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Thermodynamic Analysis of Inclusion Complexation between r-Cyclodextrin-Based Molecular Tube and Sodium Alkyl Sulfonate Taichi Ikeda, Etsuko Hirota, Tooru Ooya, and Nobuhiko Yui* School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan Received August 21, 2000. In Final Form: October 12, 2000 Thermodynamics on inclusion complexation of R-cyclodextrin-based molecular tube (MT) with sodium octyl-, decyl-, and dodecylsulfonates (C8-, C10-, and C12SO3Na) were measured by isothermal titration calorimetry (ITC) to evaluate the effects of alkyl chain length of CnSO3Na and temperature. It was found that CnSO3Na with a longer alkyl chain formed more stable inclusion complex with MT. The association constant of inclusion complexation between MT and C12SO3Na at 298 K was found to be on the order of 105. The stoichiometric number ([host]/[guest] ratio) was found to be 0.50 ( 0.05. This result indicates that one MT forms an inclusion complex with two CnSO3Na molecules, regardless of the alkyl chain length of CnSO3Na. Enthalpy-entropy compensation was also observed in MT/CnSO3Na systems, and it was confirmed that the free energy change was not affected by the change in temperature (288-308 K). From the thermodynamic parameters, it was considered that both hydrophobic interaction and van der Waals interaction were mainly contributed to inclusion complexation of MT with CnSO3Na. The large and negative entropy change (T∆S) attributed to the strong binding of MT was observed. The strong T∆S dependence on the alkyl chain length of CnSO3Na was found to be a characteristic for inclusion complexation of MT.
Introduction Cyclodextrin (CD) is a cyclic oligomer of R-D-glucose linked by R-1,4-glycoside bonds.1 The most familiar members are R-, β-, and γ-CDs consisting of six, seven, and eight glucose units, respectively. CD has the hollow truncated cone-like structure, and the environment inside the cavity of CD is hydrophobic due to C3H, C5H, and C6H hydrogen and O4 ether oxygen. The most intriguing feature of CDs is the ability to form an inclusion complex with many types of hydrophobic compounds. The inclusion complexation phenomena of CDs have been studied in various field, e.g., analytical chemistry, enzymology,2 pharmaceuticals,3 food industry,4 and so on. The thermodynamic analysis is important for the characterization of inclusion complexation with CDs. Various methods have been applied to analyze the thermodynamics on inclusion complexation of CD, e.g., electrochemical, spectroscopic, solubility, calorimetric, kinetic methods, and so on.5 Among these methods, isothermal titration calorimetry (ITC) is extensively applied in recent years. This is because ITC allows simultaneous determination of both free energy and enthalpy changes at a given temperature, as well as precise determination of heat capacity changes from measurements at different temperatures. The thermodynamics on inclusion complexation of CD have been analyzed and discussed by many research groups.6-10 * To whom correspondence should be addressed. Telephone: +81761-51-1640. Fax: +81-761-51-1645. E-mail:
[email protected]. (1) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; SpringerVerlag: Berlin, 1978. (2) Breslow, R.; Dong, S. D. Chem. Rev. 1998, 98, 1997. (3) Uekawa, K.; Hirayama, F.; Irie, T. Chem. Rev. 1998, 98, 2045. (4) Walter, R. H. Polysaccharide Association Structures in Food; Marcel Dekker: New York, 1998. (5) Schneider, H.-J.; Yatsimirsky A. Principles and Methods in Supramolecular Chemistry; John Wiley & Sons: New York, 2000. (6) Inoue, Y.; Hakushi, T.; Liu, Y.; Tong, L.-H.; Shen, B.-J.; Jin, D.-S. J. Am. Chem. Soc. 1993, 115, 475. (7) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875. (8) Rekharsky, M. V.; Mayhew, M. P.; Goldberg, R. N.; Ross, P. D.; Yamashoji, Y.; Inoue, Y. J. Phys. Chem. B 1997, 101, 87.
