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Langmuir 2002, 18, 5974-5976
Temperature Induced Crystallization Transition in Aqueous Solutions of β-Cyclodextrin, Heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB), and Heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) Studied by Differential Scanning Calorimetry Joachim Frank,*,† Josef F. Holzwarth,† and Wolfram Saenger‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, and Institut fu¨ r Kristallographie, Freie Universita¨ t Berlin, Takustrasse 6, D-14195 Berlin, Germany Received January 31, 2002. In Final Form: May 15, 2002 Methylated cyclodextrins were investigated to shed light on their solubility behavior in water where they exhibit a negative temperature coefficient, contrasting the positive temperature coefficient of unmodified cyclodextrins. Both heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and heptakis(2,3,6-tri-O-methyl)β-cyclodextrin (TRIMEB) show two different conditions of crystallization. At low temperatures around 18 °C, highly hydrated clathrates are formed, whereas, at high temperatures around 60-70 °C, DIMEB crystallizes as an anhydrate and TRIMEB as a monohydrate. The crystallization at high temperature is driven by entropy gain of water to compensate for the positive enthalpy change associated with this crystallization process, as could be shown by differential scanning calorimetry experiments in H2O and D2O.
Introduction
Materials and Methods
Compared with unsubstituted β-cyclodextrin (β-CD), methylated β-cyclodextrins (mβ-CDs) show different physical properties, and their tendencies to form inclusion complexes are increased.1 In particular, mβ-CDs are manyfold more soluble in water than β-CD and exhibit a negative temperature coefficient for their aqueous solubility,2 contrasting the positive coefficient of β-CD. If concentrated cold solutions of mβ-CD are heated to approximately 70 °C, crystals appear which redissolve upon cooling. This effect is reflected by the solubility of heptakis(2,6-di-O-methyl)-β-cyclodextrin (hereafter abbreviated as DIMEB), 60 g/100 mL at room temperature and only 70 °C,3 a property that is shared by heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (hereafter abbreviated as TRIMEB). DIMEB and TRIMEB crystallize from heated aqueous solutions as anhydrous DIMEB4 and TRIMEB‚H2O,5 but at 18 °C, DIMEB crystallizes as heavily hydrated semiclathrate DIMEB‚15H2O,6 suggesting that aqueous solutions of DIMEB and TRIMEB are dehydrated upon heating. The crystallization process was studied in greater detail, using differential scanning calorimetry to derive the crystallization transition temperature and the heats of crystallization of DIMEB and TRIMEB in H2O/D2O and to obtain information on the influence of salt, in the presence of 0.5 M (NH4)2SO4.
DIMEB, TRIMEB, and β-CD were purchased from Aldrich (Germany). DIMEB was purified by treatment with charcoal followed by repeated recrystallization in hot water as described previously.7 TRIMEB and β-CD were used without further purification. (NH4)2SO4 was p. a. quality (Merck, Germany). DIMEB and TRIMEB were dissolved either in deionizied water of Millipore-Q quality or in 99.75% D2O (Merck, Germany). The temperature induced crystallization transitions of DIMEB and TRIMEB were studied by differential scanning calorimetry (DSC) experiments performed in a MicroCal MC-2 instrument (MicroCal, Northampton, MA) between 4 and 95 °C at the scan rate 20-30 °C per hour (up-scans) followed by down-scans with the scan rate -10 to -20 °C per hour. The β-CD and mβ-CD concentrations were 50 and 100 mg/mL and 30-200 mg/mL, respectively, in H2O or D2O with and without (NH4)2SO4 as additive. The heat capacity cp (J/°C) was measured as a function of temperature (°C). cp was converted to the molar heat capacity cpm (kJ mol-1 °C-1), accounting for cell volume (V ) 1.2249 mL) and sample concentration. The phase transition temperatures Tcryst (°C) of the crystallization processes were obtained from the DSC curves at the maximum of the molar heat capacity cpm. The area under the DSC curve corresponds to the molar enthalpy change ∆Hcal (kJ/mol) of crystallization.
