Dospectral Analyses of Interaction between Congo Red and Cyclodextrins at Various pH

The absorbance of 500 nm of various concentration of cyclodextrin (CD) containing 0.02 mM congo red (CR) solution were obtained. The absorbance was plotted toward the concentration of CDs to obtain curves showing the relation of the variation of the absorbance to the concentration of CDs. In the case of the α-CD series (α-CD, G₁-α-CD and G₂-α-CD), the absorbance increased in accordance with the CDs’ concentration, and the increasing ratio of G₂-α-CD was considerably higher than those of α- and G₁-α-CD. On the other hand, in the case of the β-CD and γ-CD series, the absorbance curves showed that they reached a plateau at a certain concentration. It was almost same among the β-CD series. A certain interaction, which was maximum at a ratio less than CR: β-CD/1:250, might occur between CR and β-CD, except inclusion complex formation. The absorbance curves for the γ-CD series showed that it reached a plateau at a considerably lower concentration than that of β-CD. The formation ratio of CR: γ-CD complex was calculated as 1:2 at pH 5.0. We inferred that cavity size and the character of CDs affected accessing style to CR molecules and that the CR molecule was unable to access α-CD because the cavity size was too small to incorporate a CR molecule. The cavity of β-CD may be able to attract a CR molecule, but is unable to incorporate it. The cavity of γ-CD may be able to incorporate a CR molecule. As a conclusion, the α- and β-CD series were able to interact with CR molecules except in inclusion complexes, whereas, the γ-CD series were able to form inclusion complexes. In the NMR spectroscopy of CR in CDs containing D₂O, the ¹H NMR profile of CR was greatly changed by the addition of the γ-CD series, slightly changed by the addition of the β-CD series and not changed by the addition of the α-CD series. The chemical shift of the γ-CD series was also greatly changed by the addition of CR, but not the α-CD or β-CD series. By the addition of the same molar concentration of γ-CD to CR in D₂O, 1:1 complex formation was achieved, and the pH of this solution was around 8. We inferred that the complex formation ratios’ difference between NMR of spectroscopy and spectrophotometry depends on the solutions’ pH.