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A facile one-pot synthesis of novel amphiphilic perfluoroalkyl ester functionalized γ-cyclodextrin and complex formation with anionic surfactants
Journal of Fluorine Chemistry 127 (2006) 730–735 www.elsevier.com/locate/fluor
A facile one-pot synthesis of novel amphiphilic perfluoroalkyl ester functionalized g-cyclodextrin and complex formation with anionic surfactants Kwon Taek Lim a,*, Hullathy Subban Ganapathy a, Min Young Lee a, Haldorai Yuvaraj a, Won-Ki Lee b, Hoon Heo a a
Division of Image Science and Information Engineering, Pukyong National University, Pusan 608-739, South Korea b Division of Chemical Engineering, Pukyong National University, Pusan 608-739, South Korea Received 11 January 2006; received in revised form 31 January 2006; accepted 8 February 2006 Available online 6 March 2006
Abstract g-Cyclodextrin (g-CyD) was hydrophobically modified by selective functionalization at the C-6 position (primary face) with fluoroalkyl ester groups. This new amphiphilic g-CyD was prepared in a facile one-pot synthesis by direct esterification of g-CyD with perfluorobutanoic acid on the narrow rim of macrocylic molecule. The selective per-substituted product, octakis (6-O-perfluorobutanoyl)-g-cyclodextrin (g-CyD-F), was confirmed by 1H NMR, 13C NMR and 19F NMR spectroscopic methods. The complexing ability of the g-CyD-F was investigated with different types of anionic surfactant having single or double hydrophobic tail groups. Predominant complex formation was observed in all cases with an equimolar mixture of surfactant and g-CyD-F in methanol irrespective of the type of the surfactant. # 2006 Elsevier B.V. All rights reserved. Keywords: Cyclodextrin; Amphiphilic; Perfluoroalkyl ester; Complex formation; Anionic surfactants
1. Introduction It is well known that cyclodextrins (CyDs) form inclusion complexes with a number of organic molecules without any covalent bonds. CyDs as cyclic oligosaccharides are classified as a-, b- and g-CyD according to the number of (1, 4) linked aD-glucopyranose units: six, seven and eight, respectively, and have a truncated hydrophobic cavity into which a guest molecule of appropriate size and shape is incorporated in aqueous media [1]. However, the potential use of CyDs in biological system requires amphiphilic properties, several modifications have been made on CyDs with the aim of providing versatile carriers and delivery systems for hydrophilic and lipophilic drugs [2]. Amphiphilic CyDs can be obtained by the introduction of lipophilic groups at primary face and/or secondary face of the CyD and they have been shown to form monolayers at the air–water interface [3] and micelles in water [4]. Different self-organized amphiphilic
* Corresponding author. Tel.: +82 51 620 1692; fax: +82 51 625 2229. E-mail address: [email protected] (K.T. Lim). 0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2006.02.004
CyDs, such as nanospheres [5], solid-lipid nanoparticles [6], liquid crystals [7] and vesicles [8] were prepared with varying length of hydrophobic chains for their promising properties for pharmaceutical applications. Furthermore, bilayer vesicles composed of amphiphilic CyDs have been shown to bind specific guests to recognize molecular signals [9,10]. This suggests that amphiphilic cyclodextrins may prove valuable in the development of biological receptors and may help to understand more clearly on the complicated mechanism of the molecular recognition by living cells and bacteria. While a large variety of amphiphilic CyDs were synthesized by modifying with long alkyl chains as hydrophobic substituents, [3–10] only few works based on fluoroalkyl functionalized CyDs were reported [11–13]. Recently, the fluorine-containing organic compounds have attracted much attention owing to their potential importance in industrial as well as in biomedical researches. Because of the unique properties conferred by the fluorinated chains to molecules, several fluorocarbons have been reported for their promising pharmaceutical application, such as oxygen delivery (liquid ventilation) and temporary blood substitutes and they are currently being investigated in phase-III clinical trials for the treatment of the diseases like
K.T. Lim et al. / Journal of Fluorine Chemistry 127 (2006) 730–735
respiratory distress syndrome [14]. More recently, nanocapsules and nanospheres prepared from amphiphilic perfluoro-bcyclodextrins and perfluoroalkylthio-b-cyclodextrins were investigated for their potential role as oxygen carriers [12,13]. Besides these applications, fluorinated compounds were shown to be highly soluble in densified carbon dioxide, a promising environmentally benign medium for protein extraction, bioconversion, polymer processing, nano-material synthesis and drug delivery applications [15,16]. While a wide range of inclusion complexes were described for unmodified cyclodextrin with small organic molecules, polymers and surfactants, to our knowledge, there have been no reports on the inclusion phenomena of fluorinated amphiphilic cyclodextrins. Thus, the combination of inclusion properties of the CyDs and the useful amphiphilic properties of fluorocarbon chains are expected to give molecules possessing novel physical, chemical and biological properties compared to their hydrocarbon analogs. Herein, we describe the synthesis of a new amphiphilic g-cyclodextrin, selectively functionalized at the primary face (six-position) with perfluoro butanoate groups. Additionally, a detailed investigation on the complexing ability of this hydrophobically modified g-CyD in methanol with three different anionic surfactants is also presented. 2. Results and discussion 2.1. Synthesis and characterization of g-CyD-F The g-CyD-F was successfully synthesized by selective functionalization at C-6 position of CyD by esterification with heptafluorobutyric acid (Scheme 1). While fluoro amphiphilic CyDs were synthesized previously by multistep reactions, gCyD-F was prepared by a relatively straightforward, one-pot synthesis by avoiding additional steps, such as halogenation and protection/deprotection of hydroxy groups [12,13]. In a typical synthesis experiment, g-cyclodextrin was reacted with an excess of heptafluorobutyric acid in a round bottom flask fitted with distillation accessories. The esterification reaction was conducted at 110 8C in order to remove water formed during the reaction. After completion of the reaction, the unreacted heptafluorobutyric acid was removed by vacuum evaporation and the resulting product was washed with benzene and water several times to remove the trace of heptafluorobutyric acid and unreacted g-CyD. The success in the selective
Scheme 1. Synthesis of fluoroalkyl ester functionalized amphiphilic cyclodextrin.
