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Monitoring Processes for the Heat-Induced Crystallization of Heptakis(2,6-di-O-methyl)-β-cyclodextrin in Water Eun Chul Cho*,† and Hyung Jun Lim‡ † ‡
Department of Chemical Engineering, Division of Chemical and Bioengineering, Hanyang University, Seoul 133-791, Korea Amorepacific Corporation/R&D Center, Yongin City 449-729, Korea
bS Supporting Information ABSTRACT: Although some structural analyses of heptakis(2,6-di-O-methyl)-β-cyclodextrin hydrates have been well conducted, little has been known about processes for the crystallization of this molecule in water upon heating. This study first monitors processes for the heat-induced crystallization of heptakis(2,6-di-O-methyl)-β-cyclodextrin in water from a dynamic thermal analysis. Results showed that the crystallization is induced by slow dehydration of water molecules from DM-β-CD molecules, and this process is followed by the fast crystallization of DM-β-CD molecules.
S
ome hydrophobically modified cyclodextrins have been known to have negative temperature coefficients in water.1 A typical example is heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD, Figure 1a). Unlike other types of cyclodextrin compounds, these molecules decrease their solubilities in water and finally crystallize (Figure 1b and c). Saenger and co-workers thoroughly investigated the structures of these CDs, and they proposed that large amounts of water molecules surround the CDs at low temperatures.2,3 By contrast, at high temperatures, the molecules are completely dehydrated or only a few water molecules are remaining in the CDs. In addition, they also found that the crystallization of these CDs in water brought large endothermic heats, especially for DM-β-CD.4 However, considering most compounds accompany exothermic heats during their crystallizations, the endothermic heat implies that more than two processes are involved in the crystallization of DM-βCD. Moreover, the processes of crystallization for DM-β-CD in water are not clearly understood, although the mechanism will provide an important clue to expand the application of this molecule to a variety of biomedical fields. Here we investigate the processes involved in the crystallization of DM-β-CD in water. For this study, we introduced temperature-modulated differential scanning calorimetry (TMDSC). The principles of this technique are well-described elsewhere.5 TMDSC has been used to separate a heat which reflects complex processes. For example, it is useful to separate a peak into melting and crystallization peaks of semicryastalline polymers5,6 or to explain a lower critical solution temperature (LCST) behavior of a polymer in water.7 Unlike other calorimetric methods, TMDSC increases temperature in a sinusoidal manner with a certain frequency. As such, we expect that the single endothermic peak of the DM-β-CD solution may be separated if at least two different thermal events would exist in the peak and if these events would have different relaxation rates. r 2011 American Chemical Society
We first observed the crystallization of DM-β-CD in water. An aqueous solution of DM-β-CD was heated from room temperature, where the solution is clear and homogeneous (Figure 1b). At a certain temperature during heating, flake or needle-like crystals started to form (Figure 1c; see also Figure S1 in the Supporting Information). When the crystal-containing solution was cooled to room temperature, the crystals were resolubilized and the solution became homogeneous, indicating that the crystallization/resolubilization process is reversible, with some hysteresis as reported in the other reference.4 The LCST of the DM-β-CD is highly dependent on the concentration of DM-βCD, as shown in the phase diagram in Figure 1d. The shape of the crystals is also dependent on their concentrations (see Figure S1 in the Supporting Information). We next examined the thermal behavior of DM-β-CD in water. We recorded conventional differential scanning calorimetry (DSC) thermograms for 3, 5, and 10 wt % aqueous solutions of DM-β-CD by heating from 0 to 100 °C at a scan rate of 0.5 °C/min (Figure 2a). All the aqueous solutions have endothermic peaks with onset temperatures of 66.4, 59.4, and 55.5 °C, respectively. Meanwhile, the DSC thermograms for other CDs under the identical condition showed no significant indication of a transition (Figure 2b). Only heptakis(2,3,6-tri-O-methyl)-βcyclodextrin (TM-β-CD), which is also known to have a negative temperature coefficient in water,8 showed a small endothermic peak around 90 100 °C. To explore the origin of this large endothermic peak for the DM-β-CD in water during its crystallization, we further recorded TMDSC thermograms. In this experiment, the temperature is increased in a sinusoidal manner as shown in Figure 3a and with the following equation: T(t) = T0 + β0t + A sin ωt, where T0 is Received: July 27, 2011 Published: August 24, 2011 4296
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Figure 1. (a) Chemical structure of DM-β-CD. (b) A photograph of DM-β-CD aqueous solutions (10 wt %) at 80 °C and room temperature. (c) An optical microscope image of DM-β-CD aqueous solutions (10 wt %) at 80 °C. (d) A phase diagram of the DM-β-CD aqueous solution.
