Heterodimerization of Dye-Modified Cyclodextrins with Native Cyclodextrins

The heterodimerization behavior of dye-modified β-cyclodextrins (1−6) with native cyclodextrins (CDs) was investigated by means of absorption and induced circular dichroism spectroscopy in an aqueous solution. Three types of azo dye-modified β-CDs (1−3) show different association behaviors, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl red-modified β-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4‘ position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its β-CD cavity. It also associates with the native α-CD by inserting the moiety of 1 into the α-CD cavity. The association constants for such heterodimerization are 198 M-1 at pH 1.00 and 305 M-1 at pH 6.59, which are larger than the association constant of 1 for β-CD (43 M-1 at pH 1.00). Methyl red-modified 2, which has the same dye moiety as that for 1 although its substituent position is different from that of 1, does not associate even with α-CD due to the stable self-intramolecular complex, in which the dye moiety is deeply included in its own cavity of β-CD. Alizarin yellow-modified CD (3), which has an azo dye moiety different from that of 1 and 2, caused a slight spectral variation upon addition of α-CD, suggesting that the interaction between 3 and α-CD is weak. On the other hand, phenolphthalein-modified β-CD (4), which forms an intermolecular association complex in its higher concentrations, binds with β-CD with an association constant of 787 M-1 at pH 10.80, where 4 exists as the dianion monomer in the absence of β-CD. p-Nitorophenol-modified β-CDs (5 and 6), each having p-nitorophenol moieties with a different connecting part with an amido and amidophenyl group, respectively, associated with α-CD with association constants of 66 and 16 M-1 for 5 and 6, respectively. The phenyl unit in the connecting part of 6 may prevent the smooth binding with α-CD. All these results suggest that the dye-modified CDs, in which the dye part is not tightly included in its CD cavity, associate with the native CD to form heterodimer composed of two different CD units by inserting the dye moiety into the native CD unit. The resulting heterodimers have a cavity that can bind another appending moiety of host molecules. On this basis, more ordered molecular arrays or the supramolecular hereropolymers can be constructed.