Self-Organization of Bolaamphiphile Bearing Biphenyl Mesogen and

Beijing 100084, People's Republic of China ... heads, and study its self-organization behavior. ... the effect of hydrogen bonding on self-organizatio...
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J. Phys. Chem. C 2008, 112, 3308-3313

Self-Organization of Bolaamphiphile Bearing Biphenyl Mesogen and Aspartic-Acid Headgroups Senlin Chen, Bo Song, Zhiqiang Wang,* and Xi Zhang* Key Lab of Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: NoVember 8, 2007; In Final Form: December 18, 2007

A novel bolaamphiphile (DCBP-10-L-Asp) that bears the biocompatible aspartic acid as hydrophilic heads has been designed and synthesized. It is found that DCBP-10-L-Asp can self-organize into supramolecular fiber-like structures in aqueus solution. These fiber-like structures have a good thermal stability that can bear a temperature as high as 110 °C, because of the collective interaction, including hydrogen bonding, π-π interaction, and van der Waals interaction. The hydrogen bonding is indispensable to the formation of the supramolecular fiber-like structures, which is confirmed by adding urea or Cu2+ ion into the solution or by changing the pH of the solution to break the hydrogen bonding. It is anticipated that this line of research may provide a way to fabricate biocompatible nanomaterials.

Introduction

Experimental Section

Self-organized supramolecular entities stand for a new type of nanomaterials due to their flexibility and reversibility.1 The building blocks of these entities are mainly anisotropic molecules, especially amphiphiles.2,3 Usually, the self-organized structures of the amphiphiles have defined sizes and shapes,2,4 which may represent good candidates for templates of nanostructure material synthesis or biomimic process. Introducing functional groups into the building block molecules is a smart way to change the morphology of the entities due to the different interactions of these groups;5 at the same time, the introduction of these groups may generate new application prospects. Among those building block molecules, the bolaamphiphiles6 have been widely studied and blossomed into a hot research area7 after the report that the bacteria membranes composed of this kind of molecule have good stability in low pH value and high temperature.8 In our previous work, we have successfully engineered the stability of the self-organized structure and obtained different micellar structures by introducing the groups that can produce different intermolecular interactions.9 Herein, we would like to introduce amino acids into the bolaamphiphile as hydrophilic heads, and study its self-organization behavior. For this purpose, we attempt to design and synthesize a new bolaamphiphile bearing a mesogenic group and two L-aspartic acid groups, noted as DCBP-10-L-Asp, as shown in Scheme 1. We expect that this molecule can form a stable supramolecular structure through the combined interaction, including hydrogen bonding, π-π interaction, and van der Waals interaction. Moreover, by adjusting the solution conditions, we also attempt to understand the effect of hydrogen bonding on self-organization of this bolaamphiphile. The self-organized structures could have biocompatible ability due to the outer amino acid groups, so that a potential application as template of cell growth is greatly anticipated.10

Materials. 11-bromoundecanoic acid, biphenyl-4,4′-diol, pyrene, L-aspartic acid (Asp), (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), N,Ndiisopropylethylamine (DIEA), and Pd/C (palladium, 5% on activated carbon powder) were purchased from Aldrich and used without further purification. Methanol, sulfuric acid, acetone, dichloromethane, and K2CO3 were all analytical-grade products purchased from Beijing Chemical Reagent Company. The synthetic route of bolaamphiphile, (S)-2-(11-{4′-[10-((S)-1,2dicarboxy- ethylcarbamoyl)-decyloxy]-biphenyl-4-yloxy}-undecanoylamino)-succinic acid, noted as DCBP-10-L-Asp, is shown in Scheme 1. The experimental details are shown below. Synthesis of Methyl 11-Bromoundecanoate (1). 11-bromoundecanoic acid (0.53 g, 2 mmol) was dissolved in methanol (30 mL), and then 5 mL sulfuric acid was added dropwise. After the mixture solution was refluxed for 24 h, the solvent was evaporated under reduced pressure to remove methanol. The mixture was extracted with CH2Cl2/H2O. The organic phase was washed with saturated NaHCO3 aqueous solution and then dried over anhydrous Na2SO4. The product (0.53 g, 94%) was obtained by removing the solvent. 1H NMR (300 Hz, CDCl3): δ (ppm) 3.60 (s, 3H, CH3), 3.34 (t, 2H, BrCH2), 2.24 (t, 2H, CH2COOCH3), 2.24 (m, 16H, BrCH2(CH2)8CH2COOCH3). Synthesis of 11, 11′-(Biphenyl-4,4′-diylbis(oxy))diundecanoic Acid (2). Methyl 11-bromoundecanoate (0.53 g, 1.8 mmol), biphenyl-4,4′-diol (0.16 g, 0.8 mmol), K2CO3 (0.8 g, 5.8 mmol) and a catalytic amount of 18-crown-6 were added into acetone (30 mL). After the solution was refluxed for 20 h under stirring, the solvent was removed. The crude product was purified by column chromatography (SiO2, CH2Cl2/acetic ester ) 20:1) to get a white solid. Then the white solid was dissolved into 20 mL THF, and KOH (0.56 g, 10 mmol) was added. After the mixture solution was stirred for 5 h at 60 °C, the solution was acidified with hydrochloric acid, and the solvent was removed. The obtained solid was dissolved into 5 mL CH3OH and dropwise added into 100 mL water. The produced precipitation was filtered, washed several times with water, and dried to yield a white solid (0.23 g, 67%). 1H NMR (300 Hz, DMSO-