Fig. 1.
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C NMR Spectra of g-CyD and g-CyD-F in DMSO-d6.
functionalization of hydroxy group at C-6 position mainly relies on the difference in the reactivity between primary and secondary hydroxy groups present in the CyD molecule. The primary hydroxy groups are relatively more nucleophilic than secondary hydroxy groups and the reactivity is approximately 10:1 towards nucleophilic substitution reactions [17]. The fact that the primary OH group reacts selectively is inferred from 13 C NMR spectrum (Fig. 1). The resonance of the C6 changes from 59.9 ppm for g-cyclodextrin to 67.81 ppm for g-CyD-F, whereas the resonance of the C5 changes from 72.06 ppm for gcyclodextrin to 69.8 ppm for g-CyD-F. The shift of C6 to lower field and that of C5 to higher field is known in the literature for compounds substituted in C6 [18]. The 1H NMR spectra confirmed the total substitution of primary face, in particular, by the disappearance of the primary hydroxy groups attached to C6 at 4.5 ppm and also with considerable shift in upfield by protons in the C6 due to the esterification (Fig. 2). Additionally, the formation of g-CyD-F was verified by the carbonyl ester peak at 1750 cm1 (C O) in IR spectra and appearance of extra peaks for fluorine atoms by 19 F NMR spectroscopy. While g-CyD is well known for its water solubility, by esterification of OH groups at six-position, the g-CyD-F becomes insoluble in water owing to the high hydrophobicity of eight fluoroalkyl chains (–CF2–CF2–CF3) substituted on the narrow rim of the CyD. However, it is readily soluble in organic solvents, such as methanol and DMSO and a fluorinated solvent, trifluorotoluene. Though the unmodified cyclodextrins are practically insoluble in supercritical CO2, g-CyD-F readily dissolves in liquid and supercritical carbon dioxide at low temperatures and pressure because of the CO2-philic fluoroalkyl ester groups. For example, 1 wt.% of g-CyD-F was found to be soluble at around 179 bar and 40 8C, which presents an opportunity for preparing CO2-soluble amphiphilic cyclodextrins. We are currently investigating the solubility of
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Fig. 2. 1H NMR Spectra of g-CyD and g-CyD-F in DMSO-d6.
fluorine functionalized CyDs and inclusion complex phenomena with various surfactants in densified CO2. 2.2. Formation of complex with surfactants The g-CyD-F/anionic surfactant complexes were obtained by mixing equimolar amount of each compound in methanol. The solution was then stored quiescently for 3 h and the methanol was removed from the solution. The resulting complex compound was dried and stored for further analysis. The physical mixtures of g-CyD-F and surfactant were prepared by mixing both solids at 1:1 molar ratio at room temperature for comparison. The anionic surfactants used in this study are shown in Fig. 3. To confirm the complex formation of g-CyD-F with the anionic surfactants, standard analytical methods such as 1H NMR, differential scanning calorimetry (DSC) and thin layer chromatography (TLC) were used. TLC followed by I2
Fig. 3. Molecular structures of surfactants (a) bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT), (b) N-lauroylsarcosine sodium salt and (c) octadecanoic acid sodium salt.
Fig. 4. DSC curves for (a) AOT, (b) g-CyD-F, (c) physical mixture of AOT and g-CyD-F and (d) g-CyD-F/AOT complex.
development or UV light observation is an important tool to prove the formation of a complex. For example, the Rf values of g-CyD-F (Rf, methanol = 0.7) and the free bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) (Rf, methanol = 0.78) were significantly different from the value determined for the complex, g-CyD-F/AOT (Rf, methanol = 0.64), indicating the existence of a stable complex under the conditions of chromatographic flow. Similar trend was observed in the other surfactant complexes, g-CyD-F/SOD (Rf, methanol = 0.65) and g-CyD-F/NLS (Rf, methanol = 0.64). For further investigation, the complexes were subjected to DSC analysis. The samples were heated from 20 to 300 8C at heating rate of 10 8C min1. Results from the DSC of bulk AOT, bulk g-CyD-F, the physical mixture of AOT and g-CyD-F and the complex g-CyD-F/AOT formed in methanol are shown in Fig. 4. Because of the hygroscopic nature of AOT, it was dried thoroughly by freeze-drying method before subjecting it to the DSC analysis. Free AOT exhibits endothermic peak at 170–180 8C corresponding to its melting point. While a peak was observed in the same range for the physical mixture of equimolar amount of AOT and g-CyD-F, the complex compound prepared with the same composition in methanol did not show any peak. According to Loukas et al., the disappearance or shifting of endo or exothermic peaks is generally an indication of the formation of inclusion complexes [19]. Therefore, the absence of the characteristic peak of AOT in the complex suggests a clear evidence for the formation of complex between the surfactant and the fluoroalkyl ester functionalized g-cyclodextrin at the molecular level. This trend in the DSC pattern was consistent for the results obtained in the complex formation with other single-tailed hydrocarbon surfactants. Fig. 5 shows the DSC curves for g-CyD-F, the physical mixture of g-CyD-F and octadecanoic acid sodium salt (SOD) or N-lauroylsarcosine sodium salt (NLS), and the
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Fig. 5. DSC profiles for g-CyD-F/SOD (A) and g-CyD-F/NLS (B) complexes; (a) free SOD or NLS, (b) g-CyD-F, (c) physical mixture of surfactants and g-CyD-F and (d) g-CyD-F/SOD or NLS complexes.