* Corresponding author. Phone: +49-30-8413-5517. Fax: +4930-8413-8353. E-mail:
[email protected]. † Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡ Freie Universita ¨ t Berlin. (1) Rekharsky, M.; Inoue, Y. Chem. Rev. 1988, 98, 875. (2) Szejtli, J. Cyclodextrin Technology; Kluwer Academic Publishers: Dortrecht, 1988. (3) Uekama, K.; Irie, T. In Cyclodextrins and Their Industrial Uses; Ducheˆne, D., Ed.; Editions de Sante´: Paris, 1987; p 395. (4) Steiner, T.; Saenger, W. Cabohydr. Res. 1995, 275, 73. (5) Caira, M. R.; Griffith, V. J.; Nassimbeni, L. R.; Oudtshoorn, B. J. Chem. Soc., Perkin Trans. 1994, 2, 2071. (6) Aree, T.; Hoier, H.; Schulz, B.; Reck, G.; Saenger, W. Angew. Chem., Int. Ed. 2000, 39, 897.
Results Figure 1 shows the molar heat capacity cpm as a function of temperature for DSC up-scans of DIMEB and TRIMEB, both dissolved in H2O at the same concentration of 40 mg/mL. Under these conditions, crystallization occurs at 66 °C for DIMEB and at 87 °C for TRIMEB. The enthalpy changes ∆Hcal associated with the crystallization processes are positive for all solutions of DIMEB and TRIMEB so far studied, indicating that crystallization is favored at high temperatures where ∆G becomes negative because the term -T∆S exceeds ∆H and crystallization takes place. The crystallization transition temperature Tcryst decreases with increasing concentrations of DIMEB and TRIMEB (7) Umbach, P.; Georgalis, Y.; Saenger, W. J. Am. Chem. Soc. 1996, 118, 9314.
10.1021/la020105a CCC: $22.00 © 2002 American Chemical Society Published on Web 07/03/2002
Letters
Langmuir, Vol. 18, No. 16, 2002 5975 Table 2. Phase Transition Temperature Tcryst (°C), Molar Enthalpy Change ∆Hcal (kJ/mol), and Molar Entropy Change (J/(mol K)) of the Thermally Induced Crystallization Transition Process of TRIMEB Dissolved in H2O or D2O with and without 0.5 M (NH4)2SO4 As Observed in DSC Experimentsa [TRIMEB] (mg/mL) 40 40 40 100 200
Figure 1. Thermally induced crystallization transition process as observed in DSC up-scans of 40 mg/mL DIMEB (---) and 40 mg/mL TRIMEB (-), both dissolved in H2O.
Tcryst (°C)
∆Hcal (kJ/mol)
Influence of Concentration in H2O 87.5 14.1 ub 86.4 17.4 u 34.9 32.3 d 75.2 16.9 u 56 33.6 u
∆S (J/(mol K)) 39.1 48.4 104.8 48.5 102.1
Influence of D2O or (NH4)2SO4 Compared to H2O 40 in D2O 83.3 31.9 u 89.5 40 in D2O 43.2 34.9 d 110.3 40 0.5 M (NH4)2SO4 in H2O 64.7 34.8 u 103 40 in H2O 87 15.5 u 44 40 in H2O 34.9 32.3 d 104.8 b
a Experimental error: T cryst, (1 °C; ∆Hcal, (10%; ∆S, (10%. u/d: up/down-scan.
