4998
2009, 113, 4998–5000 Published on Web 03/25/2009
Thermal Decomposition Behavior of Sodium Arsenite in the Presence of Carbonized β-Cyclodextrin Le Xin Song* and Zheng Dang Department of Chemistry, UniVersity of Science and Technology of China, Jin Zhai Road 96, Hefei, China 230026 ReceiVed: January 19, 2009; ReVised Manuscript ReceiVed: February 28, 2009
The thermal decomposition behavior of sodium arsenite (SA) was greatly affected by carbonized β-cyclodextrin (CD), possessing the releases of four forms of gaseous arsenic molecules according to gas chromatography coupled to time-of-flight mass spectrometry with programmed temperature operation. Also, the decomposition temperature of β-CD was lowered in the presence of SA. Owing to their unique properties, cyclodextrins (CDs) nowadays are very fascinating for supramolecular chemists in various fields. β-CD contains seven R-D-glucose units and has the form of a hollow truncate cone with one ring wider than the other.1-3 It can form inclusion complexes or adducts with many organic drugs,4,5 but there are few reports about the interaction between β-CD and traditional inorganic drugs, such as sodium arsenite (NaAsO2, SA), in solution, especially in powder states. SA, an important anticancer drug, is widely introduced into the treatment of a human acute promyelocytic leukemia and some other cancers.6 Due to its high toxicity, during the course of treatment, there are strong side effects on human bodies.6 It is well-known that introduction of β-CD into pharmaceutical processes on the molecular level can improve the properties of included drugs evidently, so as to be applied as a drug carrier.7,8 In our recent works, it was found that both phase-change temperatures of organic guests and the thermal decomposition behavior of β-CD changed upon inclusion.3,9 Can inorganic drugs influence the thermal decomposition behavior of β-CD? How does the existence of carbonized β-CD10 cause this to occur sooner for decompositions of inorganic drugs with a high thermal stability, if it does at all? Clearly, each of these questions has a significant value both in chemical research and in industrial application. The molecule-ion interaction between β-CD and SA in aqueous solution was proven by means of UV-vis absorption spectra since the absorption intensity of SA varied continuously with changing concentration of β-CD.11 The binding constant (279.8 ( 7.5 mol-1 · dm3) of β-CD to SA was determined using the Benesi-Hildebrand equation12 in terms of 1H nuclear magnetic resonance titration data.11 Four solid samples (1:1, initial molar ratio), including a physical mixture 1, a ground mixture 2 (with a ground time of 20 min), a common adduct 3 (stirring for 10 h at 298 K and dried into a powder in vacuo), and a hydrothermal adduct 4 (in an autoclave, heated at 393 K for 4 h, and dried into a powder in vacuo) were prepared. As a comparison, we also prepared the adduct of R-CD and SA (1:1, initial molar ratio); the preparation method of the adduct was the same as that of sample * To whom correspondence should be addressed. E-mail: solexin@ custc.edu.cn. Fax: +86 551 3601592. Tel: +86 551 3601804.