surfactant-g-CyD-F complex compound. As can be seen in Fig. 5, the pure SOD melts at 210 8C, whereas NLS melts at 145 8C. However, the complex compounds did not show the endothermic peak corresponding to the surfactants confirming the efficient encapsulation of surfactants in the CyD cavity at the molecular level. 1 H NMR spectra gave further insight into the formation of stable complex between the surfactants and the modified CyD. Fig. 6 shows the partial 1H NMR spectra of g-CyD-F in the absence and in the presence of AOT in methanol-d4. After 1 h of mixing, slight peak shifts in H-6 and H-5 with typical broadening were observed. Then the peaks become relatively broader with considerable shift after 3 h. No further change in the peak shapes was observed for a prolonged time. These guest-induced chemical shifts and broadening of NMR signals suggest that the guest molecule is inside the cavity of CyD and thus confirm the successful formation of complex between gCyD-F and AOT [20]. Fig. 7 shows the NMR spectrum of a single-tailed surfactant complex, SOD/g-CyD-F, in methanol at room temperature. Interaction of SOD with g-CyD-F resulted in the displacement of H-6 and H-5 signals to merge at 4.2 ppm, which again substantiates the formation of the complex.
In general, for unmodified cyclodextrin-surfactant inclusion complexes, a 1:1 stoichiometry has usually been assumed and the association constants for a number of surfactants have been reported [21]. However, in some cases, a-CyD which has ˚ ) compared to b-CyD (diameter a smaller cavity (diameter 5 A ˚ 7 A) was shown to form 2:1 type inclusion complexes with alkyl sulfate and sulfonate surfactants. Presumably, the hydrocarbon tail of the surfactant fits most snugly into bCyD, but the polar head group (sulfate or sulfonate) is accommodated most efficiently by a-CyD, with molar composition of 2:1 complex involving an encapsulation of surfactant monomer at both ends by CyDs [22]. Our observation on the predominant equimolar complex formation of g-CyD-F with both single and twin-tailed surfactants suggests that the complexing ability of this amphiphilic CyD is not hampered by the bulky fluoroalkyl chains substituted on the primary face and the association with surfactants may be similar to native CyDs. It may be noted that in some cases, the substitution on the CyD rims was to shown to greatly enhance the binding of many lipophilic host compounds [23,24]. If the phenomenon of self inclusion occurs with functionalized fluorocarbon tails, then the fluorocarbon chains would enter
Fig. 6. Partial 1H NMR spectra of g-CD-F in the absence (A) and in the presence of AOT for 1 h (B) and for 3 h (C).
Fig. 7. 1H NMR Spectra of SOD/g-CyD-F complex in methanol-d4.
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into the CyD cavity by narrow end and may create a drastic steric impact in the cavity to alter the inclusion mode, stoichiometry and binding affinity of the complex formation. However, the narrow rim might be too small to allow the bulky fluorocarbon chains thereby making the CyD cavity free for the efficient encapsulation of the surfactants [25]. These results show that the selective side chain modification of CyDs at the smaller rim (primary face) using fluoroalkyl groups leads to the useful amphiphilic CyDs, and the possibility for encapsulating water soluble surfactants makes them novel cyclodextrin compounds having different physio-chemical properties. 3. Conclusions A new amphiphilic g-cyclodextrin selectively functionalized at the six-position with perfluoro alkyl ester group was prepared by a facile one-pot synthesis by esterification. The selective per-substituted product was characterized by spectroscopic methods. The hydrophobically modified gcyclodextrin derivative was found to be soluble in common organic solvents and also in densified CO2. Additionally, equimolar complex formations of g-CyD-F with single and double-tailed hydrocarbon surfactants were demonstrated by standard analytical methods, such as TLC, 1H NMR and DSC. It was found that the bulky fluoroalkyl ester groups on the primary face (narrow rim) did not hamper the formation of complex, as was evident from the complexes with surfactants irrespective of their types. 