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Formation mechanism of supramolecular hydrogels in the presence of l -phenylalanine derivative as a hydrogelator
Journal of Colloid and Interface Science 315 (2007) 376–381 www.elsevier.com/locate/jcis
Formation mechanism of supramolecular hydrogels in the presence of L-phenylalanine derivative as a hydrogelator Xinjian Fu, Ningxia Wang, Shengzu Zhang, Hong Wang, Yajiang Yang ∗ Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, China Received 28 March 2007; accepted 9 June 2007 Available online 1 August 2007
Abstract A novel chiral hydrogelator, L-phenylalanine derivative can self-assemble in aqueous media at different pH values to form supramolecular hydrogels. The images of the FE-SEM indicate that different aggregates of TC18 PheBu in morphology were formed, which further lead to the formation of spherical crystallites as observed by polarized optical microscope (POM). The FT-IR spectra of the supramolecular hydrogels reveal that intermolecular hydrogen-bonding and hydrophobic interactions are the driving forces for the self-assembly of TC18 PheBu. Fluorescence spectra of TC18 PheBu in aqueous solutions in the presence of pyrene as a probe further confirm the importance of hydrophobic interactions for the self-assembly. The circular dichroism (CD) spectra of TC18 PheBu in supramolecular hydrogels in the presence of KF indicate that the hydrogen-bonding interaction can be disrupted by fluoride ions, which further confirm the importance of hydrogen bonding for the self-assembly of TC18 PheBu. © 2007 Elsevier Inc. All rights reserved. Keywords: L-Phenylalanine derivatives; Hydrogelator; Supramolecular hydrogels
1. Introduction In the past decade, supramolecular organogels formed by the self-assembly of low molecular weight gelators have been the subject of increasing attention [1,2]. However less attention has been paid to the supramolecular hydrogels formed by hydrogelators. The supramolecular hydrogels are a novel condensed system of water molecules, which is quite different from the common polymer hydrogels. Similar to the organogel formation, supramolecular hydrogels are also formed by the selfassembly of hydrogelators through intermolecular interactions, such as hydrogen bonding, π–π stacking, van der Waals, coordination, and charge transfer. The water molecules are immobilized by capillary force in a three-dimensional network consisting of hydrogelator aggregates. In comparison with polymer hydrogels, supramolecular hydrogels are thermally reversible. It shows potential application in the fields of nanomaterials, drug carriers, molecular recognition, and so on [3,4]. * Corresponding author. Fax: +86 27 87543632.
E-mail address: [email protected] (Y. Yang). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.013
As an appropriate hydrogelator, the precondition is that it is usually water soluble at high temperatures and insoluble below a certain temperature. Amino acid derivate hydrogelators not only have such characteristics but also are biodegradable and have low toxicity. Suzuki and co-workers synthesized some amino acid derivate hydrogelators, such as valine, isoleucine, and lysine derivatives with end groups of positive charged pyridinium or imidazolium, which can gelatinize water [5,6]. Friggeri and co-workers studied the entrapment and release of 8-aminoquinoline and 2-hydroxyquinoline in supramolecular hydrogels formed by a hydrogelator N ,N -dibenzoyl-L-cystine [7]. Motulsky and co-workers reported the gelation of vegetable oil and water in the presence of six different amphiphilic gelators based on the L-alanine derivatives. They also reported the in situ formation of hydrogels through subcutaneous injections in rats and the biodegradation and biocompatibility of hydrogels [8]. Furthermore, some surfactants such as amphiphiles [9], bola amphiphiles [10], and gemini amphiphiles [11] can be used as gelators to form supramolecular hydrogels. In this work, a chiral hydrogelator based on L-phenylalanine, tetraethylammonium 3-{[(2R)-2-(octadecylamino)-3-phenylpropanoyl]amino}butyrate (designated as TC18 PheBu), was
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synthesized and characterized. The formation mechanism of supramolecular hydrogels in the presence of the TC18 PheBu was investigated by using FT-IR, FE-SEM, fluorescence, and circular dichroism (CD). 2. Experimental 2.1. Materials BOC-L-phenylalanine, n-octadecylamine, succinic anhydride, tetraethylammonium hydroxide, dicyclohexylcarbodiimide (DCC), and 4-dimethylpyridine (DMAP) were purchased from Aldrich and used as received. Tetrahydrofuran (THF) was dried over sodium metal and distilled before use. Dichloromethane was distilled over P4 O10 . Other solvents were distilled only before use. Preparation of various pH aqueous solutions: 0.1 mol/L of hydrochloric aqueous solution was diluted to prepare the acidic solutions with corresponding pH 1, 2, 4, and 6. Similarly, 1 mol/L sodium hydroxide aqueous solution was diluted to prepare the alkaline solutions with corresponding pH 9 and 12. 2.2. Synthesis of the hydrogelator TC18 PheBu (Scheme 1) BOC-L-phenylalanine (14.04 g, 53.2 mmol) and n-octadecylamine (14.28 g, 53.2 mmol) were dissolved in 100 mL of chloroform at room temperature. Dicyclohexylcarbodiimide (DCC, 13.12 g, 63.6 mmol) and 4-dimethylaminopyridine (DMAP, 0.64 g, 5.32 mmol) were added to the solution. The mixture was stirred for 1 h in an ice bath and then for 48 h at room temperature. After filtration, the solution was washed with 0.01 mol/L HCl solutions, aqueous saturated sodium carbonate solutions and water, respectively. The organic phase was separated in a separating funnel and dried over anhydrous calcium chloride. Then the organic phase was evaporated under reduced pressure to remove the solvent. The solid product, N-BOC-N -n-octadecylamine-L-phenylalanine (designated as N-BOC-N -18-L-Phe) was obtained after washing twice with ether. N-BOC-N -18-L-Phe (20.64 g, 40 mmol) was dissolved in 100 mL of ethyl acetate and then anhydrous gaseous HCl was bubbled through the solution. The mixture was stirred at room temperature until the solid completely dissolved. Stirring was continued for 6 h. After filtration and drying in vacuum, the colorless crystalline N -n-octadecylamine-L-phenylalanine hydrochloride (designated as C18 PheHC) was obtained. C18 PheHC (9.1 g, 20 mmol) and triethylamine (2 g, 20 mmol) were dissolved in chloroform. The mixture was stirred at room temperature for 4 h. After filtration, the solution was washed three times with water and dried over anhydrous calcium chloride. Then the solution was evaporated under reduced pressure to remove the solvent. The solid product (designated as C18 Phe) was further washed twice with ether. Succinic anhydride (1 g, 10 mmol) and C18 Phe (4.16 g, 10 mmol) were added to 20 mL of dichloromethane. The solution was refluxed for 1 h and then precipitated in ether. After
Scheme 1. Synthetic scheme of the hydrogelator TC18 PheBu.
