cyclodextrin and ascorbic acid

Processes in Isotopes and Molecules Journal of Physics: Conference Series 182 (2009) 012004

IOP Publishing doi:10.1088/1742-6596/182/1/012004

Spectroscopic investigation of the interaction between β-cyclodextrin and ascorbic acid Ioan Bratu1, Marieta Muresan-Pop2, Irina Kacso1 and Sorin I Fărcaş 1

National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath, 400293 Cluj-Napoca, Romania 2

Babeş-Bolyai University, Faculty of Physics, 1 Mihail Kogălniceanu 400084 Cluj-Napoca, Romania E-mail: [email protected]

Abstract. Inclusion compound of vitamin C (ascorbic acid) with β-cyclodextrin (β-CD), prepared by different methods (kneading, co-precipitation and freeze-drying) has been caracterized by several spectroscopic techniques (FTIR, 1H NMR, UV-Vis), powder X-ray diffraction and DSC technique. Based on the chemical shifts observed in the 1H-NMR and on FTIR spectra the tentative conclusion is that vitamin C probably enters the cyclodextrin torus forming the inclusion complex.

1. Introduction Cyclodextrins and their derivatives are used to increase the solubility and to improve the drug delivery rate of the bioactive substances. Cyclodextrins are cyclic oligosaccharides having a hydrophobic external surface and an inner hydrophilic surface (see figure 1a). A bioactive compound act orally, parentherally or locally depending on the administration form (solid or liquid); in function of the auxiliary substances, its biodisponibility is influenced [1]. In order to solve this problem, the most employed method is the complexation with cyclodextrins (CD) [2-4]. H

H OH H 6

4

5

H

O

3

H

HO

O

H

1

2

OH

O

H

7 (cyclic)

(a)

(b)

Figure 1. (a) β-Cyclodextrin molecule. (b) Ascorbic acid molecule. They are capable to form more soluble guest-host systems through non-covalent bonds, Van der Waals interactions, hydrophobic effect, solvent molecules reorganization and hydrogen bonds [5]. Vitamin C (ascorbic acid) (figure 1b), known as the antiscorbutic vitamin, essential nutrient with attributes in cellular and tissular oxido-reduction processes, is implied in the glucose and collagen

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metabolism, of the folic acid or some amino acids in the neutralization of the free radicals, in immunological reactions. Based on calorimetric and spectrophotometric investigation of the interaction between HP-β-CD and ascorbic acid, the authors demonstrate an 1:1 molecular complex formation [6]. The effects of degree of substitution and of pH on complex formation between HP-α-CD and HP-β-CD with ascorbic acid have been also investigated [7]. The interaction of ascorbic acid with HP-β-CD was also investigated by FTIR, 1H NMR and UV spectroscopy [8]. 1H NMR study of α- and β-CD complexation with ascorbic acid was performed to establish the protons of α- or β-CD involved in the inclusion process [9]. The aim of this paper was to investigate the supramolecular assembly of the ascorbic acid with βCD through several spectroscopic methods (FTIR, UV-Vis and 1H NMR), DSC and powder X-ray diffraction. 2. Experimental The inclusion compounds were prepared by kneading (kn), co-precipitation (co) and freeze-drying (fd) methods. Also, physical mixtures (pm) were prepared in order to be compared with the inclusion compounds. FTIR, DSC and X-ray powder diffraction (XRD) were employed to evidence the inclusion compounds formation. FTIR measurements were performed with JASCO 6100 spectrometer. DSC thermograms were obtained with DSC 60 Shimadzu calorimeter and X-ray diffraction patterns were obtained with a Bruker D8 Advance diffractometer. The inclusion complexes of vitamin C with β-CD were prepared in aqueous solutions starting from milimolar “mother solutions” in D2O for 1H NMR and in H2O for UV-Vis, respectively. 1H NMR spectra were collected with Bruker Avance 500 spectrometer. UV-Vis spectra were obtained with JASCO V-550 spectrometer. 3. Results and discussion 3.1. FTIR The O–H stretching frequency (see figure 2) remains constant for the pm and kn products, whereas for co and fd inclusion compounds is shifted to 3414 and 3408 cm-1, respectively. The vitamin C’s peak at 3410 cm-1 can be identified in the pm and kn products, so the inclusion process is not so efficient in the case of kneaded product. As concerned the functional C=O groups, they were located at 1673 cm-1 and 1754 cm-1 for pure vitamin C, pm and kn products, whereas they are shifted to 1694 and 1763 cm-1, respectively for co and fd products.

Vitamin C

Vitamin C

β-CD

Absorbance (a.u.)

0.8

3410 3414

3405

1.0

0.8

3378 0.6

0.4

0.2

1694

0.6 1754 0.4 1763 0.2

0.0 4000

pm kn co fd

1673

Absorbance (a.u.)

