The pK_a1 and pK_a2 values were suggested to be the pK_a values of the phenolic hydroxyl group of alizarin yellow residue and the secondary amine group, respectively. Upon addition of guest species, the pK_a1 and pK_a2 values shifted to 5.11 and 7.56, respectively, indicating a larger shift in the pK_a for the amine group than for the hydroxyl group. The guest-induced pK_a shift in the alkaline region suggests that deprotonation of the amine group of ACD occurs when the alizarin yellow moiety is excluded from the cyclodextrin cavity associated with guest accommodation and exposed to an alkaline environment. The sensitivities of this host to various guests were examined by absorption changes at 475 nm in pH 8.3 phosphate buffer, and the order of the sensitivities was found to be adamantane derivatives > borneol > bile acids. This order is not parallel with that of the binding constants, suggesting that the structural features of the host - guest complexes are important. All these results demonstrate that ACD can be used as an effective chemosensor for molecules.