filtration, the product was isolated (designated as C18 PheBuA) and was washed twice with ether. Finally, C18 PheBuA (4.67 g, 9 mmol) and tetraethylammonium hydroxide (1.32 g, 9 mmol) were dissolved in ethanol. The solution was stirred at 25 ◦ C for 6 h and then precipitated in ether. After filtration, the final product TC18 PheBu was obtained in 82% yield after washing twice with ether and drying in vacuum. N-BOC-N -18-L-Phe: FT-IR (KBr, ν, cm−1 ): νN–H 3330 (N–H, amide A), 1692 (C=O), 1649 (C=O, amide I), 1529 (N–H, amide II). 1 H NMR: δ 8.311 (d, 1H; J = 7.6, BOC-NH), 7.315–7.199 (m, 5H, Ar–H), 4.256–4.238 (m, 1H, CHNHCO), 3.141–2.971 (m, 2H, ArCH 2 CH), 1.951–1.707 (m, 9H; OC(CH 3 )3 ), 1.388–1.160 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.897–0.863 (m, 3H, CH2 CH 3 ). C18 Phe: FT-IR (KBr, ν, cm−1 ): νN–H 3284 (N–H, amide A), 1634 (C=O, amide I), 1554 (N–H, amide II). 1 H NMR: δ 8.241–8.163 (d, 2H; J = 7.6, –NH2 ), 7.332–7.195 (m, 5H; Ar–H), 3.885–3.694 (m, H; ArCH2 CHNH), 3.130–2.906 (m, 2H; ArCH 2 NH), 1.318–1.162 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.859–0.825 (t, 3H; J = 6.8, CH2 CH2 CH 3 ). C18 PheBuA: FT-IR (KBr, ν, cm−1 ): νN–H 3290 (N–H, amide A), 1705 (C=O, free carboxylic), 1637 (C=O, amide I), 1549 (N–H, amide II). 1 H NMR: δ 11.998 (s, H; COOH), 8.101– 8.080 (d, H; J = 7.6, CHNHCO), 7.826–7.149 (m, 5H; Ar–H), 4.421–4.340 (m, 1H; ArCH2 CHNH), 3.467–2.906 (m, 2H; ArCH 2 CHCO), 1.327–1.271 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.859–0.825 (t, 3H; J = 6.8, CH2 CH2 CH 3 ). TC18 PheBu: m.p.: 88–90 ◦ C. FT-IR (KBr, ν, cm−1 ): 3284 (N–H, amide A), 1634 (C=O, amide I), 1554 (N–H, amide II). 1 H NMR: 8.101–8.080 (d, H; J = 7.6, CHNHCO), δ 7.42–7.12 (m, 5H; Ar–H), 4.41–4.60 (m, 1H; ArCH2 CHNHCO), 3.16–
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3.29 (m, 2H; CH2 CH 2 NH), 2.88–3.03 (m, 2H; ArCH 2 CHCO), 2.03–2.23 (m, 8H; N(CH 2 CH3 )4 ), 1.25–1.30 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.877–0.879 (m, 3H; CH2 CH2 CH2 CH 3 ). ESI-MS: m/z, calculated for C18 PheBu (C39 H71 O4 N3 ): 645.30; found: 646.40 [M + H]+ . The elementary analysis, found: C 72.42, H 11.12, N 6.52, O 9.94; calculated: C 72.51, H 11.08, N 6.50, O 9.91. 2.3. Measurement FT-IR spectra were recorded on an EQUINOX55 spectrophotometer (Bruker), KBr Pellet for the characterization of TC18 PheBu structure and D2 O solution for the characterization of hydrogen bonds. 1 H NMR spectra were recorded on a Mercury VX-300 (Varian, using TMS as internal reference, and DMSO (d6 ) as the solvent). The elemental analysis was measured by a Vario EL III, Elementar. ESI-MS were performed by LC-MS (Agilent 1100 LC/MSD Trap). The mass spectrometer equipped with an electrospray interface (ESI) was operated in the positive-ion mode. The ESI temperature was set to 350 ◦ C and the capillary voltage was 4000 V. The chromatographic separation was carried out with a column (ODS-C-18) using gradient elution at a flow of 1 mL/min. The optical analysis of the organogels was performed by using a polarized optical microscope (POM, BH-2, Olympus). The warm solution (ca. 70 ◦ C) containing 6 wt% of TC18 PheBu was dropped on a prewarmed glass plate to form a thin layer, and then allowed to spontaneously cool to room temperature. The wet gels were kept at room temperature for 6 h before testing. The morphological analysis of the xerogels was performed by field emission scanning electron microscopy (FE-SEM, Sirion 200, FEI). The gel samples were frozen in liquid nitrogen and then freeze-dried. The fractured specimens were coated with Au. The electric current was 15 mA and the accelerating voltage was 5 kV. The steady-state fluorescence spectra were recorded with a FP-6500 spectrofluorometer (JASCO) at 25 ◦ C. Saturated aqueous solutions of pyrene were used for sample preparation. The excitation wavelength was 335 nm. Wavelengths of I1 and I3 appeared at 374 and 392 nm, respectively. CD spectra were recorded on a J-810 spectrophotometer (JASCO) at 25 ◦ C. The scan rate was 500 nm/min, and all spectra were accumulated four times. Different equivalents of KF were added to aqueous solutions containing 0.02 wt% of TC18 PheBu. For a typical gelation test, a weighed TC18 PheBu was added to aqueous media in a test tube and the mixture was heated until the solid completely dissolved. The solution was allowed to cool at room temperature for 2 h and exhibited no gravitational flow on inversion of the test tube. A required minimum amount of TC18 PheBu for gelation is defined as minimum gelation concentration (MGC) [12]. Similarly, the sample solution (ca. 70 ◦ C) was placed in a constant temperature set (25 ◦ C) and allowed to cool until no gravitational flow through on inversion of the test tube. The required time for gelation is defined as time taken for gelation.