Figure 2. DSC thermograms of (a) DM-β-CD (3, 5, and 10 wt %) and (b) other CDs' (3 wt %) aqueous solutions recorded using a heating rate of 0.5 °C/min. Abbreviations: β-CD, β-cyclodextrin; HP-β-CD, (2-hydroxypropyl)-β-cyclodextrin; M-β-CD, methyl-β-cyclodextrin; γ-CD, γ-cyclodextrin.
initial the temperature, t is the time, β0 is the average heating rate (0.5 °C/min), A is the temperature amplitude (0.2 °C), and ω is the angular frequency equal to 2πf. Then, the heating rate, β, is also modulated like β = β0 + Aω cos ωt (Figure 3b). As such, we can separate the overall heat flow shown in Figure 2a into the two heat flows: the one is the reversing heat flow that can respond to the applied frequency (f) and reflects relatively fast process(es) in the transition (f, Figure 3c), and the other is the nonreversing heat flow that is not responsive to the f and thus reflects relatively slow
process(es) in the transition (Figure 3d).4 These terms should not be confused with the real and imaginary parts of the heat capacity (or heat flow) or with the concepts of reversibility and irreversibility. In this study, f was changed from 0.005 0.02 Hz. From the Figure 3, the overall heat flow in Figure 2a and Figure S2 (in the Supporting Information) consists of an exothermic shift of the baseline in the reversing heat flow and an endothermic peak in the nonreversing heat flow. Table 1 shows an analysis of these thermograms. For f = 0.02 and 0.01 Hz, the thermograms and the transition heats (ΔH) are similar. However, when the frequency is further decreased to 0.005 Hz, the reversing thermogram shows an endothermic peak at the start of the phase transition and an exothermic peak at the end of the transition. In addition, the transition heat of the nonreversing heat flow (ΔHnonrev) for this frequency differs from those for 0.02 and 0.01 Hz. Our TMDSC results revealed that at least one endothermic and one exothermic process are involved in the transition. In addition, the exothermic shift of the baseline in the reversing heat flow is indicative of the exothermic process(es) being somewhat faster than the endothermic process(es). The ΔHnonrev increases with decreasing f, and the exothermic peak at 0.005 Hz was detected with a higher intensity than the endothermic peak. This implies that the exothermic process(es) move to the reversing heat flow faster than the endothermic process(es) with decreasing f. On the basis of our experimental results and the literature, it is possible to sketch and suggest the crystallization processes of DM-β-CD (Figure 4). The reversing thermograms (Figure 3c) imply that the endothermic thermal event first takes place, which is followed by the exothermic event. Saenger et al. suggested that waters form a cage-type semiclathrate host network that encloses the DM-β-CD molecules as guests.2,3 The water network consists of (i) water molecules strongly or weakly hydrogen-bonded with oxygens of DM-β-CD, and (ii) water molecules hydrogenbonded by themselves within the network. In addition, from simulations, they also suggested that (iii) some water molecules surround the hydrophobic molecules (methyl groups in the C2 4297
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Figure 3. (a) Temperature profile of DM-β-CD (10 wt %) aqueous solutions during heating from TMDSC (solid red line). The profile from DSC is shown (dashed line) for comparison. (b) Heating rate profile of DM-β-CD aqueous solution during scan from TMDSC. For parts a and b, the modulation frequency (f) was 0.02 Hz. (c) Reversing and (d) nonreversing heat flows of the DM-β-CD aqueous solution (10 wt %) for various f values. For all the samples, the scans were conducted from 0 to 100 °C, the average heating rate was 0.5 °C/min, and the heat amplitude (A) was 0.2 °C.