CDs are also used as a building block for supramolecular structures.11 Modified cyclodextrins12 or molecular assemblies of CDs13-15 have been extensively studied by many research groups. In the 1990s, Harada et al. have proposed a new type of molecular assembly consisting of CDs and a linear polymer.16-19 They prepared a polyrotaxane, in which many cyclic molecules are threaded onto a linear polymeric chain capped with bulky blocking groups, consisting of R-CDs and poly(ethylene oxide) (PEO).20 Moreover, they successfully prepared a tubular conjugate, so-called “molecular tube (MT)”, by crosslinking adjacent hydroxyl groups of R-CDs in the polyrotaxane.21 Due to its large and hydrophobic cavity for inclusion complexation, MT is expected to form an inclusion complex with the guest molecules having a long hydrophobic segment. In our previous study, we measured the thermodynamics on inclusion complexation between MT and sodium dodecyl sulfate (SDS) by ITC.22 In that study, it was confirmed that the inclusion complex between MT and SDS was more stable than that between R-CD and SDS. MT was found to form the inclusion complex with two SDS molecules. Furthermore, we prepared a new (9) Halle´n, D.; Scho¨n, A.; Shehatta, I.; Wadso¨, I. J. Chem. Soc., Faraday Trans. 1992, 88, 2859. (10) Rekharsky, M.; Inoue Y. J. Am. Chem. Soc. 2000, 122, 4418. (11) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (12) Easton, C. J.; Lincoln, S. F. Modified Cyclodextrins; Imperial College Press: London, 1999. (13) Nepogodiev, S. A.; Stoddart, J. F. Chem. Rev. 1998, 98, 1959. (14) Yamaguchi, I.; Ishii, H.; Osakada, K.; Yamamoto, T.; Fukuzawa, S. Bull. Chem. Soc. Jpn. 1999, 72, 1541. (15) Ooya, T.; Yui, N. Crit. Rev. Ther. Drug Carrier Syst. 1999, 16, 289. (16) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821. (17) Harada, A.; Li J.; Kamachi M. Nature 1994, 370, 126. (18) Harada, A.; Okada, M.; Li J.; Kamachi, M. Macromolecules 1995, 28, 8406. (19) Harada, A.; Kawaguchi, Y.; Nishiyama, T.; Kamachi, M. Macromol. Rapid. Commun. 1997, 18, 535. (20) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325. (21) Harada, A.; Li, J.; Kamachi, M. Nature 1993, 364, 516. (22) Ikeda, T.; Ooya, T.; Yui, N. Polym. Adv. Technol. 2000, 11, 830.
10.1021/la001205o CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000
Thermodynamic Analysis of Inclusion Complexation Scheme 1. Preparation of r-Cyclodextrin-Based Molecular Tube
supramolecular architecture through inclusion complexation between MT and the amphiphilic chains grafted to dextran.23 These studies suggest that MT has potential as a good building block to construct the supramolecular assemblies in the nano-science field. In this study, the thermodynamic parameters for inclusion complexation between MT and sodium alkyl sulfonates (CnSO3Na) were measured by ITC. The alkyl chain length of CnSO3Na and temperature were changed to discuss the thermodynamics on inclusion complexation of MTs. These systematic analyses will clarify the physicochemical characteristics on inclusion complexation of MTs. Experimental Section Materials. Poly(ethylene oxide) (PEO, Mn ) 2000), dinitrofluorobenzene (DNFB), epichlorohydrin, R-cyclodextrin (R-CD), sodium octanesulfonate (C8SO3Na), sodium decanesulfonate (C10SO3Na), sodium