Table 1 Gelation of TC18 PheBu in various aqueous media Various aqueous media
MGCs (wt%)
TGS (◦ C)
Time taken for gelation (min)
Phase formation
pH 1 pH 2 pH 4 pH 6 Deionized water pH 9 pH 12
1.2 1.5 1.8 2 2 2 1.5
58 54 51 50 48 50 54
5–10 10–15 15–20 20–25 25–30 20–25 15–20
TG OG OG OG TG OG TG
TG, transparent gels; OG, opaque gels.
A small steel ball (250 mg, 4 mm) was placed on the top of the supramolecular hydrogel in a test tube ( 10 mm). Then the sample was slowly heated (2 ◦ C/min) in a thermostated water bath. When the ball falls to the bottom of the test tube, the temperature is defined as phase dissociation temperature (TGS ) of supramolecular hydrogels [13]. 3. Result and discussion 3.1. Gelation of various aqueous media in the presence of TC18 PheBu The gelation experiments indicated that TC18 PheBu was able to gelatinize water and various pHs of aqueous media. The minimum gelation concentration displays gelation ability of the gelator in aqueous media. For instance, the MGC in deionized water is 2 wt%, which reveals that one molecule of TC18 PheBu can immobilize about 1500 water molecules. With the increase of the pH values of aqueous media from pH 1 to pH 6, the MGCs increase from 1.2 to 2 wt%. The time taken for gelation also increases from about 10 to 25 min. With the increase of the pH values from pH 9 to pH 12, the MGCs decrease from 2 to 1.5 wt%. The time taken for gelation also decreases from about 25 to 15 min. These results suggest that the gelation dynamics could be controlled by the ionic strength of the media. This phenomenon can be attributed to that the increase of the ionic (H+ or Na+ ) strength in aqueous media reduced the electrostatic repulsion between the carboxyl groups of TC18 PheBu, which allows the molecule to assume the conformation required to form aggregates. Therefore, the MGCs decrease with the increase of the ionic strength [14]. 3.2. Morphology of TC18 PheBu aggregates in various aqueous media Fig. 1 shows the images of FE-SEM and POM of supramolecular hydrogels formed by TC18 PheBu. As shown in Fig. 1, TC18 PheBu self-assembles in deionized water to form fibrillike aggregates with diameters of about 40–60 nm (Fig. 1a) and forms sheet-like aggregates in pH 12 aqueous media (Fig. 1c). In pH 2 aqueous media, TC18 PheBu forms a spherical aggregate with diameter of about 600 nm (Fig. 1b). Interestingly, the images of POM indicate that the morphology of TC18 PheBu aggregates depends on the pH of the aqueous media. For instance, as shown in Fig. 1d, the characteristic
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Fig. 1. (a) FE-SEM images of dried hydrogels formed by 3.0 wt% of TC18 PheBu in deionized water, (b) in pH 2, and (c) in pH 12 aqueous media. (d and e) POM images of hydrogels formed by 5 wt% of TC18 PheBu in pH 12 and pH 2 aqueous media, respectively (magnitude 200×).
Fig. 2. FT-IR spectra of the D2 O hydrogel (solid line) formed by TC18 PheBu (3 wt%) and D2 O solution (dashed line) of TC18 PheBu (1 wt%) at 25 ◦ C.
Maltese cross reveals the formation of typical spheric crystallites resulting from further assembly of fibril-like aggregates of TC18 PheBu in the aqueous medium at pH 12. Correspondingly, dendritic crystallites can be observed for TC18 PheBu aggregates in the aqueous medium at pH 2 (Fig. 1e). POM measurements of supramolecular hydrogels indicate that TC18 PheBu forms different shapes of aggregates in acidic or alkaline media. Obviously, the hydrogel status significantly depends on the morphology of the aggregates. Water molecules are apparently entrapped in the three-dimensional network by capillary forces, leading to the formation of stable hydrogels. 3.3. The mechanism of TC18 PheBu self-assembling in water As is well known, hydrogen bonding is the main driving force for the self-assembly of gelators in organic solvents. Although FT-IR spectroscopy is a powerful tool for studying hydrogen-bonding interactions, it is difficult or almost impossible to obtain useful information on hydrogen bonding in water. Therefore, in this work, D2 O was used instead of H2 O for FT-IR measurement. Fig. 2 shows the FT-IR spectra of the hydrogels formed by TC18 PheBu (3 wt%) in D2 O and a solution of TC18 PheBu (1 wt%) in D2 O at 25 ◦ C. The absorption bands at 1645 cm−1 for the amide I region in D2 O solution shift to 1615 cm−1 in the D2 O hydrogel. While, the absorption bands at 1553 cm−1 for the amide II region in D2 O solution
shift to 1567 cm−1 in the D2 O hydrogel. Such spectral shifts are compatible with the presence of intermolecular hydrogenbonded amide groups and suggest that one of driving forces for TC18 PheBu hydrogel formation is hydrogen bonding. In addition, the absorption bands of the asymmetric (νas ) and symmetric (νs ) stretching vibrations appeared at 2928 cm−1 (νas , C–H) and 2858 cm−1 (νs , C–H) in the FT-IR spectra of a D2 O solution of TC18 PheBu. They are also red shifted to 2920 and 2852 cm−1 , respectively, in the FT-IR spectra of the hydrogel. Such low frequency shifts are induced by the interaction of the alkyl chains of TC18 PheBu [15]. It indicates that the hydrophobic interaction between the long chain alkyl groups of TC18 PheBu is one of the driving forces for the self-assembly of TC18 PheBu in water. The hydrophobic interaction between long chain alkyl groups can be further characterized by fluorescence measurements in the presence of pyrene as a probe. As shown in Fig. 3, fluorescence spectra of pyrene display five obvious vibronic bands, in which the intensity ratio of the third (394 nm) to the first (374 nm) vibronic peaks (I3 /I1 ) was usually used to estimate the polarity of the pyrene microenvironment [16]. The plot inserted in Fig. 3 shows the dependence of the fluorescence relative intensities (I3 /I1 ) of pyrene on the concentration of TC18 PheBu. A rapid increase of the ratios of I3 /I1 is found in the concentration range of 2–3 wt% of TC18 PheBu.
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Fig. 3. Fluorescence spectra and relative intensities (I3 /I1 ) of pyrene in aqueous solutions containing 0 (a), 0.5 (b), 1 (c), 1.5 (d), 2 (e), 2.5 (f), 3 (g), 3.5 (h), 4 (i), and 4.5 wt% (j) of TC18 PheBu at 25 ◦ C.
