Langmuir 1998, 14, 5389-5395
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Synthesis of New Long-Chain Fluoroalkyl Glycolipids: Relation of Amphiphilic Properties to Morphology of Supramolecular Assemblies V. Emmanouil, M. El Ghoul, C. Andre´-Barre`s, B. Guidetti, I. Rico-Lattes,* and A. Lattes Laboratoire des IMRCP, UMR au CNRS No. 5623, Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Ce´ dex, France Received March 10, 1998. In Final Form: June 12, 1998 We describe a modular synthesis for single-chain and “semi-gemini” glycolipids. We found that slight alterations in the structures of these amphiphiles led to marked differences in the morphology of their aggregates in solution. We also demonstrate, for the first time, the formation of stable gels in the structured nonaqueous solvent formamide.
Introduction After the initial description of Kunitake,1 several groups have reported the formation of tubular or helical molecular associations from synthetic amphiphiles. These compounds may be either chiral or give rise to strong attractions between monomers (e.g., hydrogen bonds between amide groups).2,3 These highly organized associations4 range in size from 100 nm to several micrometers and have stimulated considerable interest as they have led to the development of new materials.5 Linear aggregates can be viewed as the structural counterparts of spherical vesicles, which are formed from synthetic amphiphiles, with structures resembling natural phospholipids (double-chain amphiphiles). However, vesicles have also been produced from bolaform derivatives, namely, those bearing two polar heads joined by one or two hydrophobic chains. These systems have been widely studied as they can be employed as models of biological membranes or drug transport systems.6,7 However, such vectors tend to be unstable, which has limited their application. The formation and properties of vesicles, tubules, or helixes from the self-assembly of fluorinated amphiphiles have also been investigated by numerous authors. Molecules containing fluorinated chains have unusual behavior due to the geometrical characteristics and the hydrophobic and lipophobic nature of the fluorinated linkages. For example, under conditions in which a single short-chain hydrogenated compound only forms micelles, the fluorinated homologue forms vesicles.8 Other single or double-chain fluorinated derivatives have been shown to form tubular or helicoidal structures without recourse (1) Kunitake, T. J. Am. Chem. Soc. 1981, 103, 5401. (2) Fuhrhop, J. H.; Helfrich, W. Chem. Rev. 1993, 93, 1565 and references therein. (3) Schnur, J. M. Science 1993, 262, 1669 and references therein. (4) (a) Kimizuka, N.; Kawasaki, T.; Hirata, T.; Kunitake´, T. J. Am. Chem. Soc. 1995, 117, 6360. (b) Kimizuka, N.; Fujikawa, S.; Kuwahara, H.; Kunitake´, T.; Marsh, A.; Lehn, J. M. J. Chem. Soc., Chem. Commun. 1995, 2103. (5) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (6) Bangham, A. D.; Hill, M. W.; Miller, N. G. A. In Methods in membrane biology; Korn, E. D., Ed.; Plenum Press: New York, 1974; p 1. (7) Lasic, D. D. In Liposomes: from physics to applications; Elsevier: Amsterdam, 1993. (8) Kunitake´, T.; Higashi, N. J. Am. Chem. Soc. 1985, 107, 692.
to chiral effects or hydrogen bonding.9 Among the fluorinated amphiphiles, nonionic derivatives, especially those bearing sugar-based polar heads, are of interest as they have potential biochemical and pharmaceutical applications.10 Such applications require compounds of high purity, which involves long and costly synthesis due to the demand for protection and deprotection of the sugar hydroxyl residues. Furthermore, perfluoroalkyl segments are not readily introduced, as conventional methods of alkylation cannot always be employed due to the strong electronegativity of the perfluoroalkyl groups. In recent publications we have described a variety of new and simple methods for the preparation of surfactants derived from sugars including fluoroalkyl derivatives,11 without protection of the sugar residues. We describe here an adaptation of these methods for preparing three families of fluoroalkyl glycolipids of general structure
They included (i) single-chain compounds (2) bearing a single polar head derived from lactobionic acid, (ii) singlechain compounds (1) derived from gluconolactone, and (iii) bolaform, or “semi-gemini” compounds (3) with one of the polar heads based on lactose and the other bearing a carboxyl group. We also examine the mode of aggregation of these compounds in solution. We present a method of modular synthesis to prepare a series of compounds designed to (9) Giulieri, F.; Krafft, M. P.; Riess, J. G. Angew. Chem., Int. Ed. Engl. 1994, 33, 1514. (10) Riess, J. G. Colloids Surf. 1994, 84, 33. (11) (a) Rico-lattes, I.; Lattes, A. International Festschrift for Pr S. E. Friberg Colloids Surf. 1997, 123-124, 37 and references therein. (b) El Ghoul, M.; Escoula, B.; Rico, I.; Lattes, A. J. Fluorine Chem. 1992, 59, 107.
S0743-7463(98)00286-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/21/1998
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study the relationships between amphiphile structure and the shape of the molecular aggregates formed in solution. The objective was to prepare “designer assemblies”. Experimental Section General Methods. NMR spectra were recorded on a Brucker AC 200 or AC 400 spectrometer. Chemical shifts are reported in parts per million relative to tetramethylsilane for 1H and CF3COOH for 19F. IR spectra were recorded on a Perkin-Elmer 683 spectrometer and the frequencies are expressed in cm-1. Mass spectra were recorded on ZAB-HS instrument (WGanalytical, Manchester) using FAB mode (fast atom bombardment) with a glycerol matrix and a Nermag R10-10 instrument for DCI/NH3 mode. Elemental analyses were carried out at the ENSCT (Toulouse France) or at the CNRS central facilities in Vernaison (France). They are expressed in g/100 g of compound. Silica gel plates (Merck 60 F254) were used to follow the progress of the reactions. Silica gel Geduran SI60 (Merck) was used to perform chromatographic purifications. Surface tensions were measured at 25 or 37 °C using the stirrup detachment method with a Prolabo Tensimat no. 3 instument. The formation of supramolecular aggregates was observed through transmission electron microscopy using Philips EM 301 (80 kV) and CM 20 (200 kV) microscopes. The samples with compounds 3 were examined after sonication using a Sonics 600w apparatus (2.5 power) at 50% active cycles, 5 min, 0 °C. (1) Synthesis of N-[2-(Perfluoroalkyl)ethyl]gluconamides (1) and N-[2-(Perfluoroalkyl) ethyl]lactobionamides (2). A solution of 2-(F-alkyl) ethyl azide (3.45 mmol) in 5 mL of anhydrous THF, placed in a 100 mL two-necked flask equipped with a stirrer, was cooled at 0 °C. A solution of triphenylphosphine (3.16 mmol) in 5 mL of anhydrous THF was added dropwise under argon. The reaction mixture was stirred for 1 h at 0 °C and then for 3 h at room temperature. THF was evaporated and the residue taken up in 10 mL of methanol. Gluconolactone or lactobionic acid (2.65 mmol) and 100 µL of water were added, and the mixture was heated at 75 °C for 4 h. The solvent was evaporated and the residue washed in copious hot toluene. The precipitate was filtered and dried under vacuum. N-[2-(Perfluorohexyl)ethyl]gluconamide (1a). IR (KBr) νCONH ) 1650. NMR 1H (200 MHz), DMSO-d6: 7.90 (m, 1H, NH); 3.02-5.50 (m,15H, CHOH, CH2OH, C2H4-RF). NMR 13C (100 MHz), DMSO-d6: 172.87 (CO); 73.35 (CHOH in β position of CO); 72.17 (CHOH in δ position of CO); 71.41 (CHOH in R position of CO); 69.96 (CHOH in γ position of CO); 63.26 (CH2OH); 30.50 (CH2N); 29.74 (CH2-RF). NMR 19F (376 MHz), DMSO-d6: -5.78 (s, 3F, CF3); -38.89 (s, 2F, CF2 in R position of CH2); -47.15 (s, 2F, CF2 in β position of CH2); -48.10 (s, 2F, CF2 in γ position of CH2); -48.71 (s, 2F, CF2 in β position of CF3); -51.25 (s, 2F, CF2 in R position of CF3). DCI/NH3: MH+ ) 542; RF - C2H4 - NH3+ ) 364. Anal. Calcd for C14F13H16NO6: C, 31.05; H, 2.95; N, 2.58. Found: C, 31.15; H, 2.92; N, 2.59. N-[2-(Perfluorooctyl)ethyl]gluconamide (1b). IR (KBr) νCONH ) 1660. NMR 1H (200MHz), DMSO-d6: 7.89 (m, 1H, NH); 3.20-5.50 (m, 15H, CHOH, CH2OH, C2H4-RF). NMR13C (100 MHz), DMSO-d6: 172.81 (CO); 72.35 (CHOH in β position of CO); 72.17 (CHOH in δ position of CO); 71.60 (CHOH in R position of CO); 70.19 (CHOH in γ position of CO); 63.36 (CH2OH); 30.60 (CH2N); 29.98 (CH2-RF). NMR 19F (376 MHz), DMSO-d6: -5.69 (s, 3F, CF3); -38.84 (s, 2F, CF2 in R position of CH2); -47.09 (s, 6F, CF2 in β, γ, δ position of CH2); -47.88 (s, 2F, CF2 in γ position of CF3); -48.61 (s, 2F, CF2 in β position of CF3); -51.18 (s, 2F, CF2 in R position of CF3). DCI/NH3: MH+ ) 642; RF - C2H4 - NH3+ ) 464. Anal. Calcd for C16F17H16NO6: C, 29.95; H, 2.50; N, 2.18. Found: C, 30.23; H, 2.50; N, 2.09. N-[2-Perfluorodecyl)ethyl]gluconamide (1c). IR (KBr) νCONH ) 1650. NMR1H (200MHz), DMSO-d6: 7.55 (m, 1H, NH); 3.28-5.50 (m, 15H, CHOH, CH2OH, C2H4-RF). NMR 13C (100 MHz), DMSO-d6: 172.80 (CO); 73.35 (CHOH in β position of CO); 72.29 (CHOH in δ position of CO); 71.59 (CHOH in R position
Emmanouil et al. of CO); 70.19 (CHOH in γ position of CO); 63.36 (CH2OH); 30.50 (CH2N); 29.99 (CH2-RF). NMR 19F (376 MHz), DMSO-d6: -5.43 (s, 3F, CF3); -38.70 (s, 2F, CF2 in R position of CH2); -46.79 (s, 10F, CF2 in β, γ, δ position of CH2 and CF2 in δ, position of CF3); -47.72 (s, 2F, CF2 in γ position of CF3); -48.52 (s, 2F, CF2 in β position of CF3); -50.99 (s, 2F, CF2 in R position of CF3). DCI/NH3: MH+ ) 742; RF - C2H4 - NH3+ ) 564. Anal. Calcd for C18F21H16NO6: C, 29.15; H, 2.16; N, 1.89. Found: C, 29.46; H, 2.13; N, 1.80. N-[2-(Perfluoroalkyl)ethyl]lactobionamides (2a-c). The synthesis of these compounds was previously described.11b (2) Syntheses of N-(ω-(Sodium oxycarbonyl)alkyl)-N(perfluoroalkyl ethylcarbonyl)-1-amino-1-deoxy lactitol (3) and N-(ω-(Sodium oxycarbonyl)alkyl)-1-amino-1-deoxy lactitol (5). To a solution of sodium ω-amino alkanoate (22 mmol) in 60 mL of methanol is added 13.7 mmol of lactose monohydrate dissolved in 30 mL of water. The mixture is stirred 48 h at room temperature. The crude product is immediately reduced by sodium borohydride (15 mmol) added at room temperature in little portions. The mixture is stirred during 12 h at room temperature. The residual solution is evaporated in a vacuum. The crude product is purified by chromatography on silica gel. N-(ω-(Sodium oxycarbonyl)dodecanyl)-1-amino-1-deoxylactitol (5a). Eluent: CHCl3/CH3OH/ammonia solution 30% ) 6/3/1, Rf ) 0.25. NMR 1H (400 MHz), DMSO-d6-D2O (1/1): 1.2 (16H, 8 CH2 in β-ι position of COO-); 1.5 (t, 3JHCCH ) 7.5 Hz, 2H, CH2 in κ position of COO-); 2.2 (t, 3JHCCH ) 7.5 Hz, 2H, CH2 in R position of COO-); 3.4-3.9 (m, 14H sugar moiety and 2H, CH2 in R position of NH); 4.5 (d, 3JHCCH ) 7.5 Hz, 1H, 1 position on galactose). NMR 13C (100 MHz), DMSO-d6-D2O (1/1): 28.131.5 (8s, CH2 in β-κ position of COO-); 50.2 (s, CH2 in R position of COO-); 52.4 and 53.5 (2s, CH2 in 1 position on glucitol and CH2 in R position of NH); 63.4 and 64.4 (2sd, 2 CH2 in 6 position on galactose and glucitol); 70.4, 71.4, 73.1, 73.6, 75.3, 77.4, and 78.0 (7s, 7 CH sugar moiety); 80.9 (s, CH, 4 position on glucitol); 105.7 (s, CH in 1 position on galactose); 186.1 (s, COO-). FAB < 0 (GLY): 540 ) M - Na; 378 ) M - Na - Gal. N-(ω-(Sodium oxycarbonyl)undecanyl)-1-amino-1-deoxylactitol (5b). Eluent: CHCl3/CH3OH/ammonia solution 30% ) 6/3/1, Rf ) 0.23. NMR 1H (400 MHz), DMSO-d6-D2O (1/1): 1.2 (12H, 6 CH2 in β-η position of COO-); 1.5 (t, 3JHCCH ) 7.5 Hz, 2H, CH2 in θ position of COO-); 1.6 (t, 3JHCCH ) 7.5 Hz, 2H, CH2 in ι position of COO-); 2.1 (t, 3JHCCH ) 7.5 Hz, 2H, CH2 in R position of COO-); 3.5-3.8 (m, 14H, sugar moiety and 2H of CH2 in R position of NH); 4.4 (d, 3JHCCH ) 7.5 Hz, 1H in 1 position on galactose). NMR 13C (100 MHz), DMSO-d6-D2O (1/1): 28.031.4 (8s, CH2 in β-ι position of COO-); 50.5 (s, CH2 in R position of COO-); 52.1 and 53.7 (2s, CH2, 1 position on glucitol and CH2 in R position of NH); 63.9 and 64.6 (2sd, 2 CH2, in 6 position on galactose and glucitol); 70.4, 71.4, 73.2, 73.7, 75.1, 77.7, and 78.0 (7s, 7 CH of sugar moiety); 80.9 (s, CH in 4 position on glucitol); 105.5 (s, CH in 1 position on galactose); 186.3 (s, COO-). FAB < 0 (GLY): 526 ) M - Na; 364 ) M - Na - Gal. 3-Perfluorooctylpropanoyl Chloride (7). Twenty-two millimoles of 3-perfluorooctyl propanoı¨c acid is dissolved in 60 mL of chloroform at 40 °C. The solution is cooled to room temperature and 55 mL of oxalyl chloride (2 M solution in CH2Cl2) is then added carefully under argon. The mixture is refluxed for 4 h, and the solvents are evaporated under reduced pressure. The obtained solid residue is washed with chloroform and dried under reduced pressure (yield ) 34%). 3-Perfluorooctylpropanoylthiazolidine-2-thione (6). Triethylamine (20 mmol) is added to a solution of mercaptothiazolidine-2-thione (18 mmol) in 40 mL of dichloromethane. The mixture is stirred at 35 °C to solubilize the mercaptothiazolidine2-thione. Acid chloride (7) (20 mmol) is then added, and the mixture is heated at 65 °C for 4 h. The reaction mixture is then washed with water and the organic layer is dried and evaporated. The obtained yellow powder is dried under vacuum (yield ) 90%). NMR 1H (400 MHz), CDCl3: 2.4 (m, 2H, CH2 in β position of CdO); 3.0 (t, 3JHCCH ) 7.5 Hz, 2H, CH2 in R position of CdO); 3.4 (t, 3JHCCH ) 7.5 Hz, 2H, CH2-S); 3.9 (t, 3JHCCH ) 7.5 Hz, 2H, CH2-N). NMR 13C (100 MHz), CDCl3: 26.7 (s, CH2-S); 27.2 (t, 2J CF ) 22 Hz, CH2 in β position of CdO); 33.7 (s, CH2 in R position
Synthesis of Fluoroalkyl Glycolipids
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Table 1. Long Fluoroalkyl Chain Glycolipids Prepared X
Y
Z
R
m
compound
yield (%)
CO-N CO-N CO-N CO-N CO-N CO-N N-CO N-CO
-C2H4-C6F13 -C2H4-C6F13 -C2H4-C8F17 -C2H4-C8F17 -C2H4-C10F21 -C2H4-C10F21 -C2H4-C8F17 -C2H4-C8F17
H H H H H H C10H21-COO-Na+ C11H22-COO-Na+
H β-D-Gal H β-D-Gal H β-D-Gal β-D-Gal β-D-Gal
0 0 0 0 0 0 1 1
1a 2a 1b 2b 1c 2c 3a 3b
90 95 92 95 97 95 16 20
Scheme 1. Synthesis of N-(2-(Perfluoroalkyl)ethyl)gluconamides (1) and N-(2-(Perfluoroalkyl)ethyl)lactobionamides (2)
of CdO); 51.5 (s, CH2-N); 108.1-120.8 (m, CF3(CF2)7); 175.8 (s, CdO); 201.8 (s, CdS). NMR 19F (376 MHz), CDCl3: -4.1 (t, 3J FCCF ) 9.5 Hz, 3F, CF3); -37.9 (m, 2F, CF2 in R position of CH2); -44.9 (m, 2F, CF2 in β position of CH2); -45.1 (m, 4F, CF2 in γ, δ position of CH2); -46.0 (m, 2F, CF2 in position of CH2); -46.7 (m, 2F, CF2 in ζ position of CH2); -49.4 (m, 2F, CF2 in η position of CH2). Anal. Calcd for C14H8F17S2ON: C, 28.33; H, 1.35; N, 2.36; S, 10.79. Found: C, 28.54; H, 1.57; N, 2.42; S, 10.57. N-(ω-(Sodium oxycarbonyl)alkyl)-N-(perfluoroalkylethylcarbonyl)-1-amino-1-deoxylactitol (3a,b). To a solution of compounds 5a,b (1.77 mmol) in 100 mL of dimethylformamide, triethylamine (2.65 mmol) and acylating agent 6 (2.65 mmol) are added. The mixture is stirred for 3 days at 60-65 °C. After evaporation to dryness, the residue is purified by chomatography on silica gel. Both the 1H and 13C NMR spectra show the doubling of some
signals, which indicates the presence of two conformational isomers in solution due to the amide bound, as its has been already described.12 N-(ω-(Sodium oxycarbonyl)undecanyl)-N-(perfluorooctylethylcarbonyl)-1-amino-1-deoxylactitol (3a). EluentEluent: CHCl3/CH3OH/ammonia solution 30% ) 6/3/1, Rf ) 0.15. NMR 1H (400 MHz), DMSO-d6: 1.2 (m, 12H, 6 CH2 in δ-ι position of COO-); 1.4 (m, 4H, 2 CH2 in β and γ position of COO-); 2.1 (m, 2H, CH2 in R position of COO-); 2.6 (m, 2H, CH2 in R position of RF); 3.1-3.7 (m, 14H, sugar moiety 2H, CH2 in κ position of COO-, and 2H, CH2 in R position of RF); 4.2 (dd, 3JHCCH ) 7.5 Hz, 1H in 1 position on galactose). NMR 13C (100 MHz), DMSO-d6: 23.3-28.7 (8s, 8 CH2 in β-ι position of COO-); 33.7 (s, CH2 in β position of RF); 45.3 (s, CH2 in R position of RF); 48.1 (s, CH2 in R position of COO-); 49.0 (s, (12) Locknoff, O. Angew. Chem., Int. Ed. Engl. 1991, 30, 1611.
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Scheme 2. Synthesis of N-(ω-(Sodium oxycarbonyl)alkyl)N-(perfluoroalkylethylcarbonyl)-1-amino-1-deoxylactitol (3a,b)
CH2 in κ position of COO-); 60.0 and 62.0 (2s, 2 CH2 in 6 position on galactose and glucitol); 67.8, 68.6, 70.1, 70.3, 71.1, 73.2, and 75.1 (7s, 7 CH, sugar moiety); 83.9 (s, CH in 4 position on glucitol); 104.4 (sd, CH in 1 position on galactose); 105.0-118.6 (m, (CF3(CF2)7); 169.0 (s, COO-); 174. 5 (sd, CdO). NMR 19F (376 MHz), DMSO-d6: -3.2 (t, 3JFCCF ) 9.5 Hz, 3F, CF3); -36.2 (m, 2F, CF2 in R position of CH2); -44.4 (m, 2F, CF2 in β position of CH2);
-44.0 (m, 2F, CF2 in γ position of CH2); -45.4 (m, 2F, CF2 in δ position of CH2); -45.9 (m, 2F, CF2 in position of CH2); -46.1 (m, 2F, CF2 in ζ position of CH2); -48.7 (m, 2F, CF2 in η position of CH2). FAB < O (GLY): 1000 ) M - Na; 838 ) M - Na - Gal; 674 ) RF - C2H4 - CONH - C10H20 - COO. Anal. Calcd for C34H47O13F17NNa: C, 39.88; H, 4.59; N, 1.37; F, 31.57. Found: C, 40.05; H, 4.97; N, 1.70; F, 31.15.
Synthesis of Fluoroalkyl Glycolipids
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Table 2. Gel Formation with N-Fluoroalkyl Gluconamides 1 and Lactobionamides 2 in Aqueous and Formamide Solution compound
RF
concentration (% g/ml)
solvent
2a 2b 2c 1a 1b 1c
C6F13 C8F17 C10F21 C6F13 C8F17 C10F21
30 20 10 10 5 2
H2O H2O H2O HCONH2 HCONH2 HCONH2
Figure 2. Plot of surface tension against the logarithm of the concentration for compound 3b in water at 37 °C. position on galactose and glucitol); 67.7, 68.2, 68.5, 70.3, 71.1, 73.1, and 75.3 (7s, 7 CH sugar moiety); 84.1 (s, CH in 4 position on glucitol); 105.2-119.4 (m, CF3(CF2)7); 104.4 (sd, CH in 1 position on galactose); 169.0 (s, COO-); 174.6 (sd, CdO). NMR 19F (376 MHz), DMSO-d : -3.2 (t, 3J 6 FCCF ) 9.5 Hz, 3F, CF3); -36.2 (m, 2F, CF2 in R position of CH2); -44.4 (m, 2F, CF2 in β position of CH2); -44.6 (m, 2F, CF2 in γ position of CH2); -45.4 (m, 2F, CF2 in δ position of CH2); -45.9 (m, 2F, CF2 in position of CH2); -46.1 (m, 2F, CF2, in ζ position of CH2); -48.7 (m, 2F, CF2,in η position of CH2). FAB < O (GLY): 1014 ) M - Na; 852 ) M - Na - Gal; 688 ) RF - C2H4 - CONH - C11H22 - COO; 540 ) M - Na - (RFC2H4CO) + H. Anal. Calcd for C35H49O13F17NNa: C, 40.50; H, 4.72; N, 1.35; F, 31.14. Found: C, 40.67; H, 5.11; N, 1.71; F, 30.72.
Results and Discussion
Figure 1. Electron micrograph from negative staining method of 10% in water N-perfluorodecyl lactobionamide (2c) gel (A) and 2% in formamide N-perfluorodecyl gluconamide 1c gel (B). The bars represent 500 nm. N-(ω-(Sodium oxycarbonyl)dodecanyl)-N-(perfluorooctylethylcarbonyl)-1-amino-1-deoxylactitol (3b). Eluent: CHCl3/CH3OH/ammonia solution 30% ) 6/3/1, Rf ) 0.18. NMR 1H (400 MHz), DMSO-d : 1.2 (m, 14H, 7 CH in δ-κ position of 6 2 COO-); 1.4 (m, 4H, 2 CH2 in β and γ position of COO-); 2.1 (m, 2H, CH2 in R position of COO-); 2.7 (m, 2H, CH2 in β position of RF); 3.1-3.9 (m, 14H, sugar moiety, 2H, CH2 in λ position of COO-, and 2H, CH2 in R position of RF); 4.2 (dd, 3JHCCH ) 7.5 Hz, 1H in 1 position on galactose). NMR 13C (100 MHz), DMSOd6: 24.5-28.9 (9s, 9 CH2 in β-κ position of COO-); 33.9 (s, CH2 in β position of RF); 45.2 (s, CH2 in R position of RF); 48.0 (s, CH2 in R position of COO-); 48.9 (s, CH2 in λ position of COO-); 49.7 (s, CH2 in 1 position on glucitol); 60.0 and 61.9 (2s, 2 CH2 in 6
(1) Synthesis. The method used in our laboratory for synthesis of hydrocarbon derivatives11 was adapted for a modular synthesis of three families of fluoroalkyl glycolipids. The details of the compounds prepared are listed in Table 1. They included (i) single-chain compounds (2) bearing a single polar head derived from lactobionic acid, (ii) single-chain compounds (1) derived from gluconolactone, and (iii) bolaform, or “semi-gemini” compounds13-15 (3) with one of the polar heads based on lactose and the other bearing a carboxyl group. The methods for the synthesis of these three families of fluoroalkyl glycolipids are outlined in Schemes 1 and 2. The single-chain amphiphiles 1a-c and 2a-c were readily prepared in good yields (Scheme 1, Table 1) by an aza-Wittig reaction of the 2-(fluoroalkyl)ethyl azides, obtained from the corresponding commercial iodides, with lactobionic acid or gluconolactone via an iminophosphorane intermediate.11 The amphiphiles 3a-b were prepared in three stages (Scheme 2) from unprotected lactose: the sodium salts of the R,ω-amino acids were reacted with lactose to form the lactosylamines 4a-b, which were reduced directly by sodium borohydride into the corresponding lactitols 5a-b. Compounds 5a-b were isolated in yields ranging from 30% to 43% after purification by chromatography on a silica gel column. The perfluoroalkyl chain was introduced by selective acylation of the nitrogen atom in the presence of reagent 6 in DMF. This acylating reagent derived from mercaptothiazoline, developed by Brown,16 reacts preferentially with amines. (13) Song, Li D.; Rosen, M. J. Langmuir 1996, 12, 1149. (14) (a) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Karthau¨ser, J.; Van Os, N. M.; Zana, R. Science 1994, 266, 254. (b) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (15) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083.
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Figure 3. Transmission electron micrograph of aggregates formed from semi-gemini compounds 3: (A) spherical closed vesicles of 3b; (B) fibers of 3a. The bars represent 200 nm.
This selectivity has been exploited for acylation of amino sugars and aminophenols under mild conditions.11,17 We adapted this method for the fluorinated derivatives: commercial perfluorooctyl propanoic acid was transformed into its acid chloride 7 in the presence of 5 equiv of oxalyl
chloride in a mixture of chloroform and dichloromethane (1/1, v/v). Compound 7 was then reacted with mercaptothiazoline to give 3-perfluorooctylethylcarbonylthiazolidine-2-thione (6) in 84% yield. The “demi-gemini” compounds 3a-b were purified by chromatography on a
Synthesis of Fluoroalkyl Glycolipids
silica gel column and obtained in 40%-45% yields. The final products (3) were obtained with overall yields ranging from 16% to 20% over the three stages (Table 1). (2) Physicochemical Study. We investigated the molecular aggregation of the compounds to discern relationships between the structures of the amphiphiles and the shapes of the aggregates. (i) Single-Chain Compounds. Micellization. The derivatives of gluconolactone 1a-c were insoluble in water and so micellization could not be investigated in aqueous solution. The single-chain derivatives of lactobionic acid 2a-c self-associated in water above the critical micellar concentration (cmc ) 10-4 M determined at 25 °C by tensiometry). It should be noted that the surface tension at the cmc is not very low (40 mN m-1) in contrast to that generally found with fluorinated amphiphiles.18 These compounds are thus poor surfactants. Formation of Gels. It is known that the hydrocarbon analogues form gels on sudden cooling to 0 °C of aqueous solutions after heating them to around 100 °C at concentrations as low as 1-2%.19,20 Gels were obtained on application of these conditions to the N-fluoroalkyl lactobionamides (2a-c) at concentrations ranging from 10% to 30% depending on the length of the fluoroalkyl chain (Table 2). Similar experiments were carried out on the gluconamides 1a-c in formamide, instead of water. We and other workers have shown that this highly cohesive solvent (in common with glycerol and ethylene glycol) can be employed instead of water for study of self-association.21,22 Gels were obtained after heating solutions of 1a-c in formamide (concentrations from 2% to 10%) to 155 °C and sudden cooling to 0 °C. To our knowledge, this is the first demonstration of gel formation in this medium. The gels were analyzed by electron microscopy using 2% uranyl acetate as contrast agent. The presence of fibers was demonstrated for derivatives 2a-c in water (Figure 1A) and 1a-c in formamide (Figure 1B). Analysis without contrast agent of these gels in formamide showed the presence of twisted lamellae of widths ranging from 50 to 120 nm and an infinite length with a helical pitch ranging from 160 to 600 nm. Our results are in agreement with literature reports on the behavior of analogous hydrocarbon derivatives in water.19,20 These assemblies were formed from lamellae of a bilayer of molecules in which the surfactants were oriented head to head or tail to tail. The hydrogen bonds between the amide groups favor formation of such lamellae and stabilize the aggregates. The lamellae are twisted due to the presence of an asymmetric carbon atom. Selective interactions involving chiral groups have been proposed to account for the formation of twists.23,24 (16) Brown, E.; Joyeau, R.; Paterne, M. Tetrahedron Lett. 1977, 30, 2575. (17) Nagao, Y.; Seno, K.; Kawabaru, K.; Yasaka, T. M.; Takao, S.; Fujisu, E. Tetrahedron Lett. 1980, 21, 841. (18) Yoshimo, N.; Morita, M.; Ito, A.; Abe, M. J. Fluorine Chem. 1995, 70, 187. (19) (a) Fuhrhop, J. H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (b) Fuhrhop, J. H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem.. Soc. 1988, 110, 2861. (c) Fuhrhop, J. H.; Svenson, S.; Boettcher, C.; Rossler, E.; Vieth, H. M. J. Am. Chem. Soc. 1990, 112, 1768 and 4307. (20) Zabel, V.; Muller Fahrnow, A.; Hilgenfeld, R.; Saenger, W.; Pfannemu¨ller, B.; Enkelman, V.; Welte, W. Chem. Phys. Lipids 1986, 39, 313. (21) (a) Perche, T.; Auvray, X.; Petipas, C.; Anthore, R.; Rico-Lattes, I.; Lattes, A. Langmuir 1997, 13, 1475. (b) Auvray, X.; Petipas, C.; Lattes, A.; Rico-Lattes, I. International Festschrift for Pr S. E. Friberg Colloids Surf. 1997, 123-124, 247. (22) (a) Nagarajan, R.; Wang, C. C. J. Colloid Interface Sci. 1996, 178, 471. (b) Mukherjee, K.; Mukherjee, D. C. J. Phys. Chem. 1994, 98, 4713. (c) Ceglie, A.; Colafemmina, G.; Monica, M. D.; Olsson, U.; Jo¨nsson, B. Langmuir 1993, 9, 1449. (23) Tachibana, T.; Kambara, H. J. Am. Chem. Soc. 1965, 87, 3015.