dodecanesulfonate (C12SO3Na), dimethylformamide (DMF), diethyl ether, dimethyl sulfoxide (DMSO), 2 N sodium hydroxide aqueous solution (NaOH aq), hydrochloric acid (HCl), and pure water were commercially available. C8-, C10-, and C12SO3Na were purified by recrystallization from ethanol. DMF was distilled by the usual method. DMSO in HPLC grade was purchased from Kishida Chemical Co., Ltd. A HPLC system equipped with an intelligent HPLC pump PU-980 and a two-line degasser DG-980-51 was used for characterization (Japan Spectroscopic Co., Ltd.). The signals were detected by a refractive index detector RI-930 and a chiral detector OR-990 (Japan Spectroscopic Co., Ltd.). 1H NMR spectra were measured by a 300-MHz 1H NMR instrument Gemini 300 (Varian, Unity plus, CA). Preparation of r-ω-Diamino Poly(ethylene oxide). R-ωDiamino poly(ethylene oxide) (PEO-BA) (Mn ) 2000) was prepared from PEO (Mn ) 2000) according to the method reported by Pillai et al.24 The conversion to the amino group was confirmed to be 85% by titration. Preparation of Polyrotaxane. The polyrotaxane (2 in Scheme 1) was prepared according to the method reported by Harada et al.20 The pseudopolyrotaxane (1) was prepared from PEO-BA (2.5 g, 1.25 × 10-3 mol) and R-CD (500 mL of R-CD saturated solution). The pseudopolyrotaxane was obtained as white powder. Yield: 90%. The polyrotaxane (2) was prepared by the capping reaction of the pseudopolyrotaxane (25 g, 9.4 × 10-4 mol) with DNFB (25 (23) Ikeda, T.; Ooya, T.; Yui, N. Macromol. Rapid. Commun., in press. (24) Pillai, V. N. R.; Mutter, M.; Bayer E.; Catfield I. J. Org. Chem. 1980, 45, 5346.
Langmuir, Vol. 17, No. 1, 2001 235 g, 1.3 × 10-1 mol) in 200 mL of DMF. The product was washed by ether, DMF, and water. The precipitate was collected by the centrifuge and dried under vacuum to give the polyrotaxane (2). Yield: 25%. Finally, the product was purified by GPC (Sephadex G-50, 7 × 80 cm) with DMSO as an eluent. The product was characterized by HPLC (column, Shodex GF-510HQ; solvent, DMSO). The refractive index detector and the chiral detector were used for the detection. 1H NMR (DMSO-d ): δ 8.86 (s, 2 H, aromatic), 8.24 (d, 2 H, 6 aromatic), 7.25 (d, 2 H, aromatic), 5.65 (s, 6 H × 18, O2H of R-CD), 5.49 (d, 6 H × 18, O3H of R-CD), 4.78 (d, 6 H × 18, C1H of R-CD), 4.43 (s, 6 H × 18, O6H of R-CD), 3.60-3.80 (m, 24 H × 18, C3H, C6H, and C5H of R-CD), 3.51 (s, 4 H × 45, CH2 of PEG), 3.25-3.45 (m, 12 H × 18, C2H and C4H of R-CD). Preparation of Molecular Tube. MT (4 in Scheme 1) was prepared according to the method reported by Harada et al.21 The polyrotaxane (2.0 g, 1.0 × 10-4 mol) was allowed to react with epichlorohydrin (25 mL, 3.2 × 10-1 mol) in 2 N NaOH aq (155 mL) for 36 h at 20 °C. To remove the intermolecular crosslinked product, the product (3) was purified by GPC (Sephadex G-75 column, 5.0 × 90 cm) with water as an eluent. Further, the product (3) was treated with 2 N NaOH aqueous solution (150 mL) at 45 °C for 48 h to remove the dinitrophenyl groups. Finally, the prepared MT (4) was fractionated into eight portions by GPC (Sephadex G-50, 5.0 × 90 cm) with water as an eluent. Each fraction was lyophilized to give MT (4). Total yield: 25%. The product was characterized by GPC (column, Sephadex G-50; solvent, water). The chiral detector was used for the detection. 