Fig. 4. The CD spectra of TC18 PheBu aqueous solutions (3 × 10−4 mol/L) in the presence of KF. (1) 0.0 e.q. KF; (2) 0.5 e.q. KF; (3) 1.0 e.q. KF.
Apparently, there are not enough hydrophobic domains formed by the alkyl groups of TC18 PheBu for effective solubilization of pyrene when the concentration of TC18 PheBu is lower than 2 wt%. In this case, the TC18 PheBu disperses in the solution. The exciplex through the interaction between TC18 PheBu molecules is hard formed, which resulted in lower fluorescence relative intensities (I3 /I1 ). By contrast, when the concentration of TC18 PheBu is higher than 2 wt%, more pyrene can be solubilized resulting in higher ratios of I3 /I1 . The λmax of pyrene shows a red shift from 454 to 408 nm as shown in Fig. 3. Such fluorescence behavior is usually observed when the pyrene solubilized in a hydrophobic environment, namely, the interior of the strands in the self-assembled nanofibers is a hydrophobic microdomain. Fluorescence spectra indicate that ca. 2.5 wt% is a critical concentration for the aggregation of TC18 PheBu. It is in accord with the minimum gelation concentration of TC18 PheBu described above. Therefore hydrophobic interaction between the alkyl groups of TC18 PheBu also is one of the driving forces for the self-assembling of TC18 PheBu in water. Considering the chiral structure of TC18 PheBu, circular dichroism spectroscopy was applied for the characterization of supramolecular chirality originated from the chiral monomer TC18 PheBu. The use of fluoride ions is based on a possibility that the hydrogen of amide moieties is able to bind fluoride ions, thereby destroying hydrogen bonds. It could be an evidence of hydrogen-bonding formation for chiral compounds.
Fig. 4 shows CD spectra of TC18 PheBu in the presence of fluoride ions (F− ). The intensity peak at 220–225 nm could be attributed to the π–π ∗ transition of the amide bonds, and the shoulder peaks at longer wavelengths could be attributed to the n–π ∗ transition of the amide bonds [17]. These transitions are extremely sensitive to coupling with neighboring amide moieties. The λmax of the UV absorption of TC18 PheBu is about 221 nm. In the CD spectra of aqueous solutions of TC18 PheBu, the λθ=0 value also appears at about 221 nm. This indicates that λ = 221 nm is the wavelength of the exciton coupling, in accord with TC18 PheBu aggregates. The CD spectrum of TC18 PheBu aggregates exhibits a positive Cotton effect as shown in Fig. 4. This implies that the dipole moments of TC18 PheBu aggregates possess an orientation in a clockwise direction. Therefore the CD spectra confirm that the aggregates have right-handed helical structures. The CD intensities of TC18 PheBu aqueous solutions were much stronger than those in the presence of varied e.q. KF. As shown in Fig. 4, the intensities of the CD bands at about 221 nm decrease in the presence of a solution with strong ionic strength, such as potassium fluoride, due to the disintegration of the self-assembly. Potassium fluoride, as an impurity, can destroy the amide-bonding interactions between TC18 PheBu molecules during the process of TC18 PheBu self-assembling. The CD analysis further confirms that hydrogen bonding is one of the driving forces for the self-assembly of TC18 PheBu. 4. Conclusion A novel amphiphilic hydrogelator TC18 PheBu based on was synthesized. The gelation of water and aqueous solutions with various pH values in the presence of TC18 PheBu was investigated. The mechanism of the formation of the supramolecular hydrogels was studied by using FESEM, POM, FT-IR, fluorescence, and CD. The results indicate that TC18 PheBu can self-assemble in aqueous media into different aggregates in morphology. The main driving forces for self-assembly are both hydrogen-bonding and hydrophobic interactions. L -phenylalanine
Acknowledgments The work was financially supported by National Natural Science Foundation of China (No. 20474022). We thank the Analytic and Testing Center of HUST for the measurements of FT-IR, FE-SEM, fluorescence, and circular dichroism. References [1] [2] [3] [4] [5]
P. Terech, R.G. Weiss, Chem. Rev. 97 (1997) 3133. D.J. Abdallah, R.G. Weiss, Adv. Mater. 12 (2000) 1237. L.A. Estroff, A.D. Hamilton, Chem. Rev. 104 (2004) 1201. M. de Loos, B.L. Feringa, J.H. van Esch, Eur. J. Org. Chem. (2005) 3615. M.T. Sato, A. Kurose, H. Shirai, K. Hanabusa, Tetrahedron Lett. 46 (2005) 2741. [6] M. Suzuki, M. Yumoto, M. Kimura, H. Shirai, K. Hanabusa, Chem. Eur. J. 9 (2003) 348. [7] A. Friggeri, B.L. Feringa, J. van Esch, J. Controlled Release 97 (2004) 241.
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[8] A. Motulsky, M. Lafleurb, A.C. Couffin-Hoaraua, D. Hoarauc, F. Bouryd, J.P. Benoitd, J.C. Leroux, Biomaterials 26 (2005) 6242. [9] M. Suzuki, S. Owa, M. Kimura, A. Kurose, H. Shiraib, K. Hanabusa, Tetrahedron Lett. 46 (2005) 303. [10] T. Shimizu, R. Iwaura, M. Masuda, T. Hanada, K. Yase, J. Am. Chem. Soc. 123 (2001) 5947. [11] F.M. Menger, H. Zhang, K.L. Caran, V.A. Seredyuk, R.P. Apkarian, J. Am. Chem. Soc. 124 (2002) 1140.
381
[12] K. Hanabusa, K. Okui, K. Karaki, M. Kimura, H. Shirai, J. Colloid Interface Sci. 195 (1997) 86. [13] A. Carré, P. Le Grel, M. Baudy-Floc’h, Tetrahedron Lett. 42 (2001) 1887. [14] L.A. Estroff, A.D. Hamilton, Angew. Chem. Int. Ed. 39 (2000) 3447. [15] E.J. Van, F. Schoonbeek, M. de Loos, H. Kooijman, A.L. Spek, R.M. Kellogg, B.L. Feringa, Chem. Eur. J. 5 (1999) 937. [16] L. Bromberg, D.P. Barr, Macromolecules 32 (1999) 3649. [17] Z. Yang, B. Xu, Chem. Commun. 21 (2004) 2424.