10.1021/jp900550d CCC: $40.75
4.11 Then, the solid samples were characterized by Fourier transform infrared spectra (FTIR), powder X-ray diffraction (PXRD), and thermogravimetry (TG) analyses.11 Initially, the stretching vibration band of OH groups due to β-CD at about 3427 cm-1 in the FTIR spectra of the samples 2, 3, and 4 was found to red shift to a less extent compared to that of OH groups of free β-CD, and the shift extent increased in the order of 1 < 2 < 3 < 4.11 This suggest that the hydrogen bondings between β-CD molecules were weakened upon interaction with SA.13 Similarly, with the change in the FTIR spectrum of β-CD before and after adduct, the stretching vibration band of OH groups of R-CD also indicated a small red shift (about 16 cm-1) upon adduct with SA.11 Next, PXRD analyses showed that the existence of SA has affected the structural arrangement of β-CD molecules to a different extent dependent on the preparation methods of the samples. For example, several main peaks due to β-CD in 4 shifted toward a lower 2θ angle in comparison with those in 1 and 2, meaning that the introduction of SA has led to an increase of the intermolecular distance of β-CD.11 The increase can be explained as the effect of the introduction of SA into the openings of the β-CD cavity. This observation is in good accordance with the result of FTIR. In addition, the molecule-ion interaction between β-CD and SA in the powder state was demonstrated by the fact that there were two new peaks at the 2θ values of 28.0 and 34.1° in the spectra of 3 and 4 which did not appear in the PXRD spectra of free β-CD, SA, 1 and 2.11 Also, according to the comparison of PXRD patterns between R-CD and its adduct, the stacking form of R-CD has been changed by the presence of SA in the adduct, from a cage form to a channel form. Nevertheless, the significant difference in stacking forms was not found between β-CD and its adduct of SA, suggesting that the molecule-ion interaction between SA and R-CD might be stronger than that between SA and β-CD.11 Subsequently, TG profiles (Figure 1) of samples 2, 3, and 4 indicated that the rapid release temperature (349 K) of water in samples 2 and 3 was highly similar to that of water in free β-CD,3 but the adduct 4 lost its water molecules at a higher temperature of about 367 K (∆T1 ) 17 K). The phenomena implied that there existed different distribution forms of water in the two adducts, which was also found in some inclusion 2009 American Chemical Society
Letters
Figure 1. TG profiles of samples 2, 3, and 4 at the heating rate of 5.0 K · min-1.
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Figure 3. Mass spectra of 4 at 31.35 (A) and 35.62 min (B).
Figure 4. MID curve of the fragment at m/z 299.685. Figure 2. TIC curves of free β-CD (A) and adduct 4 (B).
complexes of CDs.14 Further, in the temperature range from 406 to 509 K, both free β-CD and the ground mixture 2 did not lose any mass, but the two adducts 3 and 4 lost their respective masses of about 2%. Moreover, the decomposition behavior of β-CD in the three samples is rather different from each other. After 613 K, the residue mass (RM) of the remains increased in the order of 2 < 3 < 4 at any temperature, and the three curves in the range from 613 to 773 K exhibited a larger slope than free β-CD. This may be caused by the different decomposition modes of β-CD. The analyses of Figure 1 gave a strong impression that the presence of SA influenced the thermal decomposition behavior of β-CD and that there was a large difference in decomposition behavior between β-CD and 4. Therefore, it is necessary to further evaluate the direct effect of the molecule-ion interaction on the thermal decomposition of the two reactants. Gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS) is a soft ionization technique, which therefore allows for the transfer of noncovalent adducts from a liquid phase into a gaseous phase.9,15 GC-TOF-MS measurements of free β-CD and 4 with the same programmed temperature were carried out, with the objective of trying to find details regarding the decomposition process of SA. Initially, the plots of the variation of total ion current (TIC) in intensity with the heating time of free β-CD and the adduct 4 are shown in panels A and B, repectively, of Figure 2. The decomposition