1H NMR (DMSO-d ): δ 5.25-5.80 (broad, O2H and O3H of 6 R-CD), 4.90 (s, C1H of R-CD with bridge), 4.73 (s, C1H of R-CD), 4.51 (s, OH of bridge), 4.40 (s, O6H of R-CD), 3.10-3.90 (broad, C3H, C6H, C5H, C2H, C4H of R-CD, CH, and CH2 of bridge). Isothermal Titration Calorimetry. The isothermal titration calorimetric experiments were carried out using an Omega isothermal titration calorimeter (MicroCal, Inc., Northampton, USA). MT with Mn of 6400 (Mw/Mn ) 1.67) was used in this study. The reaction cell and an injection syringe were filled with CnSO3Na aqueous solution (1.0 × 10-4 M) and MT aqueous solution (1.0 × 10-3 M), respectively. This arrangement minimized the contributions from the heat of dilution of CnSO3Na. The pure water was used as a solvent throughout the work. Twenty portions, 10 µL each, of the MT solution were injected to the CnSO3Na solution with intervals of 3 min and the binding heat was recorded. Heat of dilution was also observed to correct the observed binding heats. Calorimetric data were processed by the computer program Origin for ITC, Version 2.9 (MicroCal, Inc., Northampton, USA).
Results Characterization of Polyrotaxane. From the 1H NMR measurement of the polyrotaxane, all the peaks attributed to PEO, R-CD, and DNFB were confirmed. The peaks were broader than those attributed to free R-CD and PEO. This result indicates that the mobility of R-CD and PEO is restricted. As for the alternative characterization, HPLC measurement was also carried out. Since R-CD is optically active, the peak attributed to the polyrotaxane consisting of R-CD and PEO should be detected by the chiral detector. Figure 1 shows the HPLC charts of (a) the polyrotaxane, (b) PEO (Mn ) 2000), and (c) R-CD. The polyrotaxane was detected by the refractive index detector and the chiral detector at elution volume of 6.2 mL (Figure 1a). PEO was not detected by the chiral detector but was detected by the refractive index detector at elution volume of 7.7 mL (Figure 1b). R-CD was detected by the refractive index detector and the chiral detector at elution volume of 8.3 mL (Figure 1c). In the HPLC chart of the polyrotaxane, the peaks attributed to R-CD and PEO were not detected. These results indicate that the polyrotaxane has higher molecular weight than R-CD or PEO and did not contain free PEO and R-CD. The average number of R-CDs in the polyrotaxane was calculated to be ca. 18 from the peak integral of the 1H
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Figure 1. HPLC charts of (a) polyrotaxane, (b) poly(ethylene oxide) (Mn ) 2000), and (c) R-cyclodextrin (column, Shodex GF510HQ; solvent, DMSO). Table 1. Fractionation of MT by GPC fraction no.
elution volume,a mL
Mnb
Mw/Mnb
1 2 3 4 5 6 7 8
774-839 839-904 904-969 969-1034 1034-1099 1099-1164 1164-1229 1229-1294
18600 15000 11600 8300 6400 4100 2900 1800
1.91 1.88 1.80 1.74 1.67 1.58 1.40 1.42
a MT was fractionated by GPC, column: Sephadex G-50 (5 × 90 cm), solvent: water. b Molecular weight was estimated from GPC measurement, column: Sephadex G-50 (1 × 20 cm); solvent, water.
Figure 2. GPC charts of molecular tubes (a) Mn ) 11 600, (b) Mn ) 6400, (c) Mn ) 2900, and (d) R-cyclodextrin (column, Sephadex G-50; solvent, water).