4. Experimental g-CyD (Junsei, Japan) was vacuum dried for 24 h at 60 8C before use. Heptafluorobutyric acid (Aldrich) and the anionic surfactants, N-lauroylsarcosine sodium salt, bis(2-ethylhexyl) sulfosuccinate sodium salt and octadecanoic acid sodium salt (Aldrich), all were used as received. 1H NMR spectra were recorded using a JNM-ECP 400 (JEOL) spectrometer, with DMSO-d6 as solvent (internal reference dH = 2.5 ppm). 13C and 19 F NMR spectra were recorded on the same spectrometer with the central peak of DMSO-d6 as the internal reference (dc = 40.0 ppm). IR spectra were measured on Perkin-Elmer GX infrared spectrometer. Differential scanning calorimetry data were obtained with the aid of DSC-60 (Shimadzu, Japan) thermal analysis system. 4.1. Synthesis of octakis (6-O-perfluorobutanoyl)-gcyclodextrin (g-CyD-F) g-Cyclodextrin (1.0 g, 0.77 mmol) was reacted with an excess of heptafluorobutyric acid (7.9 g, 4.62 mmol) placed in a 50 ml flask fitted with distillation accessories. The esterification reaction was conducted at 110 8C for 9 h under argon atmosphere. After completion of the reaction, unreacted heptafluorobutyric acid was removed by vacuum evaporation and the resulting product was washed with benzene and water several times to remove the traces of heptafluorobutyric acid
and unreacted g-CyD. The final product was collected, dried in vacuum to give the compound g-CyD-F as hygroscopic white solid: yield 96%. IR nmax, cm1: 3412 (OH free), 2931 (C–H stretch), 1790 (C O stretch), 1308–1124 (C–F Stretch); 1H NMR (400 MHz, DMSO-d6) d (ppm): 3.38 (dd, 8H, H-2, J = 3.0 and 6.6 Hz), 3.44 (t, 8H, H-4, J = 9.5 Hz), 3.67 (t, 8H, H-3, J = 9.0 Hz), 3.94 (t, 8H, H-5, J = 13.0 and 10.0 Hz,), 4.54– 4.58 (m, 8H, H-60 , J = 6.0 Hz), 4.70 (d, 8H, H-6, J = 10.0 Hz); 13 C NMR (100 MHz, DMSO-d6) d (ppm): 104.5 (C1), 83.4 (C4), 74.2, 74.0, 69.8 (C2, C3, C5), 67.8 (C6), 158.8 (t, –CO– CF2, J = 23.06 Hz), 113–106 (m, –CF2–CF2, J = 33.82 Hz, J = 263.67 Hz), 123.5–114 (q, CF3, J = 33.82 Hz, J = 286.34 Hz); 19F (400 MHz, DMSO-d6), d (ppm): 82.3 (m, 3F, CF3), 121 (m, 2F, CF2), 129 (m, 2F, CF2). Acknowledgements This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce Industry and Energy of the Korean Government. This subject is supported by Ministry of Environment as ‘‘The Eco-technopia 21 project’’. References [1] J. Szejtli, Chem. Rev. 98 (1998) 1743–1754. [2] K. Uekama, F. Hirayama, T. Irie, Chem. Rev. 98 (1998) 2045–2076. [3] (a) H. Parrot-Lopez, C.C. Ling, P. Zhang, A. Baszkin, G. Albrecht, C. De Rango, A.W. Coleman, J. Am. Chem. Soc. 114 (1992) 5479–5480; (b) M.H. Greenhall, P. Lukes, R. Kataky, N.E. Agbor, J.P.S. Badyal, J. Yarwood, D. Parker, M.C. Petty, Langmuir 11 (1995) 3997–4000. [4] R. Auze´ly-Velty, F. Djedaı¨ni-Pilard, S. De´sert, B. Perly, T. Zemb, Langmuir 16 (2000) 3727–3734. [5] M. Skiba, D. Duche`ne, F. Puisieux, D. Wouessidjewe, Int. J. Pharm. 129 (1996) 113–121. [6] A. Dubes, H. Parrot-Lopez, W. Abdelwahed, G. Degobert, H. Fessi, P. Shahgaldian, A.W. Coleman, Eur. J. Pharm. Biopharm. 55 (2003) 279– 282. [7] C.C. Ling, R. Darcy, W. Risse, J. Chem. Soc. Chem. Commun. (1993) 438–440. [8] B.J. Ravoo, R. Darcy, Angew. Chem. Int. Ed. 39 (2000) 4324–4326. [9] B.J. Ravoo, J.C. Jacquier, G. Wenz, Angew. Chem. Int. Ed. 42 (2003) 2066–2070. [10] C.W. Lim, B.J. Ravoo, D.N. Reinhoudt, Chem. Commun. (2005) 5627– 5629. [11] S. Pe´roche, G. Degobert, J.-L. Putaux, M.-G. Blanchin, H. Fessi, H. Parrot-Lopez, Eur. J. Pharm. Biopharm. 60 (2005) 123–131. [12] C.E. Granger, C.P. Felix, H.P. Parrot-Lopez, B.R. Langlois, Tetrahedron Lett. 41 (2000) 9257–9260. [13] M. Skiba, M. Skiba-Lahiani, P. Arnaud, J. Inclusion Phenom. Macrocyclic Chem. 44 (2002) 151. [14] M.P. Krafft, Adv. Drug Deliv. Rev. 47 (2001) 209–228. [15] Y.-P. Sun (Ed.), Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties and Applications, Marcel Dekker, New York, 2002. [16] K.P. Johnston, K.L. Harrison, M.J. Clarke, S.M. Howdle, M.P. Heitz, F.V. Bright, C. Carlier, T.W. Randolph, Science 271 (1996) 624–626. [17] C.T. Rao, J. Pitha, Carbohydr. Res. 220 (1991) 209–213. [18] (a) Z. Shen, S.L. Huang, G. Tsai, Z. Chen, G.T. Tsao, Carbohydr. Res. 201 (1990) 241–248; (b) L.-H. Tong, Z. Hou, Y. Inoue, A. Tai, J. Chem. Soc. Perkin Trans. II (1992) 1253–1257.