processes of the samples were controlled by the temperature program as described in Supporting Information.3 According to Figure 2A, in the temperature range from 373 to 573 K, free β-CD did not lose any mass, but from Figure 2B, 4 began to release some of its mass at 17.52 min, corresponding to a small peak at 553 K. This finding indicated that in the presence of SA, some of the β-CD molecules seemed to have a lower decomposition temperature. It can be explained that some of the covalent bonds of the released β-CD molecules were weakened by the molecule-ion interaction between them and SA. Further, free β-CD produced a double peak at a range of about 25-30 min, but the main peak of 4 was a single one at 25.13 min. It should be noted that the signal of 4 rose a little earlier
compared to those of β-CD inclusion complexes formed by organic amines reported before.9 Besides, there were two new peaks at 31.52 and 35.74 min in Figure 2B, corresponding to the temperature at 773 K, which was much higher than the decomposition temperature of β-CD but still much lower than the decomposition temperature (mp, 888 K) of SA. This finding revealed that a redox reaction occurred between carbonized β-CD and SA. Next, as can be seen in Figure 3, on the one hand, the mass spectra of 4 at 31.35 (773 K) and 35.62 min (kept about 6 min at 773 K) were rather similar to each other because both of them were involved in the four fragments at m/z 74.923 (As+), 149.844 (As2+), 224.765 (As3+), and 299.685 (As4+).16 These values reflected that the liberation of arsenic from SA to the gaseous phase was presented in four kinds of forms, and the relative abundance (RA) ratios of the four simple substances are 1.0:2.2:1.2:20 at 31.35 min and 1.0:3.1:1.2:19 at 35.62 min. The stabilities of them decreased in the order As4+ > As2+ > As3+ > As+. On the other hand, differently from Figure 3B, the fragment at m/z 18.011 in Figure 3A was ascribed to H2O+. It is interesting that it appeared at 31.35 min but disappeared at 35.62 min, showing that the carbonization process of β-CD is finished before 35.62 min. Additionally, the fragment at m/z 43.990 (CO2+) had the highest RA of 100% in the two spectra. Clearly, its occurrence at 35.62 min can be attributed to the result of the redox reaction between SA and carbonized β-CD. Theoretically, tetratomic arsenic vapor has the highest symmetry and stability among the four fragments. Actually, the liberation of arsenic from SA was mainly in the form of As4, with As2 next. As and As3 were unstable because of the existence of an unpaired electron. A proposed chemical reaction describing the decomposition behavior of SA in the presence of carbonized β-CD was given in eq 1 as follows
4NaAsO2 + 3C f 2Na2CO3 + As4 + CO2
(1)
To investigate the decomposition of SA in detail, the formation of As4 was traced based on the multiple ion detection (MID, Figure 4) curve of the fragment at m/z 299.685, which is referred to as time-varying values. Interestingly, the curve in Figure 4 could be divided into three stages clearly. The first was at 25.57 min, corresponding to a relatively low temperature
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Figure 5. A schematic sketch illustrating the proposed molecule-ion interactions between SA and β-CD.
of 653 K, and just at the end of the sharp decomposition of β-CD. The fastest liberation process of As4 to the gaseous phase was at 31.49 min, with the temperature just reaching 773 K. It should be noted that all of these release temperatures were below the melting point of SA as well as the sublimation temperature (886 K) of arsenic, implying that without the decomposition and carbonization of β-CD, arsenic would not be released from the sample 4. Furthermore, according to eq 1, it was reasonable that the thermal decomposition of sodium arsenite and the release of arsenic would be observed in the presence of not only CDs such as R- and β-CD but also many other oligosaccharides. The release process of arsenic should be associated with the molecule-ion interactions occurring between SA and β-CD, which was in agreement with the result of TG measurements. The interactions between inorganic guests and β-CD were considerably weak in solution17,18 and in the solid state.19 As shown in Figure 5 and in the formation process of the adduct 4, the