Formation mechanism of supramolecular hydrogels in the presence of L-phenylalanine derivative as a hydrogelator Xinjian Fu, Ningxia Wang, Shengzu Zhang, Hong Wang, Yajiang Yang ∗ Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, China Received 28 March 2007; accepted 9 June 2007 Available online 1 August 2007
Abstract A novel chiral hydrogelator, L-phenylalanine derivative can self-assemble in aqueous media at different pH values to form supramolecular hydrogels. The images of the FE-SEM indicate that different aggregates of TC18 PheBu in morphology were formed, which further lead to the formation of spherical crystallites as observed by polarized optical microscope (POM). The FT-IR spectra of the supramolecular hydrogels reveal that intermolecular hydrogen-bonding and hydrophobic interactions are the driving forces for the self-assembly of TC18 PheBu. Fluorescence spectra of TC18 PheBu in aqueous solutions in the presence of pyrene as a probe further confirm the importance of hydrophobic interactions for the self-assembly. The circular dichroism (CD) spectra of TC18 PheBu in supramolecular hydrogels in the presence of KF indicate that the hydrogen-bonding interaction can be disrupted by fluoride ions, which further confirm the importance of hydrogen bonding for the self-assembly of TC18 PheBu. © 2007 Elsevier Inc. All rights reserved. Keywords: L-Phenylalanine derivatives; Hydrogelator; Supramolecular hydrogels
1. Introduction In the past decade, supramolecular organogels formed by the self-assembly of low molecular weight gelators have been the subject of increasing attention [1,2]. However less attention has been paid to the supramolecular hydrogels formed by hydrogelators. The supramolecular hydrogels are a novel condensed system of water molecules, which is quite different from the common polymer hydrogels. Similar to the organogel formation, supramolecular hydrogels are also formed by the selfassembly of hydrogelators through intermolecular interactions, such as hydrogen bonding, π–π stacking, van der Waals, coordination, and charge transfer. The water molecules are immobilized by capillary force in a three-dimensional network consisting of hydrogelator aggregates. In comparison with polymer hydrogels, supramolecular hydrogels are thermally reversible. It shows potential application in the fields of nanomaterials, drug carriers, molecular recognition, and so on [3,4]. * Corresponding author. Fax: +86 27 87543632.
E-mail address: [email protected] (Y. Yang). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.013
As an appropriate hydrogelator, the precondition is that it is usually water soluble at high temperatures and insoluble below a certain temperature. Amino acid derivate hydrogelators not only have such characteristics but also are biodegradable and have low toxicity. Suzuki and co-workers synthesized some amino acid derivate hydrogelators, such as valine, isoleucine, and lysine derivatives with end groups of positive charged pyridinium or imidazolium, which can gelatinize water [5,6]. Friggeri and co-workers studied the entrapment and release of 8-aminoquinoline and 2-hydroxyquinoline in supramolecular hydrogels formed by a hydrogelator N ,N -dibenzoyl-L-cystine [7]. Motulsky and co-workers reported the gelation of vegetable oil and water in the presence of six different amphiphilic gelators based on the L-alanine derivatives. They also reported the in situ formation of hydrogels through subcutaneous injections in rats and the biodegradation and biocompatibility of hydrogels [8]. Furthermore, some surfactants such as amphiphiles [9], bola amphiphiles [10], and gemini amphiphiles [11] can be used as gelators to form supramolecular hydrogels. In this work, a chiral hydrogelator based on L-phenylalanine, tetraethylammonium 3-{[(2R)-2-(octadecylamino)-3-phenylpropanoyl]amino}butyrate (designated as TC18 PheBu), was
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synthesized and characterized. The formation mechanism of supramolecular hydrogels in the presence of the TC18 PheBu was investigated by using FT-IR, FE-SEM, fluorescence, and circular dichroism (CD). 2. Experimental 2.1. Materials BOC-L-phenylalanine, n-octadecylamine, succinic anhydride, tetraethylammonium hydroxide, dicyclohexylcarbodiimide (DCC), and 4-dimethylpyridine (DMAP) were purchased from Aldrich and used as received. Tetrahydrofuran (THF) was dried over sodium metal and distilled before use. Dichloromethane was distilled over P4 O10 . Other solvents were distilled only before use. Preparation of various pH aqueous solutions: 0.1 mol/L of hydrochloric aqueous solution was diluted to prepare the acidic solutions with corresponding pH 1, 2, 4, and 6. Similarly, 1 mol/L sodium hydroxide aqueous solution was diluted to prepare the alkaline solutions with corresponding pH 9 and 12. 2.2. Synthesis of the hydrogelator TC18 PheBu (Scheme 1) BOC-L-phenylalanine (14.04 g, 53.2 mmol) and n-octadecylamine (14.28 g, 53.2 mmol) were dissolved in 100 mL of chloroform at room temperature. Dicyclohexylcarbodiimide (DCC, 13.12 g, 63.6 mmol) and 4-dimethylaminopyridine (DMAP, 0.64 g, 5.32 mmol) were added to the solution. The mixture was stirred for 1 h in an ice bath and then for 48 h at room temperature. After filtration, the solution was washed with 0.01 mol/L HCl solutions, aqueous saturated sodium carbonate solutions and water, respectively. The organic phase was separated in a separating funnel and dried over anhydrous calcium chloride. Then the organic phase was evaporated under reduced pressure to remove the solvent. The solid product, N-BOC-N -n-octadecylamine-L-phenylalanine (designated as N-BOC-N -18-L-Phe) was obtained after washing twice with ether. N-BOC-N -18-L-Phe (20.64 g, 40 mmol) was dissolved in 100 mL of ethyl acetate and then anhydrous gaseous HCl was bubbled through the solution. The mixture was stirred at room temperature until the solid completely dissolved. Stirring was continued for 6 h. After filtration and drying in vacuum, the colorless crystalline N -n-octadecylamine-L-phenylalanine hydrochloride (designated as C18 PheHC) was obtained. C18 PheHC (9.1 g, 20 mmol) and triethylamine (2 g, 20 mmol) were dissolved in chloroform. The mixture was stirred at room temperature for 4 h. After filtration, the solution was washed three times with water and dried over anhydrous calcium chloride. Then the solution was evaporated under reduced pressure to remove the solvent. The solid product (designated as C18 Phe) was further washed twice with ether. Succinic anhydride (1 g, 10 mmol) and C18 Phe (4.16 g, 10 mmol) were added to 20 mL of dichloromethane. The solution was refluxed for 1 h and then precipitated in ether. After
Scheme 1. Synthetic scheme of the hydrogelator TC18 PheBu.