NMR spectrum (C1H protons of R-CD vs CH2 protons of PEO). The average number of R-CDs in the polyrotaxane was also calculated from the absorbance attributed to DNFB ( ) 18 000 at 360 nm). From UV-vis spectrum measurement, the average number of R-CDs in the polyrotaxane was calculated to be ca. 17. These results are consistent with those reported by Harada et al.20 Characterization of MT. MT with wide molecular weight distribution was obtained after removing the dinitrophenyl groups of the polyrotaxane. This result was supported by a broad peak on the GPC chart (data not shown).22 To analyze the thermodynamics on inclusion complexation of MT precisely, the molecular weight distribution of MT should be minimized as much as possible. From this reason, MT was fractionated by GPC into eight portions. As shown in Table 1, the molecular weight of the fractionated MTs ranged from 1800 to 18600. Figure 2 shows the GPC charts of some fractionated MTs and R-CD. The fractionated MTs were optically active and the elution volumes of them were smaller than that of R-CD. When the solution of MT was added to an I3solution, the absorbance over 500 nm increased (data not shown). This result suggests that MT has a capillary-like
Figure 3. Isothermal titration calorimetry for inclusion complexation between a molecular tube and sodium dodecyl sulfonate at 298 K. Signal of titration (a) and binding isotherm (b).
structure. It was difficult to evaluate the degree of bridging from NMR peak intensities, because all the peaks attributed to MT were broadened and overlapped each other. These results were consistent with those reported by Harada et al.21 The degree of bridging was evaluated by the MALDI-TOF mass spectrum in our previous study.22 That result indicates that the adjacent R-CDs are crosslinked by three bridges on average. MT in the fifth fraction (Mn ) 6400, Mw/Mn ) 1.67) was used in the ITC experiment. This is because the molar yield of the fifth fraction was the highest of all the fractionated MTs. MT with Mn of 6400 corresponds to be a pentamer of R-CDs. Effect of Alkyl Chain Length on Thermodynamics of Inclusion Complexation between MT and CnSO3Na. The thermodynamics on inclusion complexation of MT with C8-, C10-, and C12SO3Na were analyzed by ITC at 298 K. Figure 3 shows signals of the titration (a) and a binding isotherm (b) for inclusion complexation between MT and C12SO3Na. Strong exothermic signals were observed by the titration of the MT solution to the CnSO3Na solution. It was confirmed that the heat of dilution was so small to be negligible. The enthalpy change (∆H), the association constant (Ka ) [complex]/[host][guest]), and the stoichiometric number (ncom) were evaluated by the nonlinear regression fitting of the binding isotherm. Furthermore, the free energy change (∆G) and the entropy change (∆S) were obtained from eqs 1 and 2.
∆G ) -RT ln Ka
(1)
∆G ) ∆H - T∆S
(2)
Table 2 summarizes the thermodynamic parameters for inclusion complexation in MT/CnSO3Na systems at 298 K. In Figure 4, ∆H, -T∆S, and ∆G for inclusion complexation in MT/CnSO3Na systems are plotted against the number of alkyl carbon atoms in CnSO3Na (nc) (298 K). As shown in Figure 4, ∆H decreased with increasing nc. On the contrary, -T∆S increased with nc. ∆G slightly decreased with increasing nc. Although the change in ∆G with nc was small, Ka in MT/C12SO3Na system was ca. 10 times larger than that in MT/C8SO3Na system. Ka in MT/ C12SO3Na system at 298K was found to be on the order of 105. The stoichiometric number of the inclusion complex between MT and sodium alkyl sulfonates is 0.50 ( 0.05 in this experiment. This result suggests that one MT forms the inclusion complex with two CnSO3Na molecules. -∆G, -∆H, and -T∆S increments per CH2 were 0.75, 4.4, and 3.6 kJ mol-1, respectively, at 298 K. It should be
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Table 2. Effect of Alkyl Chain Length on Thermodynamic Parameters for Inclusion Complexation between MT and CnSO3Na. ((SEM, n ) 3) guest molecule
T, K
∆H,a kJ mol-1
Ka,a,b 10-4 M-1
∆G,c kJ mol-1
T∆S,c kJ mol-1
ncoma,d
C8SO3Na C10SO3Na C12SO3Na
298 298 298
-47.3 ( 1.7 -68.7 ( 4.2 -82.8 ( 7.4
1.08 ( 0.08 4.64 ( 0.05 12.5 ( 0.4
-23.0 ( 0.2 -26.6 ( 0.1 -29.0 ( 0.1
-24.4 ( 1.9 -42.1 ( 4.3 -53.8 ( 7.5
0.47 0.43 0.50
a ∆H, K , and n b a com were evaluated directly from the titration data using the Origin software. Association constant (Ka ) [inclusion complex]/[host][guest]). c ∆G and T∆S were calculated using ∆G ) -RT ln Ka and ∆G ) ∆H - T∆S. d Stoichiometric number (host/guest ratio).