effects of the interactions on electrostatic forces between Na+ and AsO2- were different from one another. We thought that it was the different interactions in the adduct of β-CD that resulted in different environments around SA20 as well as β-CD molecules, which is considered the fundamental cause of the difference between a supramolecular inclusion complex and a molecule-ion adduct. GC-TOF-MS measurements of the adduct of R-CD and SA were performed as comparison experiments, and the release of arsenic in four forms was also observed. However, the release temperature of the arsenic forms was greatly changed when compared with those of sample 4. The largest release of As4 to a gaseous phase was delayed to 36.45 min. The result is ascribed to different interaction intensities between the adducts of SA with R-CD and β-CD. Conclusions The molecule-ion interaction between β-CD and SA in aqueous solution was proven by UV-vis absorption spectra and 1 H nuclear magnetic resonance titration. The adduct interaction was also characterized by the aid of FTIR and PXRD in the solid state. TG, especially TOF-GC-MS, confirmed that the thermal behaviors of β-CD and SA were affected by the formation of the adduct between them. This effect could be ascribed to different molecule-ion interactions existing in the adduct of
Letters β-CD and SA, and the comparison experiments of R-CD also supported the viewpoint. The observation provided important insight into the distinction between an adduct and an inclusion complex. It can be expected that the advanced decomposition behavior of SA in the presence of β-CD as well as the unpredictable decomposition of adducted β-CD in the presence of SA has important value in the application of CD in industrial production. Acknowledgment. We would like to acknowledge the assistance of H. Yin in the GC-TOF-MS measurements of this investigation and useful discussions. Supporting Information Available: UV-vis absorption spectra and 1H nuclear magnetic resonance titration data for the system of β-CD and SA in solution, as well as the preparation, FTIR spectra, and PXRD data of solid adducts 3 and 4. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Rekharsky, M. V.; Inoue, Y. Chem. ReV. 1998, 98, 1875–1917. (2) Xu, P.; Song, L. X. Acta Phys. Chim. Sin. 2008, 24, 729–736. (3) Song, L. X.; Wang, H. M.; Guo, X. Q.; Bai, L. J. Org. Chem. 2008, 73, 8305–8316. (4) Lysik, M. A.; Wu, P. S. J. Pharm. Sci. 2003, 92, 1559–1573. (5) Marquesa, J.; Anjoa, L.; Marquesb, M. P. M.; Santosa, T. M.; Almeida Paza, F. A.; Braga, S. S. J. Organomet. Chem. 2008, 693, 3021– 3028. (6) Niu, C.; Yan, H.; Yu, T.; Sun, H. P.; Liu, J. X.; Li, X. S.; Wu, W.; Zhang, F. Q.; Chen, Y.; Zhou, L.; Li, J. M.; Zeng, X. Y.; Ou Yang, R. R.; Yuan, M. M.; Ren, M. Y.; Gu, F. Y.; Cao, Q.; Gu, B. W.; Su, X. Y.; Chen, G. Q.; Xiong, S. M.; Zhang, T. d.; Waxman, S.; Wang, Z. Y.; Chen, Z.; Hu, J.; Shen, Z. X.; Chen, S. J. Blood 1999, 94, 3315–3324. (7) Davis, M. E.; Brewster, M. E. Nat. ReV. Drug DiscoVery 2004, 3, 1023–1035. (8) Machut-Binkowski, C.; Hapiot, F.; Cecchelli, R.; Martin, P.; Monflier, E. J. Inclusion Phenom. Macrocyclic Chem. 2007, 57, 567–572. (9) Song, L. X.; Xu, P. J. Phys. Chem. A 2008, 112, 11341–11348. (10) Denicourt-Nowicki, A.; Roucoux, A.; Wyrwalski, F.; Kania, N.; Monflier, E.; Ponchel, A. Chem.sEur. J. 2008, 14, 8090–8093. (11) See Supporting Information. (12) Yoshikiyo, K.; Matsui, Y.; Yamamoto, T.; Okabe, Y. Bull. Chem. Soc. Jpn. 2007, 80, 1124–1128. (13) Magyar, A.; Szendi, Z.; Kiss, J. T.; linko, I. P. J. Mol. Struct.: THEOCHEM 2003, 666, 163–168. (14) Petrovski, Z.; de Matos, M. R. P. N.; Braga, S. S.; Pereira, C. C. L.; Matos, M. L.; Goncalves, I. S.; Pillinger, M.; Alves, P. M.; Romao, C. C. J. Organomet. Chem. 2008, 693, 675–684. (15) Xu, P.; Song, L. X.; Wang, H. M. Thermochim. Acta 2008, 469, 36–42. (16) Lynch, D. C. Metall. Trans. B 1980, 11B, 623–629. (17) Matsui, Y.; Ono, M.; Tokunaga, S. Bull. Chem. Soc. Jpn. 1997, 70, 535–541. (18) Yamashoji, Y.; Fujiwara, M.; Tanaka, M. Chem. Lett. 1993, 1029– 1032. (19) Chierotti, M. R.; Gobetto, R. Chem. Commun. 2008, 1621–1634. (20) Yan, X. L.; Chen, T. B.; Liao, X. Y.; Huang, Z. C.; Pan, J. R.; Hu, T. D.; Nie, C. J.; Xie, H. EnViron. Sci. Technol. 2008, 42, 1479–1484.
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