K.T. Lim et al. / Journal of Fluorine Chemistry 127 (2006) 730–735 [19] Y.L. Loukas, V. Vraka, G. Gregoriadis, J. Pharm. Biomed. Anal. 16 (1997) 263–268. [20] H.-J. Schneider, F. Hacket, V. Ru¨diger, H. Ikeda, Chem. Rev. 98 (1998) 1755–1785. [21] (a) I. Satake, T. Ikenoue, K. Takeshita, K. Hayakawa, T. Maeda, Bull. Chem. Soc. Jpn. 58 (1986) 2746–2750; (b) I. Satake, S. Yoshida, K. Hayakawa, T. Maeda, Y. Kusumoto, Bull. Chem. Soc. Jpn. 59 (1986) 3991–3993.
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A facile one-pot synthesis of novel amphiphilic perfluoroalkyl ester functionalized g-cyclodextrin and complex formation with anionic surfactants Kwon Taek Lim a,*, Hullathy Subban Ganapathy a, Min Young Lee a, Haldorai Yuvaraj a, Won-Ki Lee b, Hoon Heo a a
Division of Image Science and Information Engineering, Pukyong National University, Pusan 608-739, South Korea b Division of Chemical Engineering, Pukyong National University, Pusan 608-739, South Korea Received 11 January 2006; received in revised form 31 January 2006; accepted 8 February 2006 Available online 6 March 2006
Abstract g-Cyclodextrin (g-CyD) was hydrophobically modified by selective functionalization at the C-6 position (primary face) with fluoroalkyl ester groups. This new amphiphilic g-CyD was prepared in a facile one-pot synthesis by direct esterification of g-CyD with perfluorobutanoic acid on the narrow rim of macrocylic molecule. The selective per-substituted product, octakis (6-O-perfluorobutanoyl)-g-cyclodextrin (g-CyD-F), was confirmed by 1H NMR, 13C NMR and 19F NMR spectroscopic methods. The complexing ability of the g-CyD-F was investigated with different types of anionic surfactant having single or double hydrophobic tail groups. Predominant complex formation was observed in all cases with an equimolar mixture of surfactant and g-CyD-F in methanol irrespective of the type of the surfactant. # 2006 Elsevier B.V. All rights reserved. Keywords: Cyclodextrin; Amphiphilic; Perfluoroalkyl ester; Complex formation; Anionic surfactants
1. Introduction It is well known that cyclodextrins (CyDs) form inclusion complexes with a number of organic molecules without any covalent bonds. CyDs as cyclic oligosaccharides are classified as a-, b- and g-CyD according to the number of (1, 4) linked aD-glucopyranose units: six, seven and eight, respectively, and have a truncated hydrophobic cavity into which a guest molecule of appropriate size and shape is incorporated in aqueous media [1]. However, the potential use of CyDs in biological system requires amphiphilic properties, several modifications have been made on CyDs with the aim of providing versatile carriers and delivery systems for hydrophilic and lipophilic drugs [2]. Amphiphilic CyDs can be obtained by the introduction of lipophilic groups at primary face and/or secondary face of the CyD and they have been shown to form monolayers at the air–water interface [3] and micelles in water [4]. Different self-organized amphiphilic
* Corresponding author. Tel.: +82 51 620 1692; fax: +82 51 625 2229. E-mail address: [email protected] (K.T. Lim). 0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2006.02.004
CyDs, such as nanospheres [5], solid-lipid nanoparticles [6], liquid crystals [7] and vesicles [8] were prepared with varying length of hydrophobic chains for their promising properties for pharmaceutical applications. Furthermore, bilayer vesicles composed of amphiphilic CyDs have been shown to bind specific guests to recognize molecular signals [9,10]. This suggests that amphiphilic cyclodextrins may prove valuable in the development of biological receptors and may help to understand more clearly on the complicated mechanism of the molecular recognition by living cells and bacteria. While a large variety of amphiphilic CyDs were synthesized by modifying with long alkyl chains as hydrophobic substituents, [3–10] only few works based on fluoroalkyl functionalized CyDs were reported [11–13]. Recently, the fluorine-containing organic compounds have attracted much attention owing to their potential importance in industrial as well as in biomedical researches. Because of the unique properties conferred by the fluorinated chains to molecules, several fluorocarbons have been reported for their promising pharmaceutical application, such as oxygen delivery (liquid ventilation) and temporary blood substitutes and they are currently being investigated in phase-III clinical trials for the treatment of the diseases like
K.T. Lim et al. / Journal of Fluorine Chemistry 127 (2006) 730–735
respiratory distress syndrome [14]. More recently, nanocapsules and nanospheres prepared from amphiphilic perfluoro-bcyclodextrins and perfluoroalkylthio-b-cyclodextrins were investigated for their potential role as oxygen carriers [12,13]. Besides these applications, fluorinated compounds were shown to be highly soluble in densified carbon dioxide, a promising environmentally benign medium for protein extraction, bioconversion, polymer processing, nano-material synthesis and drug delivery applications [15,16]. While a wide range of inclusion complexes were described for unmodified cyclodextrin with small organic molecules, polymers and surfactants, to our knowledge, there have been no reports on the inclusion phenomena of fluorinated amphiphilic cyclodextrins. Thus, the combination of inclusion properties of the CyDs and the useful amphiphilic properties of fluorocarbon chains are expected to give molecules possessing novel physical, chemical and biological properties compared to their hydrocarbon analogs. Herein, we describe the synthesis of a new amphiphilic g-cyclodextrin, selectively functionalized at the primary face (six-position) with perfluoro butanoate groups. Additionally, a detailed investigation on the complexing ability of this hydrophobically modified g-CyD in methanol with three different anionic surfactants is also presented. 2. Results and discussion 2.1. Synthesis and characterization of g-CyD-F The g-CyD-F was successfully synthesized by selective functionalization at C-6 position of CyD by esterification with heptafluorobutyric acid (Scheme 1). While fluoro amphiphilic CyDs were synthesized previously by multistep reactions, gCyD-F was prepared by a relatively straightforward, one-pot synthesis by avoiding additional steps, such as halogenation and protection/deprotection of hydroxy groups [12,13]. In a typical synthesis experiment, g-cyclodextrin was reacted with an excess of heptafluorobutyric acid in a round bottom flask fitted with distillation accessories. The esterification reaction was conducted at 110 8C in order to remove water formed during the reaction. After completion of the reaction, the unreacted heptafluorobutyric acid was removed by vacuum evaporation and the resulting product was washed with benzene and water several times to remove the trace of heptafluorobutyric acid and unreacted g-CyD. The success in the selective
Scheme 1. Synthesis of fluoroalkyl ester functionalized amphiphilic cyclodextrin.