Figure 4. Effect of alkyl chain length on thermodynamics for inclusion complexation between a molecular tube and sodium alkyl sulfonate at 298 K. ∆G (2); ∆H (b); -T∆S (9). Average (SEM, n ) 3.
noted that the thermodynamic parameters in MT/CnSO3Na systems are calculated based on a mole of MT. As mentioned above, one MT molecule forms the inclusion complex with two CnSO3Na molecules. If one CH2 group is added to the CnSO3Na molecule, the increment of CH2 per MT is 2. Thus, the increments of the thermodynamic parameters per CH2 correspond to the half values of the slopes in Figure 4. For instance, the -∆H increment per CH2 for inclusion complexation with MT was 4.4 kJ mol-1 (82.8 - 47.3 ) 35.5 and 35.5/(4 × 2) ) 4.4). Effect of Temperature on Thermodynamics of Inclusion Complexation between MT and CnSO3Na. The thermodynamics on inclusion complexation between MT and C12SO3Na were analyzed by ITC at 288, 298, and 308 K. Table 3 summarizes the thermodynamic parameters for inclusion complexation in the MT/C12SO3Na system at 288, 298, and 308 K. In Figure 5, ∆G, ∆H, and -T∆S for inclusion complexation in MT/C12SO3Na system are plotted against temperature. ∆H decreased with increasing temperature. On the other hand, -T∆S increased with temperature. ∆G was almost independent of temperature. The temperature coefficient of ∆H, i.e., heat capacity change (∆Cp), was large and negative (-1.0 kJ K-1 mol-1). These results are consistent with those for inclusion complexation of R-CD.9 Discussion Inclusion Complexation between MT and CnSO3Na. The critical micelle concentrations (cmc’s) of C8-, C10-, and C12SO3Na were reported to be 1.55 × 10-1, 3.8 × 10-2, and 1.0 × 10-2 M, respectively.25 In this study, the concentration of CnSO3Na (1.0 × 10-4 M) was set far below their cmc’s. Thus, the process observed at ITC measurements did not involve the process of micelle deformation. The stability of the inclusion complex between MT and C12SO3Na was found to be comparable to that for the inclusion complex of β-CD with a 1-adamantane derivative (e.g., 1-adamantanecarboxylic acid)26 or β-CD with steroids (25) Tartar, H. V. J. Colloid Sci. 1959, 14, 115. (26) Eftink, M. R.; Andy, M. L.; Bystrom, K.; Perlmutter, H. D.; Kristol, D. S. J. Am. Chem. Soc. 1989, 111, 6765.