1.0

β-CD

pm kn co fd

0.0 3500

3000

2500

1850

1800

1750

1700

1650

1600

1550

Wavenumber (1/cm)

Wavenumber (1/cm)

Figure 2. FTIR spectra of: vitamin C, β-CD, their 1:1 pm and the products obtained by kn, co, and fd procedures, 4000-2500 cm-1 (left) and 1850-1550 cm-1 (right) spectral ranges.

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The frequency shift toward higher frequencies demonstrates the breaking of the hydrogen bonds during the inclusion process. 3.2. X-ray powder diffraction From powder diffraction patterns (see figure 3a) one can see that inclusion compounds are formed in a different degree. Both crystalline and amorphous phases are present in the so obtained compounds.

(a)

(b)

Figure 3. (a) X-ray patterns of: vitamin C, β-CD, and the products obtained by kn, co and fd procedures; (b) DSC thermograms of the vitamin C, β-CD and of the obtained inclusion compounds. The difference between these patterns is due to different ratios between crystalline and amorphous phases. 3.3. DSC DSC trace of vitamin C (figure 3b) presents a sharp endothermic melting peak at 197oC, followed by an exothermic peak at around 240oC, possible the start of degradation. The thermogram of the β-CD shows an endothermic transition between 70-110oC and an endothermic peak at around of 300oC. By comparing the thermograms of the pure compounds with those of the products, the decreasing of dehydration endothermic peak of cyclodextrin, as well as a disappearance of the melting peak of the vitamin C in the inclusion compounds were observed. 3.4. 1H NMR The 1H NMR measurements are used to prove the inclusion phenomena of C vitamin in β-CD identifying the protons implied in the complexation process and belonging both to the guest and to the host molecules, respectively. The inclusion of a guest molecule into a host one is shown by changes in chemical shifts of some of the guest and host protons in comparison with the chemical shifts of the same protons in the free compounds (figure 4). The complexation is a dynamic process and the exchange between the free and bound molecules is very rapid. In any case, it was observed a variation of the chemical shifts of the same protons in the case of NMR spectrum of the free and bound molecules. This was the aim of the preliminary measurements, and the results are in good accordance with the other measurements presented in this paper. 3.5. UV-Vis One can observe a hypsochromic shift of the maximum (see figure 5 – the arrows suggest the absorption maxima shifts with the increase of the β-CD concentration) in comparison with the corresponding one for the pure substance (265 nm), with the increase of the β-cyclodextrin excess.

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This maximum frequency shift demonstrates the inclusion complex formation and can be used to determine its stoichiometry and the stability constant.

Figure 4. 1H NMR spectra of the inclusion complex of vitamin C and β-CD. Bottom: vitamin C; middle: β-CD; top: inclusion complex of vitamin C:β-CD (1:1).

Figure 5. UV-Vis spectra of the inclusion complex of vitamin C (ascorbic acid) with β-CD ([C]=0.08 mM in H2O, [β-CD] = variable).

4. Conclusions FTIR, X-ray powder diffraction and DSC data reveal that an inclusion compound between ascorbic acid and β-CD was formed. The changes appearing in the DSC data of the pure substance as compared with those of the kneaded, freeze-dried and co-precipitated products confirm the inclusion compound formation, all employed preparation methods being successfully in obtaining these supramolecular assemblies. The 1H NMR and UV-Vis data demonstrates also the inclusion complex formation in aqueous solution. Acknowledgments This work received financial support from the Romanian Research and Education Ministry under the Core Project PN-09-44 02 01. References [1] Leucuta S E 1989 Farmacocinetica şi biodisponibilitatea în terapia medicamentoasă (Bucureşti: Editura Medicală) [2] Albers E and Muller B W 1992 J. Pharm. Sci. 81 756-61 [3] Ficarra R, Ficarra P and Rameri D 2000 J. Pharm. Biomed. Anal. 23 231-6 [4] Acerbi D, Bovis G, Carli F, Pasini M, Pavesi L and Peveri T 1990 Drug Investigation 2, 29-36 [5] Lopata A, Darvas F, Stadler-Szóke A and Szejtli J 1995 J. Pharm. Sci. 74 211-3 [6] Terekhova I V and Obukhova N A 2005 Mendeleev Commun. 15 38-40 [7] Terekhova I V, Obukhova N A, Agafonov A V, Kurochkina G I, Syrtsev A N and Gratckev M K 2005 Russ. Chem. Bull. Int. Ed. 54 1883-6 [8] Garnero G and Longhi M 2007 J. Pharm. Biomed. Anal. 45 536-45 [9] Terekhova I V, Kulikov O V, Kumeev R S, Nikiforov M Yu and Al’per G A 2005 Russ. J. Coord. Chem. 31 218-20.

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