Fig. 1.
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C NMR Spectra of g-CyD and g-CyD-F in DMSO-d6.
functionalization of hydroxy group at C-6 position mainly relies on the difference in the reactivity between primary and secondary hydroxy groups present in the CyD molecule. The primary hydroxy groups are relatively more nucleophilic than secondary hydroxy groups and the reactivity is approximately 10:1 towards nucleophilic substitution reactions [17]. The fact that the primary OH group reacts selectively is inferred from 13 C NMR spectrum (Fig. 1). The resonance of the C6 changes from 59.9 ppm for g-cyclodextrin to 67.81 ppm for g-CyD-F, whereas the resonance of the C5 changes from 72.06 ppm for gcyclodextrin to 69.8 ppm for g-CyD-F. The shift of C6 to lower field and that of C5 to higher field is known in the literature for compounds substituted in C6 [18]. The 1H NMR spectra confirmed the total substitution of primary face, in particular, by the disappearance of the primary hydroxy groups attached to C6 at 4.5 ppm and also with considerable shift in upfield by protons in the C6 due to the esterification (Fig. 2). Additionally, the formation of g-CyD-F was verified by the carbonyl ester peak at 1750 cm1 (C O) in IR spectra and appearance of extra peaks for fluorine atoms by 19 F NMR spectroscopy. While g-CyD is well known for its water solubility, by esterification of OH groups at six-position, the g-CyD-F becomes insoluble in water owing to the high hydrophobicity of eight fluoroalkyl chains (–CF2–CF2–CF3) substituted on the narrow rim of the CyD. However, it is readily soluble in organic solvents, such as methanol and DMSO and a fluorinated solvent, trifluorotoluene. Though the unmodified cyclodextrins are practically insoluble in supercritical CO2, g-CyD-F readily dissolves in liquid and supercritical carbon dioxide at low temperatures and pressure because of the CO2-philic fluoroalkyl ester groups. For example, 1 wt.% of g-CyD-F was found to be soluble at around 179 bar and 40 8C, which presents an opportunity for preparing CO2-soluble amphiphilic cyclodextrins. We are currently investigating the solubility of
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Fig. 2. 1H NMR Spectra of g-CyD and g-CyD-F in DMSO-d6.
fluorine functionalized CyDs and inclusion complex phenomena with various surfactants in densified CO2. 2.2. Formation of complex with surfactants The g-CyD-F/anionic surfactant complexes were obtained by mixing equimolar amount of each compound in methanol. The solution was then stored quiescently for 3 h and the methanol was removed from the solution. The resulting complex compound was dried and stored for further analysis. The physical mixtures of g-CyD-F and surfactant were prepared by mixing both solids at 1:1 molar ratio at room temperature for comparison. The anionic surfactants used in this study are shown in Fig. 3. To confirm the complex formation of g-CyD-F with the anionic surfactants, standard analytical methods such as 1H NMR, differential scanning calorimetry (DSC) and thin layer chromatography (TLC) were used. TLC followed by I2
Fig. 3. Molecular structures of surfactants (a) bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT), (b) N-lauroylsarcosine sodium salt and (c) octadecanoic acid sodium salt.
Fig. 4. DSC curves for (a) AOT, (b) g-CyD-F, (c) physical mixture of AOT and g-CyD-F and (d) g-CyD-F/AOT complex.
development or UV light observation is an important tool to prove the formation of a complex. For example, the Rf values of g-CyD-F (Rf, methanol = 0.7) and the free bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) (Rf, methanol = 0.78) were significantly different from the value determined for the complex, g-CyD-F/AOT (Rf, methanol = 0.64), indicating the existence of a stable complex under the conditions of chromatographic flow. Similar trend was observed in the other surfactant complexes, g-CyD-F/SOD (Rf, methanol = 0.65) and g-CyD-F/NLS (Rf, methanol = 0.64). For further investigation, the complexes were subjected to DSC analysis. The samples were heated from 20 to 300 8C at heating rate of 10 8C min1. Results from the DSC of bulk AOT, bulk g-CyD-F, the physical mixture of AOT and g-CyD-F and the complex g-CyD-F/AOT formed in methanol are shown in Fig. 4. Because of the hygroscopic nature of AOT, it was dried thoroughly by freeze-drying method before subjecting it to the DSC analysis. Free AOT exhibits endothermic peak at 170–180 8C corresponding to its melting point. While a peak was observed in the same range for the physical mixture of equimolar amount of AOT and g-CyD-F, the complex compound prepared with the same composition in methanol did not show any peak. According to Loukas et al., the disappearance or shifting of endo or exothermic peaks is generally an indication of the formation of inclusion complexes [19]. Therefore, the absence of the characteristic peak of AOT in the complex suggests a clear evidence for the formation of complex between the surfactant and the fluoroalkyl ester functionalized g-cyclodextrin at the molecular level. This trend in the DSC pattern was consistent for the results obtained in the complex formation with other single-tailed hydrocarbon surfactants. Fig. 5 shows the DSC curves for g-CyD-F, the physical mixture of g-CyD-F and octadecanoic acid sodium salt (SOD) or N-lauroylsarcosine sodium salt (NLS), and the
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Fig. 5. DSC profiles for g-CyD-F/SOD (A) and g-CyD-F/NLS (B) complexes; (a) free SOD or NLS, (b) g-CyD-F, (c) physical mixture of surfactants and g-CyD-F and (d) g-CyD-F/SOD or NLS complexes.