filtration, the product was isolated (designated as C18 PheBuA) and was washed twice with ether. Finally, C18 PheBuA (4.67 g, 9 mmol) and tetraethylammonium hydroxide (1.32 g, 9 mmol) were dissolved in ethanol. The solution was stirred at 25 ◦ C for 6 h and then precipitated in ether. After filtration, the final product TC18 PheBu was obtained in 82% yield after washing twice with ether and drying in vacuum. N-BOC-N -18-L-Phe: FT-IR (KBr, ν, cm−1 ): νN–H 3330 (N–H, amide A), 1692 (C=O), 1649 (C=O, amide I), 1529 (N–H, amide II). 1 H NMR: δ 8.311 (d, 1H; J = 7.6, BOC-NH), 7.315–7.199 (m, 5H, Ar–H), 4.256–4.238 (m, 1H, CHNHCO), 3.141–2.971 (m, 2H, ArCH 2 CH), 1.951–1.707 (m, 9H; OC(CH 3 )3 ), 1.388–1.160 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.897–0.863 (m, 3H, CH2 CH 3 ). C18 Phe: FT-IR (KBr, ν, cm−1 ): νN–H 3284 (N–H, amide A), 1634 (C=O, amide I), 1554 (N–H, amide II). 1 H NMR: δ 8.241–8.163 (d, 2H; J = 7.6, –NH2 ), 7.332–7.195 (m, 5H; Ar–H), 3.885–3.694 (m, H; ArCH2 CHNH), 3.130–2.906 (m, 2H; ArCH 2 NH), 1.318–1.162 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.859–0.825 (t, 3H; J = 6.8, CH2 CH2 CH 3 ). C18 PheBuA: FT-IR (KBr, ν, cm−1 ): νN–H 3290 (N–H, amide A), 1705 (C=O, free carboxylic), 1637 (C=O, amide I), 1549 (N–H, amide II). 1 H NMR: δ 11.998 (s, H; COOH), 8.101– 8.080 (d, H; J = 7.6, CHNHCO), 7.826–7.149 (m, 5H; Ar–H), 4.421–4.340 (m, 1H; ArCH2 CHNH), 3.467–2.906 (m, 2H; ArCH 2 CHCO), 1.327–1.271 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.859–0.825 (t, 3H; J = 6.8, CH2 CH2 CH 3 ). TC18 PheBu: m.p.: 88–90 ◦ C. FT-IR (KBr, ν, cm−1 ): 3284 (N–H, amide A), 1634 (C=O, amide I), 1554 (N–H, amide II). 1 H NMR: 8.101–8.080 (d, H; J = 7.6, CHNHCO), δ 7.42–7.12 (m, 5H; Ar–H), 4.41–4.60 (m, 1H; ArCH2 CHNHCO), 3.16–
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3.29 (m, 2H; CH2 CH 2 NH), 2.88–3.03 (m, 2H; ArCH 2 CHCO), 2.03–2.23 (m, 8H; N(CH 2 CH3 )4 ), 1.25–1.30 (m, 30H; CH2 (CH 2 )15 CH2 CH3 ), 0.877–0.879 (m, 3H; CH2 CH2 CH2 CH 3 ). ESI-MS: m/z, calculated for C18 PheBu (C39 H71 O4 N3 ): 645.30; found: 646.40 [M + H]+ . The elementary analysis, found: C 72.42, H 11.12, N 6.52, O 9.94; calculated: C 72.51, H 11.08, N 6.50, O 9.91. 2.3. Measurement FT-IR spectra were recorded on an EQUINOX55 spectrophotometer (Bruker), KBr Pellet for the characterization of TC18 PheBu structure and D2 O solution for the characterization of hydrogen bonds. 1 H NMR spectra were recorded on a Mercury VX-300 (Varian, using TMS as internal reference, and DMSO (d6 ) as the solvent). The elemental analysis was measured by a Vario EL III, Elementar. ESI-MS were performed by LC-MS (Agilent 1100 LC/MSD Trap). The mass spectrometer equipped with an electrospray interface (ESI) was operated in the positive-ion mode. The ESI temperature was set to 350 ◦ C and the capillary voltage was 4000 V. The chromatographic separation was carried out with a column (ODS-C-18) using gradient elution at a flow of 1 mL/min. The optical analysis of the organogels was performed by using a polarized optical microscope (POM, BH-2, Olympus). The warm solution (ca. 70 ◦ C) containing 6 wt% of TC18 PheBu was dropped on a prewarmed glass plate to form a thin layer, and then allowed to spontaneously cool to room temperature. The wet gels were kept at room temperature for 6 h before testing. The morphological analysis of the xerogels was performed by field emission scanning electron microscopy (FE-SEM, Sirion 200, FEI). The gel samples were frozen in liquid nitrogen and then freeze-dried. The fractured specimens were coated with Au. The electric current was 15 mA and the accelerating voltage was 5 kV. The steady-state fluorescence spectra were recorded with a FP-6500 spectrofluorometer (JASCO) at 25 ◦ C. Saturated aqueous solutions of pyrene were used for sample preparation. The excitation wavelength was 335 nm. Wavelengths of I1 and I3 appeared at 374 and 392 nm, respectively. CD spectra were recorded on a J-810 spectrophotometer (JASCO) at 25 ◦ C. The scan rate was 500 nm/min, and all spectra were accumulated four times. Different equivalents of KF were added to aqueous solutions containing 0.02 wt% of TC18 PheBu. For a typical gelation test, a weighed TC18 PheBu was added to aqueous media in a test tube and the mixture was heated until the solid completely dissolved. The solution was allowed to cool at room temperature for 2 h and exhibited no gravitational flow on inversion of the test tube. A required minimum amount of TC18 PheBu for gelation is defined as minimum gelation concentration (MGC) [12]. Similarly, the sample solution (ca. 70 ◦ C) was placed in a constant temperature set (25 ◦ C) and allowed to cool until no gravitational flow through on inversion of the test tube. The required time for gelation is defined as time taken for gelation.