(Ka ≈ 104-106).27 These inclusion complexes are known to be quite stable. It was found that MT forms an inclusion complex with CnSO3Na in 1:2 stoichiometry. The lengths of fully extended octyl, decyl, and dodecyl chain are estimated to be ca. 11, 13, and 16 Å, respectively.25 The depth of the R-CD cavity was reported to be 8 Å from X-ray crystallography.28 Assuming that the length between the adjacent R-CDs in MT is 2 Å, the length of MT (pentamer of R-CD) is estimated to be ca. 50 Å. From the geometrical consideration, MT can bind many guest molecules inside the cavity. However, the number of the guest molecules included in the cavity is only two. Since the environment of R-CD inner cavity is hydrophobic,29 the hydrophilic SO3- headgroup should be unfavorable to be included in the cavity of MT. Figure 5 shows a possible structure of the inclusion complex between MT and CnSO3Na. Two CnSO3Na are considered to bind at both ends of MT, exposing the hydrophilic SO3- headgroups to the solvent. The changes in ∆H and -T∆S in MT/C12SO3Na systems against temperature were compensatory (Table 3 and Figure 5). It was found that inclusion complexation of MT also followed the enthalpy-entropy compensation.6,7 Binding Mode for Inclusion Complex of MT. The driving force for inclusion complexation of CD has been extensively discussed in the last 4 decades. Based on the thermodynamic studies on inclusion complexation of R-CD, we can obtain some insights into the factors that govern inclusion complexation of MT. Various kinds of factors have been proposed, e.g., hydrophobic interaction, van der Waals interaction, conformational changes of host and guest molecules, hydrogen bonds, electrostatic interaction, release of “high-energy” water from the cyclodextrin cavity,30 relief of conformational strain energy possessed by the uncomplexed CD,31 and so on. Recently, the last two factors are considered to be the minor driving forces for inclusion complexation.32 CnSO3Na and MT at pH 7 do not have the site for the hydrogen bonds and the ionic groups, respectively. Thus, the hydrogen bonds and the electrostatic interaction do not participate in the driving force of inclusion complexation between MT and CnSO3Na. The arguments mainly center on hydrophobic and van der Waals interactions. In the hydrophobic interaction process, which is the transfer of the hydrocarbons from water to liquid organic phases, ∆G is negative and is proportional to the surface area of the solute.33 ∆H is small and positive, and ∆S is large and positive, which is an indication for the occurrence (27) Yang, Z.; Breslow, R. Tetrahedron Lett. 1997, 38, 6171. (28) Saenger, W.; Jacob, J.; Gessler, K.; Steiner, T.; Hoffman, D.; Sanbe, H.; Koizumi, K.; Smith, S. M.; Takaha, T. Chem. Rev. 1998, 98, 1787. (29) Hamai, S. J. Phys. Chem. 1990, 94, 2595. (30) Griffiths, D. W.; Bender, M. L. Adv. Catal. 1973, 23, 209. (31) Manor, P. C.; Saenger, W. J. Am. Chem. Soc. 1974, 96, 3630. (32) Connors, K. A. Chem. Rev. 1997, 97, 1325. (33) Tanford, C. The Hydrophobic Effect; Formation of Micelles and Biological Membranes, 2nd ed.; Wiley-Interscience: New York, 1980.
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Table 3. Effect of Temperature on Thermodynamic Parameters for Inclusion Complexation between MT and C12SO3Na. ((SEM, n ) 3) guest molecule
T, K
∆H,a kJ mol-1
Ka,a,b 10-4 M-1
∆G,c kJ mol-1
T∆S,c kJ mol-1
ncoma,d
C12SO3Na C12SO3Na C12SO3Na
288 298 308
-71.8 ( 1.7 -82.8 ( 7.4 -92.7 ( 1.6
6.26 ( 0.44 12.5 ( 0.4 9.88 ( 0.69
-26.4 ( 0.2 -29.0 ( 0.1 -29.4 ( 0.2
-45.4 ( 1.9 -53.8 ( 7.5 -63.3 ( 1.8
0.57 0.50 0.52
a ∆H, K , and n b a com were evaluated directly from the titration data using the Origin software. Association constant (Ka ) [inclusion complex]/[host][guest]). c ∆G and T∆S were calculated using ∆G ) -RT ln Ka and ∆G ) ∆H - T∆S. d Stoichiometric number (host/guest ratio).
Figure 5. Effect of temperature on thermodynamics for inclusion complexation between a molecular tube and sodium dodecyl sulfonate. ∆G (2); ∆H (b); -T∆S (9). Average (SEM, n ) 3.