surfactant-g-CyD-F complex compound. As can be seen in Fig. 5, the pure SOD melts at 210 8C, whereas NLS melts at 145 8C. However, the complex compounds did not show the endothermic peak corresponding to the surfactants confirming the efficient encapsulation of surfactants in the CyD cavity at the molecular level. 1 H NMR spectra gave further insight into the formation of stable complex between the surfactants and the modified CyD. Fig. 6 shows the partial 1H NMR spectra of g-CyD-F in the absence and in the presence of AOT in methanol-d4. After 1 h of mixing, slight peak shifts in H-6 and H-5 with typical broadening were observed. Then the peaks become relatively broader with considerable shift after 3 h. No further change in the peak shapes was observed for a prolonged time. These guest-induced chemical shifts and broadening of NMR signals suggest that the guest molecule is inside the cavity of CyD and thus confirm the successful formation of complex between gCyD-F and AOT [20]. Fig. 7 shows the NMR spectrum of a single-tailed surfactant complex, SOD/g-CyD-F, in methanol at room temperature. Interaction of SOD with g-CyD-F resulted in the displacement of H-6 and H-5 signals to merge at 4.2 ppm, which again substantiates the formation of the complex.
In general, for unmodified cyclodextrin-surfactant inclusion complexes, a 1:1 stoichiometry has usually been assumed and the association constants for a number of surfactants have been reported [21]. However, in some cases, a-CyD which has ˚ ) compared to b-CyD (diameter a smaller cavity (diameter 5 A ˚ 7 A) was shown to form 2:1 type inclusion complexes with alkyl sulfate and sulfonate surfactants. Presumably, the hydrocarbon tail of the surfactant fits most snugly into bCyD, but the polar head group (sulfate or sulfonate) is accommodated most efficiently by a-CyD, with molar composition of 2:1 complex involving an encapsulation of surfactant monomer at both ends by CyDs [22]. Our observation on the predominant equimolar complex formation of g-CyD-F with both single and twin-tailed surfactants suggests that the complexing ability of this amphiphilic CyD is not hampered by the bulky fluoroalkyl chains substituted on the primary face and the association with surfactants may be similar to native CyDs. It may be noted that in some cases, the substitution on the CyD rims was to shown to greatly enhance the binding of many lipophilic host compounds [23,24]. If the phenomenon of self inclusion occurs with functionalized fluorocarbon tails, then the fluorocarbon chains would enter
Fig. 6. Partial 1H NMR spectra of g-CD-F in the absence (A) and in the presence of AOT for 1 h (B) and for 3 h (C).
Fig. 7. 1H NMR Spectra of SOD/g-CyD-F complex in methanol-d4.
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into the CyD cavity by narrow end and may create a drastic steric impact in the cavity to alter the inclusion mode, stoichiometry and binding affinity of the complex formation. However, the narrow rim might be too small to allow the bulky fluorocarbon chains thereby making the CyD cavity free for the efficient encapsulation of the surfactants [25]. These results show that the selective side chain modification of CyDs at the smaller rim (primary face) using fluoroalkyl groups leads to the useful amphiphilic CyDs, and the possibility for encapsulating water soluble surfactants makes them novel cyclodextrin compounds having different physio-chemical properties. 3. Conclusions A new amphiphilic g-cyclodextrin selectively functionalized at the six-position with perfluoro alkyl ester group was prepared by a facile one-pot synthesis by esterification. The selective per-substituted product was characterized by spectroscopic methods. The hydrophobically modified gcyclodextrin derivative was found to be soluble in common organic solvents and also in densified CO2. Additionally, equimolar complex formations of g-CyD-F with single and double-tailed hydrocarbon surfactants were demonstrated by standard analytical methods, such as TLC, 1H NMR and DSC. It was found that the bulky fluoroalkyl ester groups on the primary face (narrow rim) did not hamper the formation of complex, as was evident from the complexes with surfactants irrespective of their types. 