Table 1 Gelation of TC18 PheBu in various aqueous media Various aqueous media
MGCs (wt%)
TGS (◦ C)
Time taken for gelation (min)
Phase formation
pH 1 pH 2 pH 4 pH 6 Deionized water pH 9 pH 12
1.2 1.5 1.8 2 2 2 1.5
58 54 51 50 48 50 54
5–10 10–15 15–20 20–25 25–30 20–25 15–20
TG OG OG OG TG OG TG
TG, transparent gels; OG, opaque gels.
A small steel ball (250 mg, 4 mm) was placed on the top of the supramolecular hydrogel in a test tube ( 10 mm). Then the sample was slowly heated (2 ◦ C/min) in a thermostated water bath. When the ball falls to the bottom of the test tube, the temperature is defined as phase dissociation temperature (TGS ) of supramolecular hydrogels [13]. 3. Result and discussion 3.1. Gelation of various aqueous media in the presence of TC18 PheBu The gelation experiments indicated that TC18 PheBu was able to gelatinize water and various pHs of aqueous media. The minimum gelation concentration displays gelation ability of the gelator in aqueous media. For instance, the MGC in deionized water is 2 wt%, which reveals that one molecule of TC18 PheBu can immobilize about 1500 water molecules. With the increase of the pH values of aqueous media from pH 1 to pH 6, the MGCs increase from 1.2 to 2 wt%. The time taken for gelation also increases from about 10 to 25 min. With the increase of the pH values from pH 9 to pH 12, the MGCs decrease from 2 to 1.5 wt%. The time taken for gelation also decreases from about 25 to 15 min. These results suggest that the gelation dynamics could be controlled by the ionic strength of the media. This phenomenon can be attributed to that the increase of the ionic (H+ or Na+ ) strength in aqueous media reduced the electrostatic repulsion between the carboxyl groups of TC18 PheBu, which allows the molecule to assume the conformation required to form aggregates. Therefore, the MGCs decrease with the increase of the ionic strength [14]. 3.2. Morphology of TC18 PheBu aggregates in various aqueous media Fig. 1 shows the images of FE-SEM and POM of supramolecular hydrogels formed by TC18 PheBu. As shown in Fig. 1, TC18 PheBu self-assembles in deionized water to form fibrillike aggregates with diameters of about 40–60 nm (Fig. 1a) and forms sheet-like aggregates in pH 12 aqueous media (Fig. 1c). In pH 2 aqueous media, TC18 PheBu forms a spherical aggregate with diameter of about 600 nm (Fig. 1b). Interestingly, the images of POM indicate that the morphology of TC18 PheBu aggregates depends on the pH of the aqueous media. For instance, as shown in Fig. 1d, the characteristic
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Fig. 1. (a) FE-SEM images of dried hydrogels formed by 3.0 wt% of TC18 PheBu in deionized water, (b) in pH 2, and (c) in pH 12 aqueous media. (d and e) POM images of hydrogels formed by 5 wt% of TC18 PheBu in pH 12 and pH 2 aqueous media, respectively (magnitude 200×).
Fig. 2. FT-IR spectra of the D2 O hydrogel (solid line) formed by TC18 PheBu (3 wt%) and D2 O solution (dashed line) of TC18 PheBu (1 wt%) at 25 ◦ C.
Maltese cross reveals the formation of typical spheric crystallites resulting from further assembly of fibril-like aggregates of TC18 PheBu in the aqueous medium at pH 12. Correspondingly, dendritic crystallites can be observed for TC18 PheBu aggregates in the aqueous medium at pH 2 (Fig. 1e). POM measurements of supramolecular hydrogels indicate that TC18 PheBu forms different shapes of aggregates in acidic or alkaline media. Obviously, the hydrogel status significantly depends on the morphology of the aggregates. Water molecules are apparently entrapped in the three-dimensional network by capillary forces, leading to the formation of stable hydrogels. 3.3. The mechanism of TC18 PheBu self-assembling in water As is well known, hydrogen bonding is the main driving force for the self-assembly of gelators in organic solvents. Although FT-IR spectroscopy is a powerful tool for studying hydrogen-bonding interactions, it is difficult or almost impossible to obtain useful information on hydrogen bonding in water. Therefore, in this work, D2 O was used instead of H2 O for FT-IR measurement. Fig. 2 shows the FT-IR spectra of the hydrogels formed by TC18 PheBu (3 wt%) in D2 O and a solution of TC18 PheBu (1 wt%) in D2 O at 25 ◦ C. The absorption bands at 1645 cm−1 for the amide I region in D2 O solution shift to 1615 cm−1 in the D2 O hydrogel. While, the absorption bands at 1553 cm−1 for the amide II region in D2 O solution
shift to 1567 cm−1 in the D2 O hydrogel. Such spectral shifts are compatible with the presence of intermolecular hydrogenbonded amide groups and suggest that one of driving forces for TC18 PheBu hydrogel formation is hydrogen bonding. In addition, the absorption bands of the asymmetric (νas ) and symmetric (νs ) stretching vibrations appeared at 2928 cm−1 (νas , C–H) and 2858 cm−1 (νs , C–H) in the FT-IR spectra of a D2 O solution of TC18 PheBu. They are also red shifted to 2920 and 2852 cm−1 , respectively, in the FT-IR spectra of the hydrogel. Such low frequency shifts are induced by the interaction of the alkyl chains of TC18 PheBu [15]. It indicates that the hydrophobic interaction between the long chain alkyl groups of TC18 PheBu is one of the driving forces for the self-assembly of TC18 PheBu in water. The hydrophobic interaction between long chain alkyl groups can be further characterized by fluorescence measurements in the presence of pyrene as a probe. As shown in Fig. 3, fluorescence spectra of pyrene display five obvious vibronic bands, in which the intensity ratio of the third (394 nm) to the first (374 nm) vibronic peaks (I3 /I1 ) was usually used to estimate the polarity of the pyrene microenvironment [16]. The plot inserted in Fig. 3 shows the dependence of the fluorescence relative intensities (I3 /I1 ) of pyrene on the concentration of TC18 PheBu. A rapid increase of the ratios of I3 /I1 is found in the concentration range of 2–3 wt% of TC18 PheBu.
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Fig. 3. Fluorescence spectra and relative intensities (I3 /I1 ) of pyrene in aqueous solutions containing 0 (a), 0.5 (b), 1 (c), 1.5 (d), 2 (e), 2.5 (f), 3 (g), 3.5 (h), 4 (i), and 4.5 wt% (j) of TC18 PheBu at 25 ◦ C.
