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. 2016 Oct 20:6:35764.
doi: 10.1038/srep35764.

Study to explore the mechanism to form inclusion complexes of β-cyclodextrin with vitamin molecules

Subhadeep Saha  1 , Aditi Roy  1 , Kanak Roy  1 , Mahendra Nath Roy  1
Affiliations

Study to explore the mechanism to form inclusion complexes of β-cyclodextrin with vitamin molecules

Subhadeep Saha et al. Sci Rep. .

Abstract

Host-guest inclusion complexes of β-cyclodextrin with two vitamins viz., nicotinic acid and ascorbic acid in aqueous medium have been explored by reliable spectroscopic, physicochemical and calorimetric methods as stabilizer, carrier and regulatory releaser of the guest molecules. Job's plots have been drawn by UV-visible spectroscopy to confirm the 1:1 stoichiometry of the host-guest assembly. Stereo-chemical nature of the inclusion complexes has been explained by 2D NMR spectroscopy. Surface tension and conductivity studies further support the inclusion process. Association constants for the vitamin-β-CD inclusion complexes have been calculated by UV-visible spectroscopy using both Benesi-Hildebrand method and non-linear programme, while the thermodynamic parameters have been estimated with the help of van't Hoff equation. Isothermal titration calorimetric studies have been performed to determine the stoichiometry, association constant and thermodynamic parameters with high accuracy. The outcomes reveal that there is a drop in ΔSo, which is overcome by higher negative value of ΔHo, making the overall inclusion process thermodynamically favorable. The association constant is found to be higher for ascorbic acid than that for nicotinic acid, which has been explained on the basis of their molecular structures.

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Figures

Figure 1
Figure 1. Molecular structure of nicotinic acid, ascorbic acid and β-cyclodextrin.
Figure 2
Figure 2
Job’s plot of different vitamin-β-CD systems at 298.15 K. (a) nicotinic acid at λmax = 260 nm and (b) ascorbic acid at λmax = 261 to 265 nm. R = [Vit]/([Vit] + [β-CD]), ΔA = absorbance difference of the vitamins without and with β-CD.
Figure 3
Figure 3
(a) Stereo-chemical configuration of β-cyclodextrin, (b) truncated conical structure of β-cyclodextrin with interior and exterior protons.
Figure 4
Figure 4. 2D ROESY spectra of 1:1 molar ratio of β-CD and nicotinic acid in D2O (correlation signals are marked by red circles).
Figure 5
Figure 5. 2D ROESY spectra of 1:1 molar ratio of β-CD and ascorbic acid in D2O (correlation signals are marked by blue circles).
Figure 6
Figure 6. Feasible and restricted inclusion of the guest into the host molecule.
Figure 7
Figure 7
Formation of inclusion complexes of (a) nicotinic acid and (b) ascorbic acid with β-CD.
Figure 8
Figure 8
Variation of surface tension of aqueous (a) nicotinic acid solution and (b) ascorbic acid solution respectively with increasing concentration of β-cyclodextrin at 298.15 K.
Figure 9
Figure 9
Variation of conductivity of aqueous (a) nicotinic acid solution and (b) ascorbic acid solution respectively with increasing concentration of β-cyclodextrin at 298.15 K.
Figure 10
Figure 10. ITC isotherms for the interaction of nicotinic acid with β-cyclodextrin at 298 K.
For each titration, β-cyclodextrin concentration in sample cell was taken as 50 μM and nicotinic acid concentration in syringe was 500 μM. The top panel represents the raw heats of binding obtained upon titration of nicotinic acid to β-cyclodextrin. The lower panel is the binding isotherm fitted to the raw data using one site model.
Figure 11
Figure 11. ITC isotherms for the interaction of ascorbic acid with β-cyclodextrin at 298 K.
For each titration, β-cyclodextrin concentration in sample cell was taken as 50 μM and ascorbic acid concentration in syringe was 500 μM. The top panel represents the raw heats of binding obtained upon titration of ascorbic acid to β-cyclodextrin. The lower panel is the binding isotherm fitted to the raw data using one site model.
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