* Corresponding authors. E-mail: [email protected]. Fax: +86010-62771149.

10.1021/jp710721u CCC: $40.75 © 2008 American Chemical Society Published on Web 02/13/2008

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SCHEME 1: Synthetic Routes of DCBP-10-L-Asp

d): δ (ppm) 7.51 (d, 4H, Ar-H meta to O(CH2)10), 6.97 (d, 4H, Ar-H ortho to O(CH2)10), 3.99 (t, 4H, ArOCH2), 2.18 (t, 4H, O(CH2)9CH2COOH). 1.25-1.14 (m, 32H, O(CH2)9CH2COOH). Synthesis of 2-(11-{4′-[10-(1,2-bis-Benzyloxycarbonyl-ethylcarbamoyl)- decyloxy]-biphenyl-4-yloxy}-undecanoylamino)succinic Acid Dibenzyl Ester (3). Compound 2 (0.23 g, 0.41 mmol) and L-aspartic acid dibenzyl ester p-toluenesulfonate (0.60 g, 1.23 mmol)synthesized as reference11 was dissolved in CH2Cl2 (20 mL). BOP reagent (0.54 g, 1.23 mmol) and DIEA (2 mL) were added. The reaction was stirred at room temperature under Ar atmosphere for 24 h. 50 mL Water was added, and the organic layer was extracted with ethyl acetate; the combined extracts were washed with water, saturated NaHCO3, dried over MgSO4, and evaporated. The crude material was purified by chromatography through silica gel (CH2Cl2/ethyl acetate ) 8:1) to get the compound 3 (0.40 g, 74%). 1H NMR (300 Hz, CDCl3): δ (ppm) 7.45 (d, 4H, Ar-H meta to O(CH2)10), 7.387.25 (m, 20H, Ar-H), 6.93 (d, 4H, Ar-H ortho to O(CH2)10), 6.43 (d, 2H, N-H), 5.15 (s, 4H, ArCH2), 5.06 (q, 4H, CH2OOCCH2), 4.93 (2×t, 2H, CHNHCO(CH2)10), 3.98 (t, 4H, CH2OAr), 3.00 (m, 4H, CH2COOBn), 2.18 (t, 4H, CH2CONH), 1.84-1.24 (m, 32H, NHCOCH2(CH2)8). Synthesis of (S)-2-(11-{4′-[10-((S)-1,2-Dicarboxy-ethylcarbamoyl)-decyloxy]- biphenyl-4-yloxy}-undecanoylamino)succinic Acid (4). Compound 3 (0.40 g, 0.35 mmol) with Pd/C (2%) in the mixture of 30 mL CH2Cl2 and 5 mL methanol was stirred under H2 for 3 h. The mixture was filtered and the filtrate was dried in vacuum to afford the product as a white solid (0.26 g, 96%). 1H NMR (300 Hz, DMSO-d): δ (ppm) 8.13 (d, 2H, N-H), 7.50 (d, 4H, Ar-H meta to O(CH2)10), 6.96 (d, 4H, Ar-H ortho to O(CH2)10), 4.50 (q, 2H, CHNHCO(CH2)10), 3.97

