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 pKa = 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 pKa values, 4.88 (pKa1) and 8.89 (pKa2), for ACD by pH titration of its absorption intensity. The pKa1 and pKa2 values were suggested to be the pKa values of the phenolic hydroxyl group of alizarin yellow residue and the secondary amine group, respectively. Upon addition of guest species, the pKa1 and pKa2 values shifted to 5.11 and 7.56, respectively, indicating a larger shift in the pKa for the amine group than for the hydroxyl group. The guest-induced pKa 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.
改性铝红黄β-
环糊精(ACD)是通过
乙二胺间隔物将铝红黄连接到β-
环糊精上合成的新型分子吸收变化指示剂。铝红黄是一种pH指示剂,在中性区和碱性区分别在360 nm和480 nm处显示吸收峰,pKa = 10.98表示两种形式之间的平衡。ACD有两个需要去质子化的部分:一是铝红黄残基的
酚羟基,另一是间隔物的二级胺基。ACD的pH依赖性与铝红黄有很大不同。通过对其吸收强度进行pH滴定,我们得到了两个pKa值,分别为4.88(pKa1)和8.89(pKa2)。pKa1和pKa2的值被认为分别是铝红黄残基的
酚羟基和二级胺基的pKa值。当添加客体物种时,pKa1和pKa2值分别 shifted至5.11和7.56,表明胺基的pKa变化幅度大于羟基。碱性区域中客体引起的pKa变化暗示,当铝红黄基团从与客体适配的
环糊精腔内排除并暴露于碱性环境中时,ACD的胺基发生去质子化。通过在pH 8.3的
磷酸盐缓冲液中观察475 nm处的吸收变化,考察了该宿主对各种客体的敏感性,敏感性的顺序为:
金刚烷衍
生物 > borneol >
胆酸。这个顺序与结合常数的顺序不相符,表明宿主-客体复合物的结构特征很重要。所有这些结果表明,ACD可以作为有效的分子
化学传感器。