4. Experimental g-CyD (Junsei, Japan) was vacuum dried for 24 h at 60 8C before use. Heptafluorobutyric acid (Aldrich) and the anionic surfactants, N-lauroylsarcosine sodium salt, bis(2-ethylhexyl) sulfosuccinate sodium salt and octadecanoic acid sodium salt (Aldrich), all were used as received. 1H NMR spectra were recorded using a JNM-ECP 400 (JEOL) spectrometer, with DMSO-d6 as solvent (internal reference dH = 2.5 ppm). 13C and 19 F NMR spectra were recorded on the same spectrometer with the central peak of DMSO-d6 as the internal reference (dc = 40.0 ppm). IR spectra were measured on Perkin-Elmer GX infrared spectrometer. Differential scanning calorimetry data were obtained with the aid of DSC-60 (Shimadzu, Japan) thermal analysis system. 4.1. Synthesis of octakis (6-O-perfluorobutanoyl)-gcyclodextrin (g-CyD-F) g-Cyclodextrin (1.0 g, 0.77 mmol) was reacted with an excess of heptafluorobutyric acid (7.9 g, 4.62 mmol) placed in a 50 ml flask fitted with distillation accessories. The esterification reaction was conducted at 110 8C for 9 h under argon atmosphere. After completion of the reaction, unreacted heptafluorobutyric acid was removed by vacuum evaporation and the resulting product was washed with benzene and water several times to remove the traces of heptafluorobutyric acid
and unreacted g-CyD. The final product was collected, dried in vacuum to give the compound g-CyD-F as hygroscopic white solid: yield 96%. IR nmax, cm1: 3412 (OH free), 2931 (C–H stretch), 1790 (C O stretch), 1308–1124 (C–F Stretch); 1H NMR (400 MHz, DMSO-d6) d (ppm): 3.38 (dd, 8H, H-2, J = 3.0 and 6.6 Hz), 3.44 (t, 8H, H-4, J = 9.5 Hz), 3.67 (t, 8H, H-3, J = 9.0 Hz), 3.94 (t, 8H, H-5, J = 13.0 and 10.0 Hz,), 4.54– 4.58 (m, 8H, H-60 , J = 6.0 Hz), 4.70 (d, 8H, H-6, J = 10.0 Hz); 13 C NMR (100 MHz, DMSO-d6) d (ppm): 104.5 (C1), 83.4 (C4), 74.2, 74.0, 69.8 (C2, C3, C5), 67.8 (C6), 158.8 (t, –CO– CF2, J = 23.06 Hz), 113–106 (m, –CF2–CF2, J = 33.82 Hz, J = 263.67 Hz), 123.5–114 (q, CF3, J = 33.82 Hz, J = 286.34 Hz); 19F (400 MHz, DMSO-d6), d (ppm): 82.3 (m, 3F, CF3), 121 (m, 2F, CF2), 129 (m, 2F, CF2). Acknowledgements This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce Industry and Energy of the Korean Government. This subject is supported by Ministry of Environment as ‘‘The Eco-technopia 21 project’’. References [1] J. Szejtli, Chem. Rev. 98 (1998) 1743–1754. [2] K. Uekama, F. Hirayama, T. Irie, Chem. Rev. 98 (1998) 2045–2076. [3] (a) H. Parrot-Lopez, C.C. Ling, P. Zhang, A. Baszkin, G. Albrecht, C. De Rango, A.W. Coleman, J. Am. Chem. Soc. 114 (1992) 5479–5480; (b) M.H. Greenhall, P. Lukes, R. Kataky, N.E. Agbor, J.P.S. Badyal, J. Yarwood, D. Parker, M.C. Petty, Langmuir 11 (1995) 3997–4000. [4] R. Auze´ly-Velty, F. Djedaı¨ni-Pilard, S. De´sert, B. Perly, T. Zemb, Langmuir 16 (2000) 3727–3734. [5] M. Skiba, D. Duche`ne, F. Puisieux, D. Wouessidjewe, Int. J. Pharm. 129 (1996) 113–121. [6] A. Dubes, H. Parrot-Lopez, W. Abdelwahed, G. Degobert, H. Fessi, P. Shahgaldian, A.W. Coleman, Eur. J. Pharm. Biopharm. 55 (2003) 279– 282. [7] C.C. Ling, R. Darcy, W. Risse, J. Chem. Soc. Chem. Commun. (1993) 438–440. [8] B.J. Ravoo, R. Darcy, Angew. Chem. Int. Ed. 39 (2000) 4324–4326. [9] B.J. Ravoo, J.C. Jacquier, G. Wenz, Angew. Chem. Int. Ed. 42 (2003) 2066–2070. [10] C.W. Lim, B.J. Ravoo, D.N. Reinhoudt, Chem. Commun. (2005) 5627– 5629. [11] S. Pe´roche, G. Degobert, J.-L. Putaux, M.-G. Blanchin, H. Fessi, H. Parrot-Lopez, Eur. J. Pharm. Biopharm. 60 (2005) 123–131. [12] C.E. Granger, C.P. Felix, H.P. Parrot-Lopez, B.R. Langlois, Tetrahedron Lett. 41 (2000) 9257–9260. [13] M. Skiba, M. Skiba-Lahiani, P. Arnaud, J. Inclusion Phenom. Macrocyclic Chem. 44 (2002) 151. [14] M.P. Krafft, Adv. Drug Deliv. Rev. 47 (2001) 209–228. [15] Y.-P. Sun (Ed.), Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties and Applications, Marcel Dekker, New York, 2002. [16] K.P. Johnston, K.L. Harrison, M.J. Clarke, S.M. Howdle, M.P. Heitz, F.V. Bright, C. Carlier, T.W. Randolph, Science 271 (1996) 624–626. [17] C.T. Rao, J. Pitha, Carbohydr. Res. 220 (1991) 209–213. [18] (a) Z. Shen, S.L. Huang, G. Tsai, Z. Chen, G.T. Tsao, Carbohydr. Res. 201 (1990) 241–248; (b) L.-H. Tong, Z. Hou, Y. Inoue, A. Tai, J. Chem. Soc. Perkin Trans. II (1992) 1253–1257.
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