Fig. 4. The CD spectra of TC18 PheBu aqueous solutions (3 × 10−4 mol/L) in the presence of KF. (1) 0.0 e.q. KF; (2) 0.5 e.q. KF; (3) 1.0 e.q. KF.
Apparently, there are not enough hydrophobic domains formed by the alkyl groups of TC18 PheBu for effective solubilization of pyrene when the concentration of TC18 PheBu is lower than 2 wt%. In this case, the TC18 PheBu disperses in the solution. The exciplex through the interaction between TC18 PheBu molecules is hard formed, which resulted in lower fluorescence relative intensities (I3 /I1 ). By contrast, when the concentration of TC18 PheBu is higher than 2 wt%, more pyrene can be solubilized resulting in higher ratios of I3 /I1 . The λmax of pyrene shows a red shift from 454 to 408 nm as shown in Fig. 3. Such fluorescence behavior is usually observed when the pyrene solubilized in a hydrophobic environment, namely, the interior of the strands in the self-assembled nanofibers is a hydrophobic microdomain. Fluorescence spectra indicate that ca. 2.5 wt% is a critical concentration for the aggregation of TC18 PheBu. It is in accord with the minimum gelation concentration of TC18 PheBu described above. Therefore hydrophobic interaction between the alkyl groups of TC18 PheBu also is one of the driving forces for the self-assembling of TC18 PheBu in water. Considering the chiral structure of TC18 PheBu, circular dichroism spectroscopy was applied for the characterization of supramolecular chirality originated from the chiral monomer TC18 PheBu. The use of fluoride ions is based on a possibility that the hydrogen of amide moieties is able to bind fluoride ions, thereby destroying hydrogen bonds. It could be an evidence of hydrogen-bonding formation for chiral compounds.
Fig. 4 shows CD spectra of TC18 PheBu in the presence of fluoride ions (F− ). The intensity peak at 220–225 nm could be attributed to the π–π ∗ transition of the amide bonds, and the shoulder peaks at longer wavelengths could be attributed to the n–π ∗ transition of the amide bonds [17]. These transitions are extremely sensitive to coupling with neighboring amide moieties. The λmax of the UV absorption of TC18 PheBu is about 221 nm. In the CD spectra of aqueous solutions of TC18 PheBu, the λθ=0 value also appears at about 221 nm. This indicates that λ = 221 nm is the wavelength of the exciton coupling, in accord with TC18 PheBu aggregates. The CD spectrum of TC18 PheBu aggregates exhibits a positive Cotton effect as shown in Fig. 4. This implies that the dipole moments of TC18 PheBu aggregates possess an orientation in a clockwise direction. Therefore the CD spectra confirm that the aggregates have right-handed helical structures. The CD intensities of TC18 PheBu aqueous solutions were much stronger than those in the presence of varied e.q. KF. As shown in Fig. 4, the intensities of the CD bands at about 221 nm decrease in the presence of a solution with strong ionic strength, such as potassium fluoride, due to the disintegration of the self-assembly. Potassium fluoride, as an impurity, can destroy the amide-bonding interactions between TC18 PheBu molecules during the process of TC18 PheBu self-assembling. The CD analysis further confirms that hydrogen bonding is one of the driving forces for the self-assembly of TC18 PheBu. 4. Conclusion A novel amphiphilic hydrogelator TC18 PheBu based on was synthesized. The gelation of water and aqueous solutions with various pH values in the presence of TC18 PheBu was investigated. The mechanism of the formation of the supramolecular hydrogels was studied by using FESEM, POM, FT-IR, fluorescence, and CD. The results indicate that TC18 PheBu can self-assemble in aqueous media into different aggregates in morphology. The main driving forces for self-assembly are both hydrogen-bonding and hydrophobic interactions. L -phenylalanine
Acknowledgments The work was financially supported by National Natural Science Foundation of China (No. 20474022). We thank the Analytic and Testing Center of HUST for the measurements of FT-IR, FE-SEM, fluorescence, and circular dichroism. References [1] [2] [3] [4] [5]
P. Terech, R.G. Weiss, Chem. Rev. 97 (1997) 3133. D.J. Abdallah, R.G. Weiss, Adv. Mater. 12 (2000) 1237. L.A. Estroff, A.D. Hamilton, Chem. Rev. 104 (2004) 1201. M. de Loos, B.L. Feringa, J.H. van Esch, Eur. J. Org. Chem. (2005) 3615. M.T. Sato, A. Kurose, H. Shirai, K. Hanabusa, Tetrahedron Lett. 46 (2005) 2741. [6] M. Suzuki, M. Yumoto, M. Kimura, H. Shirai, K. Hanabusa, Chem. Eur. J. 9 (2003) 348. [7] A. Friggeri, B.L. Feringa, J. van Esch, J. Controlled Release 97 (2004) 241.
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[8] A. Motulsky, M. Lafleurb, A.C. Couffin-Hoaraua, D. Hoarauc, F. Bouryd, J.P. Benoitd, J.C. Leroux, Biomaterials 26 (2005) 6242. [9] M. Suzuki, S. Owa, M. Kimura, A. Kurose, H. Shiraib, K. Hanabusa, Tetrahedron Lett. 46 (2005) 303. [10] T. Shimizu, R. Iwaura, M. Masuda, T. Hanada, K. Yase, J. Am. Chem. Soc. 123 (2001) 5947. [11] F.M. Menger, H. Zhang, K.L. Caran, V.A. Seredyuk, R.P. Apkarian, J. Am. Chem. Soc. 124 (2002) 1140.
381
[12] K. Hanabusa, K. Okui, K. Karaki, M. Kimura, H. Shirai, J. Colloid Interface Sci. 195 (1997) 86. [13] A. Carré, P. Le Grel, M. Baudy-Floc’h, Tetrahedron Lett. 42 (2001) 1887. [14] L.A. Estroff, A.D. Hamilton, Angew. Chem. Int. Ed. 39 (2000) 3447. [15] E.J. Van, F. Schoonbeek, M. de Loos, H. Kooijman, A.L. Spek, R.M. Kellogg, B.L. Feringa, Chem. Eur. J. 5 (1999) 937. [16] L. Bromberg, D.P. Barr, Macromolecules 32 (1999) 3649. [17] Z. Yang, B. Xu, Chem. Commun. 21 (2004) 2424.