(t, 4H, CH2OAr), 2.61 (m, 4H, CH2COOH), 2.08 (t, 4H, CH2CONH), 1.75-1.14 (m, 32H, NHCOCH2(CH2)8). 13C NMR (600 Hz, DMSO-d): δ (ppm) 25.7, 26.1, 29.1, 29.2, 29.3, 29.4, 29.5, 35.6, 36.7, 49.0, 68.0, 115.3, 127.7, 132.7, 158.3, 172.2, 172.6, 173.1. ESI-MS: [M-H]- (m/z) found, 783.58; calcd, 783.40. General Techniques. 1H NMR spectra were recorded on a JEOL JNM-ECA300 and 13C NMR spectra were recorded on a JEOL JNM-ECA600 apparatus; ESI-MS spectra were recorded on a PE Sciex API 3000 apparatus. JY FluoroMax-3 spectrofluorimeter was employed to record the fluorescence emission spectra. Mica was purchased from PLANO W. Plannet GmbH, Germany, and freshly cleaved every time before use. Silicon wafers were purchased from Wafer NetGmbH, Germany, and treated with piranha (7:3, v:v of sulfuric acid and 30% hydrogen peroxide), then washed with water, and dried with high pure nitrogen flow to get a hydrophilic surface. The samples were prepared through dropping the aqueous solution onto the substrate and drying in air. Atomic force microscopy (AFM) images were taken at room temperature with a commercial instrument Nanoscope IV of Veeco Company. Si cantilevers (Veeco Company, U.S.A.) for tapping mode in air were used (200-300 kHz). Fourier transform infrared (FTIR) transmission spectra were collected at a spectral resolution of 4 cm-1 in vacuum at around 1 to 2 mbar with an IFS-66v/S FTIR spectrometer manufactured by Bruker Company, Germany. The samples were fabricated by casting the solutions onto the CaF2 and dried in vacuum. Differential scanning calorimetry (DSC) thermogram were measured by using a DSC 822e (Mettler) thermoanalyer. Samples (4.2 mg) were directly placed in DSC pans. The

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Figure 1. Plot of pyrene I1/I3 fluorescence ratio versus logarithm concentration of DCBP-10-L-Asp. The cmc is about 1.6 × 10-5 mol/L.

measurements were performed with a temperature range from 30 to 180 °C at a heating rate of 20 °C/min. Critical Micelle Concentration. As the I1/I3 (I1 ) 372 nm, I3 ) 383 nm) intensity ratio of pyrene’s emission spectra can reflect the polarity of its microenvironment,12 we employed pyrene as a probe to detect the critical micelle concentration (cmc) of DCBP-10-L-Asp. The solution was ultrasonicated for 15 min and standing for 10 min every time before measurement. The I1/I3 intensity ratio of the fluorescence spectrum of pyrene as a function of the logarithm concentration of DCBP-10-LAsp was plotted. In the plot (Figure 1), the I1/I3 intensity ratio is rapidly decreased when the concentration of the solution is slightly below the cmc. However, when the concentration is higher than cmc, the I1/I3 intensity ratio is stable, changing slowly, which indicats the formation of micelle. Thus, the surfactant concentration where I1/I3 drops to a low and a nearly constant value marks the cmc. The cmc of DCBP-10-L-Asp obtained by fluorescence probing method is about 1.6 × 10-5 mol/L, as shown in Figure 1. Results and Discussion Stable Fiber-Like Structures Formed by Self-Organization of DCBP-10-L-Asp. DCBP-10-L-Asp consists of a mesogenic biphenyl group, two long alkyl chains and two L-aspartic acid headgroups. The rigid biphenyl group, the appropriate length of flexible alkyl chain spacer and the aspartic acid headgroups may enhance the intermolecular interaction among the bolaamphiphiles, allowing for a stable supramolecular self-organization. The self-organized structures of DCBP-10-L-Asp were adsorbed onto mica sheet and observed by ex situ AFM, the image is shown in Figure 2a. The concentration used here is 1.0 × 10-4 mol/L, about six times the cmc (about 1.6 × 10-5 mol/L). The fiber-like micelle is 50-80 nm in width, 4-5 nm in height, and several micrometers in length. In order to investigate the influence of concentration on the morphology of the selforganized structure, the analog experiments were carried at different concentrations. As shown in Figure 2b, when the concentration is decreased to 5.0 × 10-5 mol/L, about three times its cmc, the fiber-like structures have little change in width and height, while they become shorter than that formed at a higher concentration of 1.0 × 10-4 mol/L. However, when the concentration is decreased to 4.0 × 10-6 mol/L, below the cmc, we could not find any fiber-like structures on the mica sheet

Figure 2. Ex-situ AFM images of aggregate structures of DCBP-10L-Asp, on mica with different concentrations of (a) 1.0 × 10-4 mol/L, (b) 5.0 × 10-5 mol/L, (c) 4.0 × 10-6 mol/L, (the inset is a magnified image, 2 µm × 2 µm), and (d) aggregate of DCBP-10-L-Asp with the concentration of 1.0 × 10-4 mol/L on hydrophilic silicon wafer. The scan area of all images is 10 µm × 10 µm.