Figure 6. Possible structure of the inclusion complex between a molecular tube and sodium alkyl sulfonate.
of hydrophobic interaction.34 Furthermore, ∆Cp is large and negative, which is generally considered to be a decisive criterion for the occurrence of a hydrophobic interactions.34,35 ∆Cp for inclusion complexation of MT was found to be large and negative (-1.0 kJ K-1 mol-1). This result supports the contribution of hydrophobic interactions to inclusion complexation between MT and CnSO3Na. However, ∆H and T∆S were both negative in MT/CnSO3Na systems (Table 2). The signs of ∆H and T∆S for inclusion complexation of MT are inconsistent with those for the hydrophobic interaction process. The negative values of both ∆H and T∆S are attributed to other factors. The dependence of the thermodynamic parameters on alkyl chain length of the guest molecule is a valuable parameter to discuss the binding mode of inclusion complexation. The -∆H increment per CH2 for inclusion complexation of MT (4.4 kJ mol-1) was found to be consistent with that of R-CD (1.9-4.9 kJ mol-1).8 Thus, ∆H for inclusion complexation of MT may be governed by (34) Blokzijl, W.; Engberts, J. B. F. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 1545. (35) Rowe, E. S.; Zhang, F.; Leung, T. W.; Parr, J. S.; Guy, P. T. Biochemistry 1998, 37, 2430.
the same mode for inclusion complexation of R-CD. In general, the large and negative value of ∆H for inclusion complexation of CD is attributed to van der Waals interactions.8 Therefore, van der Waals interactions are considered to play an important role for inclusion complexation of MT. It is difficult to estimate the relative contribution of van der Waals and hydrophobic interactions. Presumably, both interactions are important and contribute to inclusion complexation of MT. As for -T∆S, the increment per CH2 for inclusion complexation of R-CD is less regular, and the value is 0.14 kJ mol-1 at 298 K.8 The -T∆S increment per CH2 for inclusion complexation of MT (3.6 kJ mol-1) was found to be quite different from that of R-CD. The strong -T∆S dependence on nc was a characteristic for inclusion complexation of MT. The large and negative T∆S value is attributed to two factors. One is the conformational change of CnSO3Na molecules and another is the loss of the freedom of CnSO3Na molecular motion due to strong binding with MT. The conformational change of the CnSO3Na molecule may be similar to that in inclusion complexation with R-CD. Thus, the loss of the freedom of CnSO3Na molecular motion is considered to be a predominant factor to the large and negative T∆S. The -∆G increment per CH2 for inclusion complexation of MT (0.75 kJ mol-1) was also found to be different from that of R-CD (3.1 kJ mol-1). This is attributed to the strong -T∆S dependence on nc. Conclusion The thermodynamics on inclusion complexation between MT and C8-, C10-, and C12SO3Na were analyzed by ITC. It was found that inclusion complexation between MT and CnSO3Na was quite stable. From the stoichiometric number, one MT molecule was found to form an inclusion complex with two CnSO3Na molecules, regardless of alkyl chain length. From large and negative ∆Cp value for inclusion complexation in the MT/C12SO3Na system, hydrophobic interactions may contribute to the process for inclusion complexation of MT. The large and negative value of ∆H suggests that van der Waals interactions play an important role for inclusion complexation of MT. The strong -T∆S dependence on nc was found to be a characteristic for inclusion complexation of MT. The large and negative T∆S attributes to the loss of the freedom of CnSO3Na molecular motion due to the strong binding of MT. Acknowledgment. This study was financially supported by Grants-in-Aid for Scientific Research on Priority Areas “Molecular Synchronization for Construction of New Materials System” (No. 11167238), from the Ministry of Education, Science, Sports, and Culture, Japan, from Nestle´ Science Promotion Committee, and from Micromachine Center, Japan. LA001205O
