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

Abstract

In our previous paper, 1) we reported the comparative spectral analyses of 0.014 mM CR with 4% concentration of CDs. Beyond a certain concentration of CDs, the spectra of CR were not changed. Therefore, this time, we used 0.02 mM CR and various concentrations of CDs to obtain further information on the interaction of CR with CDs. Moreover, we found that absorption spectra of CR were changed by the addition of cyclodextrins at pH 5.0.As for spectrophotometry by visible light, it was demonstrated that methyl orange formed an inclusion complex with CDs in a dissolved state and various color changes were observed with the addition of CDs and ethanol. As for the complex formation of CR with CDs, there are several reports.5 9) It is reported that the raman optical activity (ROA) spectra of CR with γ cyclodextrin were measured at the concentration of 0.05 mM (CR) and 5 mM (γ CD), and that the ROA spectra may reflect the difference in the positions of the SO3 groups in the guest molecules.5) It is revealed that the phenomenon of induced optical activity denotes the induction of optical activity in the molecules of an achiral compound when it is in contact with the molecules of a chiral compound, and that the main consequence of this interaction is asymmetric perturbation of the electronic transitions of the chromophore by the field of the inducer to be exhibited in the optical rota- Abstract: 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 1 H NMR profile of...

Evaluation of the supramolecular interaction of Congo red with cucurbiturils using mass spectrometry and spectroscopic methods

“…No signals assignable to CR-CB [7] interaction were detected by 1 H NMR. Unlike that observed for CR@cyclodextrins, 50 CB [7] does not sharpen CR signals. In fact, a precipitate was observed inside the NMR tube, which explains the absence of CR peaks and indicates that CB [7] is interacting with CR leading to its precipitation.…”

“…To obtain further insights into the interaction between CR and CB[n], the formation CR@CB [7] complexes was studied by 1 H NMR. CB [7] gave the expected 1 H NMR signals, while CR, as reported, 50 showed weak and broad signals (Fig. S2, ESI †).…”