Table 1. Thermal Parameters of 10 wt % DM-β-CD in Watera transition heat (kJ/mol) b
frequency (f, Hz)
overall heat (ΔHoverall)
nonreversing heat (ΔHnonrev)
0.02
36.9
38.5
0.01
36.8
38.5
0.005
37.0
39.7
transition temperatures (°C) c
ΔHnonrev
ΔHoverall
onset
peak
1.6
55.9
58.0
1.7
55.4
57.8
2.7
55.8
58.2
a
Thermal parameters were obtained from TMDSC thermograms shown in Figure 3 and Figure S2 in the Supporting Information. b Transition heat from overall heat flow. c Transition heat from nonreversing heat flow in TMDSC.
Figure 4. Crystallization processes of DM-β-CD in water. 4298
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Crystal Growth & Design and C6; see Figure 1a).3 The third types of waters are also found at low temperatures around small hydrophobic groups9 11 or macromolecules having these groups,12 15 and they are called clathrates, too. The structures of these bound water molecules are deviated from that of bulk waters with different hydrogen bonding energies.10,11,16 Upon heating, they rearranged their structures by breaking their hydrogen bonds and by newly forming hydrogen bonds to become bulk waters. X-ray crystallographic studies revealed that none of bound water molecules remained in DM-β-CD through this event.1 Like β-CD,17 dehydration of the DM-β-CD and the following rearrangement of bound water molecules to bulk waters may undergo an endothermic process. Therefore, the first endothermic event of the crystallization of DM-β-CD might be for the reconstruction of bound water molecules to bulk waters. The enthalpies for the crystallization of DM-β-CD from our results (ΔHoverall, ∼37 kJ/mol) and from ref 4 (35 40 kJ/mol) are smaller than that for dehydration of β-CD (75.3 kJ/mol).17 These endothermic heats seem reasonable when just considering bound water molecules directly hydrogen bonded to the two CDs (11 water molecules for β-CDs and 6 waters to DM-βCD).3 However, as stated above, DM-β-CD has two more types of bound waters (types ii and iii) different from bulk waters, and the rearrangement of these molecules may also contribute to the enthalpy.7,10,11,13,16 As such, the endothermic heat for this process should have been higher than ΔHoverall. After rearrangement of bound waters (dehydration), the OCH3 groups in the C6 of the DM-β-CD tend to move inside to reduce the contact with water,2 and the DM-β-CD molecules start to associate (crystallize) through a self-inclusion2 or a dimerization like β-CD.19 Since the OH group in C3 of DM-β-CD involves an intermolecular hydrogen bond such as O3 H 3 3 3 O2, the intra-/intermolecular hydrophobic interactions may be the main driving force for the crystallization or association. The hydrophobic interaction forces are known to be strong in water and are usually exothermic processes.7,9,13 Therefore, it is suggested that these exothermic heats compensate the endothermic heat of DM-β-CD in water, thereby resulting in ΔHoverall. The phase transition is initiated by dehydration, and then the DM-β-CD molecules begin to crystallize. From our results, the former processes (endothermic) are a little slower than the latter (exothermic) because most endothermic heats are shown in the nonreversing heat flow. Under this situation, the dehydration process would be the rate-determining step of this phase transition event. That is to say, the rearrangement of bound water molecules may play significant roles in the crystallization of DMβ-CD. For