ALIZARIN YELLOW-MODIFIED BETA-CYCLODEXTRIN AS A GUEST-RESPONSIVE ABSORPTION CHANGE SENSOR

Alizarin yellow-modified beta-cyclodextrin (ACD), in which alizarin ye llow is linked to beta-cyclodextrin via an ethylenediamine spacer, was synthesized as a new absorption change indicator for molecules, Aliza rin yellow is a pH indicator that exhibits absorption peaks at 360 and 480 nn in the neutral region and in the alkaline region, respectively , with pK(a)=10.98 for an equilibrium between two forms. ACD has two p arts to be deprotonated: one is the phenolic hydroxyl group of alizari n yellow residue and the other is the secondary amine group of the spa cer, ACD exhibits pH dependency very different from that of alizarin y ellow, 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 hydr oxyl group of alizarin yellow residue and the secondary amine group, r espectively. Upon addition of guest species, the pK(a1) and pK(a2) val ues 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 gues t-induced pK(a) shift in the alkaline region suggests that deprotonati on of the amine group of ACD occurs when the alizarin yellow moiety is excluded from the cyclodextrin cavity associated with guest accommoda tion and exposed to an alkaline environment. The sensitivities of this host to various guests were examined by absorption changes at 475 Mn in pH 8.3 phosphate buffer, and the order of the sensitivities was fou nd to be adamantane derivatives >borneol >bile acids. This order is no t parallel with that of the binding constants, suggesting that the str uctural features of the host-guest complexes are important. All these results demonstrate that ACD can be used as an effective chemosensor f or molecules.