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.
Highlights
Tiredness[16,17]
On the other hand Terekhova et al demonstrated nicotinic acid-CD interactions by volumetric and heat capacity studies[27]. In this present work the formation of host-guest inclusion complexes of these two vitamins with β-CD have been explored towards their formation, stabilization, carrying and controlled release without chemical modification by different dependable methods like 2D rotating-frame NOE spectroscopy (ROESY) NMR, UV-Vis spectroscopy, surface tension, conductivity and isothermal titration calorimetric studies, which primarily focuses on the encapsulation of the bio-molecules into the cavity of β-CD
The other fact is that in spectroscopic determination, thermodynamic parameters were estimated from association constants, which again were found out on the basis of Δεof the vitamins, that was due to the changes in the environment around the chromophore, when these go from the polar aqueous environment to the apolar cavity of β-CD, the changes in enthalpy and entropy described there were exclusively for the formation of inclusion complex (IC), not for the other solvent interactions taking place in the medium
Summary
ICs formed (Fig. 8)[44,45]. The values of γand corresponding concentrations of vitamins and β-CD at each break have been listed in Table 1, which indicate that at each break point the concentration ratio of host and guest is about 1:1, establishing the formation of 1:1 ICs between the studied vitamins and β-CD46,47. The values of ΔHo and ΔSo for the formation of ICs were found negative suggesting that the inclusion process is exothermic and entropy controlled but not entropy driven (Table 3)[48] These results may be explained on the basis of molecular association that was taking place while the ICs were being formed between β-CD and each vitamin. The other fact is that in spectroscopic determination, thermodynamic parameters were estimated from association constants, which again were found out on the basis of Δεof the vitamins, that was due to the changes in the environment around the chromophore, when these go from the polar aqueous environment to the apolar cavity of β-CD, the changes in enthalpy and entropy described there were exclusively for the formation of IC, not for the other solvent interactions taking place in the medium. The binding affinity and thermodynamic parameters of the binding process were obtained by fitting the integrated heats of binding the isotherm to the one site binding model to give the association constant (KaC), stoichiometry (NC), binding enthalpy (ΔH°C) and the entropy (ΔS°C)
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