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Chiral Glucose-Derived Surfactants: The Effect of Stereochemistry on Thermotropic and Lyotropic Phase Behavior Ben J. Boyd,†,§ Irena Krodkiewska,† Calum J. Drummond,*,† and Franz Grieser‡ CSIRO Molecular Science, Private Bag 10, Clayton Sth MDC, 3169, Australia, and Department of Chemistry, The University of Melbourne, Parkville, 3052, Australia Received October 23, 2000. In Final Form: September 4, 2001 The two chiral isomers of the surfactant (()-2-dodecyl β-D-glucoside have been prepared, and their thermotropic and lyotropic liquid crystalline behavior has been studied. Mixtures of the two pure diastereomers were prepared in known ratios to examine the effect of chiral purity on these properties using the techniques of differential scanning calorimetry and polarized light optical microscopy. It was found that the (+)-diastereomer produced crystalline and liquid crystalline phases in the presence and absence of water, which were more thermally stable than the (-)-diastereomer. The transition temperatures were reduced with increasing amounts of the (-)-diastereomer, but eutectic-like behavior was not exhibited, indicating metastable cosolubility. The large differences in thermotropic and lyotropic phase behavior may be due to the gross changes in molecular shape resulting from the very subtle difference in structure. This change in shape is likely to affect the molecular packing and headgroup interactions that stabilize the crystalline and liquid crystalline phases in other glucose-based surfactants.
Introduction The study of surfactants with chiral centers is becoming more popular as they have been shown to be useful in stereoselective synthesis1 and in the separation of chiral materials such as pharmaceuticals.2 Biological lipids often contain chiral centers also, which can dictate the nature of their self-assembly structures under physiological conditions.3 The nature of vesicular structures used for controlled drug delivery has been shown to be dependent on the stereochemistry of the chiral surfactant used.4 The effect of chirality on surfactant monolayers has been studied more extensively and has been reviewed.5 Hence, an increased knowledge of the significance of molecular stereochemistry in surfactant self-assembly is highly sought after in the biochemical field. The effect of anomeric configuration on the self-assembly behavior of linear alkyl glucosides has been well documented, where a change in the anomeric configuration can produce dramatic changes in the thermotropic liquid crystalline and binary surfactant-water phase behavior.6-8 However, in the case of headgroup positional isomers of alkyl glucoside surfactants, an additional chiral center is present at the point of headgroup attachment along the alkyl chain. This chiral center is indicated by the asterisk for (+)- and (-)-2-dodecyl β-D-glucoside shown schematically in Figure 1a. * To whom correspondence should be addressed. † CSIRO Molecular Science. ‡ The University of Melbourne. § Reckitt and Colman Scholar. (1) Zhang, Y.; Sun, P. Tetrahedron: Asymmetry 1996, 7, 3055. Takeshita, M.; Yanagihara, H.; Terada, K.; Akutsu, N. Annu. Rep. Tohoku Coll. Pharm. 1992, 39, 247. (2) Clothier J.; Tomellini, S. J. Chromatogr., A 1996, 723, 179. Desbene, P.; Fulchic, C. J. Chromatogr., A 1996, 749, 257. Mechref, Y.; Elrassi, Z. Chirality 1996, 8, 518. Peterson, A.; Ahuja, E.; Foley, J. J. Chromatogr., B 1996, 683, 15. (3) Zarif, L.; Polidori, A.; Pucci, B.; Gulik-Krzywicki, T.; Pavia, A.; Riess, J. Chem. Phys. Lipids 1996, 79, 165. Hato, M.; Minamikawa, H. Langmuir 1996, 12, 1658. Seddon, J. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 380. (4) Fuhrhop, J.; Helferich, W. Chem. Rev. 1993, 93, 1565. Jaeger, D.; Kubicz-Loring, E.; Price, R.; Nakagawa, H. Langmuir 1996, 12, 5803. (5) Stewart, M.; Arnett, E. Top. Stereochem. 1982, 13, 195.
The present authors have previously described the effect that headgroup position has on the properties of a series of positional isomers of dodecyl β-D-glucosides,9 prepared from racemic secondary alcohols. Each of these isomers exists as a pair of R- and S-diastereomers, due to the additional chiral center at the point of attachment of the headgroup to the dodecyl chain. To our knowledge, there has been no study of the effect of the R/S stereochemistry on the physicochemical properties of this class of surfactants, and there is only one mention of diastereomers in the previous literature on branched glucose-derived surfactants.7 In this paper, two diastereomeric glucose-derived surfactants, (+)-2-dodecyl β-D-glucopyranoside ((+)-2-β-C12G1) and (-)-2-dodecyl β-D-glucopyranoside ((-)-2-β-C12G1), illustrated in Figure 1b, have been prepared and their physicochemical properties have been studied. These two surfactants comprise the racemic positional isomer, (()2-dodecyl β-D-glucopyranoside or “(()-2-isomer”, whose properties have been described in detail by the present authors.9 The alkyl chain length, headgroup degree of polymerization, anomeric configuration, and position of headgroup attachment on the alkyl chain are all fixed for the two surfactants studied here, so that only the stereochemistry at the carbon atom to which the headgroup is attached has been varied. The thermotropic and lyotropic phase behavior of these surfactants has been (6) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359. Loewenstein, A.; Ignore, D.; Zehavi, U.; Zimmermann, H.; Emerson A.; Luckhurst, G. Liq. Cryst. 1990, 7, 457. Loewenstein, A.; Igner, D. Liq. Cryst. 1993, 13, 531. Loewenstein, A.; Igner, D. Liq. Cryst. 1991, 10, 457. Dorset, D.; Rosenbusch, J. Chem. Phys. Lipids 1981, 29, 299. Jeffrey, G.; Bhattacharjee, S. