Langmuir 1998, 14, 7537-7538
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Notes Spontaneous Formation of Vesicles by a Cryptand-Based Bola-Amphiphile Prasun Bandyopadhyay and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Received June 30, 1998. In Final Form: September 29, 1998
Synthesis of amphiphilic molecules that can selforganize into vesicular structures are pursued in many laboratories because of their importance in various fields of chemistry and biochemistry. In recent years, synthetic amphiphiles have been reported1-6 with different chemical structures and composition of headgroups and hydrophobic tail(s). Fuhrhop et al. introduced7 the term, “bolaamphiphile” to describe the architecture of a new series of surfactants where two headgroups are joined by hydrophobic spacers. Kunitake et al. provided the first report8 on the formation of aqueous monomolecular membranes from such two-headed amphiphiles. Since then, a number of laboratories have reported the synthesis and self-assembly of bola-amphiphiles with different headgroups that include acyclic7-11 as well as macrocyclic12 structures. In the present paper, we describe the synthesis of a new bola-amphiphile (1) (Figure 1) where two macrobicyclic cryptands are joined by three hydrophobic chains. The macrobicyclic cavity is designed to entrap a transition-metal ion like Cu(II) to have yet another new amphiphile. Compound 1 was synthesized by adding dropwise a solution of sebacoyl chloride (0.72 g, 3 mmol) in tetrahydrofuran (250 mL) to a cold solution of the cryptand (1.12 g, 2 mmol) in tetrahydrofuran (200 mL) also containing triethylamine (0.60 g, 6 mmol) to neutralize the acid formed. Addition time was 18 h and the reaction temperature was maintained at 5 °C. After the addition was complete, the reaction mixture was stirred for 5 h at room temperature and then refluxed for 1 h. Upon complete removal of the solvent, the light yellow oily liquid left behind was shaken with 250 mL of water and the desired product extracted with chloroform (5 × 50 mL). The organic layer after drying over anhydrous sodium sulfate was completely evaporated to obtain the bola-amphiphile (1) Menger, F. M.; Yamasaki, Y. J. Am. Chem. Soc. 1993, 115, 3840. (2) Schenning, A. P. H.; Bruin de, B.; Feiters, M. C.; Nolte, R. J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1662. (3) Schenning, A. P. H.; Feiters, M. C.; Nolte, R. J. M. Tetrahedron Lett. 1993, 7077. (4) Bhattacharya, S.; De, S. Chem. Commun. 1997, 2287. (5) Kimizuka, N.; Wakiyama, T.; Miyauchi, H.; Yoshimi, T.; Tokuhiro, M.; Kunitake, T. J. Am. Chem. Soc. 1996, 118, 5808. (6) Ghosh, P.; Khan, T. K.; Bharadwaj, P. K. Chem. Commun. 1996, 189. (7) Fuhrhop, J.-H.; Mathieu, J. J. Chem. Soc., Chem. Commun. 1983, 144. (8) Okahata, Y.; Kunitake, T. J. Am. Chem. Soc. 1979, 101, 5231. (9) Shimizu, T.; Masuda, M.; Shibakami, M. Chem. Lett. 1997, 267. (10) Wang, K.; Munoz, S.; Zhang, L.; Castro, R.; Kaifer, A. E.; Gokel, G. W. J. Am. Chem. Soc. 1996, 118, 6707. (11) Fahrnow, A. M.; Saenger, W.; Fritsch, D.; Schnider, P.; Fuhrhop, J.-H. Carbohydr. Res. 1993, 242, 11. (12) Munoz, S.; Mallen, J.; Nakano, A.; Chen, Z.; Gay, A.; Echegoyen, L.; Gokel, G. W. J. Am. Chem. Soc. 1993, 115, 1705.
Figure 1. Structural formula for the bola-amphiphile, 1.
Figure 2. Negative-stain transmission electron micrographs of vesicles of 1 (a) (magnification 20000×) and 2 (b) (magnification 15000×).
as a white solid. Further purification could be achieved by recrystallization from dichloromethane: petroleum ether (1:1 v/v). Yield ∼75%; mp 110° C (uncorrected). The same product could be isolated irrespective of the molar ratio of the reactants taken, although the yields vary depending upon the ratio of the reactants used. Anal. Calcd. for C96H132N10O12: C, 71.25; H, 8.22; N, 8.65. Found: C, 71.36; H, 8.15; N, 8.78. 1H NMR (300 MHz,
10.1021/la980792z CCC: $15.00 © 1998 American Chemical Society Published on Web 11/25/1998
7538 Langmuir, Vol. 14, No. 26, 1998
CDCl3, 25 °C, SiMe4 ): δ 1.2 (m, 36 H), 2.0 (t, 12 H), 2.6 (br s, 12 H), 2.9 (br s, 12 H), 3.5 (br s, 12 H), 4.2 (s, 12 H), 4.9 (br s, 12H), 7.1 (m, 24 H). FAB-MS: m/z (%) 1618 (40) 1+. IR (KBr pellet): 1640 cm-1 (s br, amide stretching). The dicopper(II) salt (2) was isolated as a green solid when copper(II) perchlorate hexahydrate was stirred at 298 K with 1 in methanol. Yield ∼90%. Anal. Calcd. for C96H132N10 O12Cu2Cl4O16: C, 53.80; H, 6.20; N, 6.53. Found: C, 53.97; H, 6.35; N, 6.42. Both EPR and electronic spectral characteristics of 2 in acetonitrile solution (not dispersion) match closely with those obtained on the wellcharacterized cupric cryptate of the parent compound.13 To form vesicles, 1 mL of ethanolic solution of either amphiphile (5 mM) was slowly added at 298 K to 24 mL of deionized water. The dispersion was not subjected to any type of mechanical agitation other than gentle mixing for ∼5 min at 298 K to obtain a turbid white suspension. The bola-amphisomes (spontaneous and ultrasonication) were characterized by their electron micrographs using a JEOL TEM-2000F instrument operating at 100 kV. The micrographs (Figure 2) confirmed the closed structures of the bola-amphisomes with the average diameter lying in the range 550-650 nm. Dynamic light scattering of (13) Chand, D. K.; Bharadwaj, P. K. Inorg. Chem. 1996, 35, 3380.
Notes
similarly prepared suspensions of 1 and 2 indicated the presence of organizates having hydrodynamic diameters of 130 and 105 nm respectively at 300 K, with polydispersity ranging within 0.91-0.94. Vesicular dispersions of either sample did not show any significant variation in static turbidity at 298 K for more than 5 days which indicates that the individual vesicles do not fuse to form clusters within the period. Bath sonication of the dispersions also gave similar results. In summary, we have shown that cryptands can be derivatized to form bola-amphiphiles easily which can spontaneously form stable vesicular aggregates. Synthesis of a number of cryptand-based bola-amphiphiles are presently in progress in our laboratory. Acknowledgment. This work was supported by the Department of Science and Technology, New Delhi, India (Grant no. SP/S1/F-08/96 to P.K.B.). P.B. wishes to thank UGC for a JRF fellowship. Supporting Information Available: 1H NMR and FAB-mass spectra of 1 (2 pages). Ordering information is available on any current masthead page. LA980792Z