Table 1. Phase Transition Temperature Tcryst (°C), Molar Enthalpy Change ∆Hcal (kJ/mol), and Molar Entropy Change (J/(mol K)) of the Thermally Induced Crystallization Transition Process of DIMEB Dissolved in H2O or D2O As Observed in DSC Experimentsa [DIMEB] (mg/mL) 30 40 40 60 100 100 200
Tcryst (°C)
∆Hcal (kJ/mol)
Influence of Concentration in H2O ≈70 b uc 65.2 35.2 u 36.4 35.4 d 64.6 35.3 u 61.4 41 u 35.9 31.5 d 52.4 40.7 u
∆S (J/(mol K)) b 104 114 104.5 122.5 102 125
Influence of D2O or (NH4)2SO4 Compared to H2O 40 in D2O 57.8 38.7 u 117 40 in D2O 34 45.6 d 148.5 40 0.5 M (NH4)2SO4in H2O 46.5 58.1 u 181.8 40 in H2O 65.9 34.2 u 101 40 in H2O 36.4 35.4 d 114.4 a Experimental error: T cryst, (1 °C; ∆Hcal, (10 %; ∆S, (10 %. ∆H not measurable because the peak was too small. c u/d: up/ down-scan.
b
(Tables 1 and 2), and Tcryst for TRIMEB (87.5-56 °C) is significantly higher than that for DIMEB (65.9-52.4 °C) with all other experimental conditions being equal. Crystals of DIMEB redissolve at (36.4-35.9 °C) if the suspensions are cooled, indicating significant hysteresis (Figure 2 and Tables 1 and 2). In particular, DIMEB dissolved in D2O exhibits significantly lower Tcryst values than those in H2O (Table 1). For TRIMEB, this effect is less pronounced (Table 2). (NH4)2SO4 at 0.5 M concentration decreases the temperature of the crystallization transition very strongly for both DIMEB and TRIMEB (Tables 1 and 2). The measured enthalpy changes ∆Hcal for solutions of DIMEB increase with increasing concentrations of DIMEB (Table 1), from 34.2 ( 3 kJ/mol at 40 mg/mL to 41 ( 4 kJ/mol at 200 mg/mL. This is because, with increasing concentrations of DIMEB, increasing amounts of DIMEB crystallize, leading to higher enthalpy changes per mole, whereas the concentration of DIMEB in the mother liquor is constant at saturation (where crystallization occurs). At 30 mg/mL DIMEB, only a very small peak could be observed at ≈70 °C, that could not be used to derive ∆Hcal, but at and above 40 mg/mL, the signal was significant.
Figure 2. Thermally induced crystallization transition process of 40 mg/mL DIMEB dissolved in H2O as observed in DSC upscans (Tcryst ) 65.9-64.4 °C). Crystals of DIMEB redissolve in down-scans at (36.4-35.6 °C) if the suspensions are cooled below the temperature required for crystallization. Table 3. Phase Transition Temperature Tcryst (°C), Molar Enthalpy Change ∆Hcal (kJ/mol), and Molar Entropy Change (J/(mol K)) of the Thermally Induced Crystallization Transition Process of β-CD Dissolved in H2O or D2O As Observed in DSC Experiments between 5 and 90 °Ca [β-CD] (mg/mL) 50 in H2O 50 in D2O 100 in H2O b
Tcryst (°C)
∆Hcal (kJ/mol)
∆S (J/(mol K))
Influence of D2O and Concentration 40.4 very small dc very smallb 49.1 0.71 d 2.2 48.5 0.59 d 1.8
a Experimental error: T cryst, (1 °C; ∆Hcal, (20%; ∆S, (20%. Could not be evaluated. c d: down-scan.