(Figure 2c), but some irregular structures. So the aggregation behavior of DCBP-10-L-Asp is concentration dependent. The fiber-like structures appear when the concentration of DCBP10-L-Asp is above the cmc, which confirms the cmc value obtained by the fluorescence measurement. We also wondered whether the formation of this kind of fiberlike aggregate structure depended on the substrate. To answer this question, we used a silicon wafer instead of mica as the substrate to examine the adsorption of DCBP-10-L-Asp. Figure 2d shows that the aggregate structures of DCBP-10-L-Asp adsorbed on the silicon wafer is similar to that adsorbed on the mica sheet (Figure 2a), with almost the same height and width. Therefore, the fiber-like structures are formed by self-organization of DCBP-10-L-Asp in the solution, but not induced by substrates. Effect of Hydrogen Bonding on the Fiber-Like Structures. To understand this point, urea is added into the system to break the hydrogen bond step-by-step, and the self-organized structures are systematically studied by changing the amounts of urea. In doing so, we kept the concentration of DCBP-10-L-Asp at 1.0 × 10-4 mol/L and changed the molar ratio of the urea and DCBP-10-L-Asp in the solution. When adding equimolar urea, although the fiber-like structures can still be observed, there are many branches extending along the fibers, as shown in Figure 3a. When the molar ratio is increased to 2:1, the border of the fibers become fuzzy and some irregular aggregates appear in the AFM image, as shown in Figure 3b. When further increasing the molar ratio to 4:1, no fiber-like structures but irregular aggregates are observed on the substrate (Figure 3c). The results indicate that the addition of urea can partially destroy the multiple hydrogen bonds between the molecules, so as to influence the organization of the molecules and their morphology of the aggregates. FTIR spectra were employed to confirm how urea affects the hydrogen bonding between the molecules. As shown in Figure

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Figure 3. AFM images of the aggregate structures of DCBP-10-L-Asp in the presence of different amounts of urea. The molar ratios of urea and DCBP-10-L-Asp are (a) 1:1, (b) 2:1, and (c) 4:1, respectively. The scan area of all images is 2 µm × 2 µm.

Figure 4. FTIR transmission spectra of DCBP-10-L-Asp with different amount of urea. From up to down, the molar ratios of urea and DCBP10-L-Asp are (a) 0:1, (b) 1:1, (c) 2:1, (d) 4:1, and (e) 1:0, respectively.

4, from up to down, the molar ratio of urea and DCBP-10-LAsp increase as 0:1, 1:1, 2:1, 4:1, and 1:0. The band at around 1724 cm-1 is attributed to the CdO stretching mode of the carboxyl groups, suggesting that the carboxyl groups are involved in the hydrogen bonding with other carboxyl groups.13 The amide I band at 1628 cm-1 and the amide II band at 1543 cm-1 also suggest the formation of the associated structure because of hydrogen bonding. When the amount of urea was further increased, the band at 1724 cm-1 was decreased and finally disappeared. The amide I band at 1628 cm-1 and the amide II band at 1543 cm-1 were shifted a little bit and also disappeared at last. The change of these three bands (1724 cm-1, 1628 cm-1, and 1543 cm-1) with addition of urea indicates that the hydrogen bonding of aspartic acid headgroups become weaker and weaker upon stepwise addition of urea. When these bands finally disappear, it is clear that the hydrogen bonding is destroyed and the fiber-like structures cannot form any more, as indicated by AFM in Figure 3c. These results suggest that the intermolecular hydrogen bonding plays an important role in the formation of the fiber-like structures. To further prove the influence of intermolecular hydrogen bonding among aspartic acids of DCBP-10-L-Asp on the formation of the fiber-like structures, we adjusted the pH value from 6.0 to 8.6 by adding KOH. As shown in Figure 5a,b, the self-organized fiber-like structures at pH ) 6.0 change into spherical structures at pH ) 8.6, and the spheres size was