example, as shown in Figure 1d, the crystallization temperature is quite dependent on the concentrations of DM-βCD. This may suggest that the different state and structure of bound waters around DM-β-CD molecules for various concentrations differently affect the crystallization of DM-β-CD. In conclusion, our TMDSC experiments suggest that the phase transition of the DM-β-CD in water is the result of dehydration of water molecules around DM-β-CD (slow and endothermic processes) followed by crystallization of DM-β-CD (fast and exothermic processes). More dynamic thermal studies are necessary because the temperature responsiveness of DM-βCD in aqueous media is potentially useful for biomedical applications. In particular, a design of molecules having a DM-β-CD moiety may enable such molecules to be temperature responsive in aqueous media as well as to deliver hydrophobic drugs.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental protocols, optical microscope images for the crystals of DM-β-CD at their different concentrations in water, and TMDSC overall thermograms for DM-β-CD in water for various frequencies. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: (+82)-2-2220-2332. Fax:(+82)-2-2298-4101. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by BK21 programs and a Manpower Development Program for Energy (MKE). The authors also acknowledge the technical support of this work from Amorepacific Corporation R&D center. ’ REFERENCES (1) Steiner, T.; Saenger, W. Carbohydr. Res. 1995, 275, 73. (2) Aree, T.; Hoier, H.; Schulz, B.; Reck, G.; Saenger, W. Angew. Chem., Int. Ed. 2000, 39, 897. (3) Starikov, E. B.; Brasicke, K.; Knapp, E. W.; Saenger, W. Chem. Phys. Lett. 2001, 336, 504. (4) Frank, J.; Holzwarth, J. F.; Saenger, W. Langmuir 2002, 18, 5974. (5) Gill, P. S.; Sauerbrunn, S. R.; Reading, M. J. Therm. Anal. 1993, 40, 931. (6) (a) Luan, C.; Urry, D. W. J. Phys. Chem. 1991, 95, 7896. (b) Okazaki, I.; Wunderlich, B. Macromolecules 1997, 30, 1758. (7) Cho, E. C.; Lee, J.; Cho, K. Macromolecules 2003, 36, 9929. (8) Caira, M. R.; Griffith, V. J.; Nassimbeni, L. R.; Oudtshoorn, B. van J. Chem. Soc., Perkin Trans. 2 1994, 2071. (9) (a) Frank, H. S.; Evans, M. W. J. Chem. Phys. 1945, 13, 507.(b) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley & Sons, Inc.: 1980. (c) Shinoda, K.; Fujihira, M. Bull. Chem. Soc. Jpn. 1968, 41, 2612. (10) Stillinger, F. H. Science 1980, 209, 451. (11) Sciortino, F.; Geiger, A.; Stanley, H. E. Nature 1991, 354, 218. (12) (a) Smith, G. D.; Bedrov, D. J. Phys. Chem. B 2003, 107, 3095. (b) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548. (13) (a) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (b) Inomata, H.; Goto, S.; Saito, S. Macromolecules 1990, 23, 4887. (c) Shibayama, M.; Mizutani, S.; Nomura, S. Macromolecules 1996, 29, 2019. (d) Grinberg, V. Y.; Dubovik, A. S.; Kuznetsov, D. V.; Grinberg, N. V.; Grosberg, A. Y.; Tanaka, T. Macromolecules 2000, 33, 8685. (14) Privalov, P. L.; Gill, S. J. Pure Appl. Chem. 1989, 61, 1097. (15) Luan, C.; Urry, D. W. J. Phys. Chem. 1991, 95, 7896. (16) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. B 1997, 101, 9767. (17) Linert, W.; Margl, P.; Renz, F. Chem. Phys. 1992, 161, 327. (18) Umbach, P.; Georgalis, Y.; Saenger, W. J. Am. Chem. Soc. 1996, 118, 9314. (19) Stezowski, J. J.; Jogun, K. H.; Eckle, E.; Bartels, K. Nature 1978, 274, 617.
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