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(ii) Bolaform or “Semi-gemini” Derivatives. In general, amphiphiles of this type form vesicles or micelles or both.14b Micellization. Measurement of surface tension (Figure 2) showed that compound 3b formed micelles in water above the cmc (0.271 mM at 37 °C). Furthermore, this compound lowered the surface tension of water to 20 mN m-1 and can thus be regarded as a good surfactant. Formation of Vesicles. We also investigated vesicle formation by the bolaform derivatives prepared. Aqueous solution of derivatives 3a and 3b (5 mM) were sonicated for 5 min, filtered (0.45 µm pore), and immediately examined in the electron microscope using 2% uranyl acetate as contrast agent. The aggregates formed with compound 3b (n ) 11) were spherical vesicles of polydispersed sizes with a maximum diameter of around 200 nm (Figure 3A). In view of the structural similarities with compound 3b, compound 3a (n ) 10) unexpectedly formed 17 nm thick filaments (Figure 3B). This could not be accounted for by differences in storage of the solutions since they were examined under identical conditions. It is known that vesicles may form linear tubular aggregates on storage at temperature below Tc.25,26 This transformation is reversible and vesicles may reform on heating the solution to a temperature above Tc.27,28 Cylindrical microstructures of this type have also been observed with galactosylceramides29 or chiral fluorinated amphiphiles bearing a galactose polar head.30 Our observations are consistent with the findings of Luisi that slight alterations in the lipid part of the monomer may give rise to marked changes in architecture (supramolecular/mesomorphic) of the aggregates formed in solution.31 Conclusion The results presented here are comparable to those obtained with hydrocarbon analogues and provide further evidence for the general nature of self-association of chiral amphiphiles. Systematic alteration of the structure of the surfactant monomer showed that slight variations could lead to marked differences in the shapes of the molecular aggregates. We also showed for the first time the formation of stable gels in formamide. Acknowledgment. The authors thank R. Bertocchio, Elf-Atochem S.A., Pierre-Benite (France), for a gift of fluoroalkyl iodides. Electron microscopy and cryomicroscopy in formamide were performed at Centre d′e´laboration des mate´riaux et d′Etudes Structurales-Laboratoire d’Optique/Electronique, Toulouse (France), by A. Boudet. Electron microscopy in water was performed by A. Moisand at Laboratoire de Pharmacologie et Toxicologie Fondamentale, Toulouse (France). LA980286+ (24) Nandi, N.; Bagchi, B. J. Am. Chem. Soc. 1996, 118, 11208. (25) Cescato, C.; Walde, P.; Luisi, P. L. Langmuir 1997, 13, 4480. (26) Yager, P.; Price, R. R.; Schnur, J. M.; Schoen, P. E.; Singh, A.; Rhodes, D. G. Chem. Phys. Lipids 1988, 46, 171. (27) Polidori, A.; Pucci, B.; Zarif, L.; Riess, J. G.; Pavia, A. Macromol. Rapid Commun. 1996, 17, 229. (28) Kunitake´, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (29) Kulkarni, V. S.; Anderson, W. H.; Brown, R. E. Biophys. J. 1995, 69, 1976. (30) Guedj, C.; Pucci, B.; Zarif, L.; Coulomb, C.; Riess, J. G.; Pavia, A. Chem. Phys. Lipids 1994, 72, 153. (31) Bonaccio, S.; Wessicken, M.; Berti, D.; Walde, P.; Luisi, P. L. Langmuir 1996, 12, 4976.