ranging from 10 to 60 nm. The morphologies change can also be reflected by FTIR spectra, as shown in Figure 6b. While the pH value is adjusted to 8.6, the band at 1724 cm-1 due to the CdO stretching mode of the carboxyl groups disappears, and the new band at 1395 cm-1 appears, suggesting that the carboxyl groups are substantially changed into carboxylate groups and the hydrogen bonding between carboxyl groups is destroyed at that time. In other words, the hydrogen bonding between carboxyl groups is also involved in the fiber-like structures. Interestingly, we also found that the aggregate structures can be adjusted by addition of Cu2+ ion. When DCBP-10-L-Asp with a concentration of 1.0 × 10-4 mol/L is in the presence of equimolar Cu2+, the aggregate structures of the bolaamphiphile become spherical structures with a diameter of about 30 nm (Figure 5c). As shown by FTIR spectrum in Figure 6c, the band at 1724 cm-1 is decreased significantly and the amide II band has a shift from 1543 cm-1 to 1568 cm-1. This change suggests that there exists coordination interactions between Cu2+ ion and aspartic acid headgroups. However, the band at 1724 cm-1 does not disappear but obviously decreases, which indicates that not all carboxyl groups of aspartic acid headgroups are involved in the coordination. This is because that aspartic acid and copper ion can coordinate into a five-membered chelate ring, with the copper ion bonded to the oxygen atom of the carboxyl group and nitrogen atom of the amino group.14 Therefore, the hydrogen bonding between the carboxyl groups without being coordinated with Cu2+ ion is not strong enough to form fiber-like structure, but is strong enough to form uniform-sized spherical structures. Thermal Stability Investigation of the Fiber-Like Structures Formed by DCBP-10-L-Asp. DSC was used to detect the thermal stability of the fiber-like structures. As shown in Figure 7, there is a narrow endothermic peak around 110 °C with an enthalpy of 14.86 kJ/mol, which should correspond to the breakage of hydrogen bonds.15 The DSC result suggests that the aggregates formed by DCBP-10-L-Asp may withstand a relatively high temperature. In order to provide further visualized evidence, the hot-stage AFM was employed to observe the morphology change with temperature. The results are shown in Figure 8, and some places are marked with circles to make sure that AFM images are scanned in the same area. No obvious changes of the fiber-like structures are observed until the substrate is heated from 25 °C (Figure 8a) to 125 °C (Figure 8b). But when the temperature is elevated to 130 °C, parts of the fiber-like structures become discrete (Figure 8c). When further increasing the temperature to 135 °C, the change of the morphology becomes much clearer where most of the structures become discrete (Figure 8d). It seems that the two results indicated by DSC and the hot-stage AFM are not consistent,

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Figure 5. AFM observation of aggregate structures self-organized by DCBP-10-L-Asp with a concentration of 1 × 10-4 mol/L. (a) Pure DCBP10-L-Asp aqueous solution, pH value is 6.0; (b) adding OH- to adjust the pH to 8.6; (c) adding equimolar Cu2+. The scan area of all images is 2 µm × 2 µm.

Figure 6. FTIR transmission spectra of (a) DCBP-10-L-Asp solution, pH 6.0; (b) adding OH- to adjust DCBP-10-L-Asp solution to pH 8.6; (c) molar ratio 1:1 mixture of DCBP-10-L-Asp/Cu2+.

Figure 8. Hot-stage AFM observation of the thermal stability of the fiber-like structures formed by self-organization of DCBP-10-L-Asp. The scan area is 2 µm × 2 µm. The arrows indicate the change of the fiber-like structure when heating. The circles show some places, as marks, to verify the in-situ AFM.

fiber-like structures can at least bear a temperature as high as 110 °C, showing a good thermal stability. Conclusion

Figure 7. DSC measurement of DCBP-10-L-Asp (second heating). The sample was 4.2 mg and the heating rate was 20 °C/min.

and AFM gives a relatively high temperature, but we believe it is reasonable. For one thing, the mica substrate is not a good thermal conductor, causing the practical temperature of the sample to be lower than that indicated by the instrument; for the other thing, although the hydrogen bond is broken at about 110 °C, the molecules in the aggregation state still need some extra energy to move around. Therefore, it is believed that these

A new bolaamphiphile (DCBP-10-L-Asp) bearing L-aspartic acid headgroups has been designed and synthesized. The biocompatible bolaamphiphile can self-organize into fiber-like structures which show a good thermal stability. Our research has demonstrated that the self-organized structures are related with the multiple intermolecular hydrogen bonds. The hydrogen bonding between the molecules, as well as the morphology of self-organized structures, can be adjusted by changing the solution condition, such as urea, pH, and Cu2+ ion. The supramolecular fiber-like nanostructures should be promising for use in template synthesis and assembly of other functional supramolecular structures and biomaterials. Acknowledgment. We thank the National Natural Science Foundation of China (20574040, 50573042, 50703022) and the National Basic Research Program (2007CB808000) for financial support.

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