For TRIMEB, ∆Hcal also increases from 14.1 ( 2 kJ/mol at a concentration of 40 mg/mL to 33.6 ( 3 kJ/mol at 200 mg/mL (Table 2). The unsubstituted β-CD shows a positive temperature coefficient. β-CD was dissolved in hot (90 °C) H2O or D2O and crystallized in down-scans. β-CD crystallizes at 40.4 and 48.5 °C, respectively, depending on concentration, 50 mg/mL and 100 mg/mL (Table 3). D2O shifted Tcryst at 50 mg/mL by ∼9 °C, which is more than observed with the methylated cyclodextrins. Cooling of concentrated solu-
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Langmuir, Vol. 18, No. 16, 2002
tions of unsubstituted β-CD shows only very small ∆Hcal values (far below 1 kJ/mol) for the crystallization transition, so that ∆Hcal and ∆S could not be determined. Discussion As the solubility coefficient of β-CD is positive, it crystallizes from saturated hot (∼80 °C) aqueous solutions upon cooling. In the obtained crystals of β-CD‚12 H2O, β-CD is highly hydrated.8,9 This contrasts the methylated forms DIMEB and TRIMEB, which crystallize as an anhydrate and a monohydrate, respectively,4,5 from hot water, as their solubility coefficients are negative. However, DIMEB also crystallizes from highly concentrated aqueous solution at 18 °C as DIMEB‚15H2O, in which the water forms a cage-type semiclathrate host network that encloses the DIMEB molecules as guests.6 This semiclathrate is unique, as it was only observed with DIMEB and with no other cyclodextrin or cyclodextrin derivative. The ∆Hcal values measured in the present study reflect these differences. For β-CD, only small ∆Hcal values around 1 kJ/mol were obtained for the phase transition solution f crystal. This we explain with comparable environments in the two states as in aqueous solution, β-CD is heavily hydrated, and a comparable situation is found in the crystals where all potential hydrogen bond donors and aceptors are hydrogen bonded, either to water or to symmetry related CD molecules.8,9 Likewise, DIMEB in aqueous solution will be well hydrated, as shown by the good solubility in water and by the crystals grown from cold solutions, DIMEB‚15H2O.6 We envisage that, with increasing temperature of these solutions, water molecules in the hydration shell of DIMEB become more mobile and diffuse into bulk water as the more favorable environment. This is associated with an increase in entropy and provides the driving force for crystallization. Upon depletion of their hydration shell, the hydrophobic DIMEB molecules aggregate7 and crystallize with no or only one water of (8) Betzel, C.; Saenger, W.; Hingerty, B. E.; Brown, G. M. J. Am. Chem. Soc. 1984, 106, 7545. (9) Zabel, V.; Saenger, W.; Mason, S. A. J. Am. Chem. Soc. 1986, 108, 3664.
Letters
hydration4,5 in the crystal asymmetric unit. The substantial ∆Hcal of crystallization in H2O (34.2-41 kJ/mol) appears to be due to the differences in environment for the methylated cyclodextrins: fully hydrated in aqueous solution at low temperature and dehydrated in the crystal lattice. This view is consistent with the temperature dependence of crystallization and the influence of D2O. The entropy gain due to dehydration of mβ-cyclodextrin upon crystallization explains the decrease of Tcryst. in the presence of D2O, since D2O forms stronger hydrogen bonds compared with those for H2O,10 yielding a larger entropy increase if D2O molecules move from the surface of the mβ-cyclodextrins into bulk D2O. The strong hysteresis observed for 40 mg/mL DIMEB between crystallization in the up-scan (Tcryst 65.9 °C (DIMEB) and 87.5 °C (TRIMEB)) and dissolution of crystals in the down-scan (Tcryst 36.4 °C (DIMEB) and 34.9 °C (TRIMEB)) must be associated with entropy contributions of water either in bulk aqueous solution or next to the mβ-cyclodextrin molecules at the surface of the crystals. The observation that crystals dissolve at much lower temperatures than those at which they are formed suggests that the entropy contribution from water in bulk solution around 35 °C is small enough to allow mβ-cyclodextrin molecules to be fully removed from the crystal lattice, hydrated, and dissolved in water. We checked this by leaving crystals of DIMEB in their mother liquor at 60 °C for several days. The crystals remained intact, and they dissolved only at temperatures of 35 °C or below. In principle, DIMEB should crystallize if aqueous solutions are cooled below 18 °C to form the DIMEB‚15H2O semiclathrate. This requires, however, very high concentrations of DIMEB and longer times (2 weeks), both conditions which could not be verified in the DSC experiments. LA020105A (10) Klinman, J. P. In Transition States of Biochemical Processes; Gandour, R. D., Schowen, R. L., Eds.; Plenum Press: New York and London, 1978; p 165.
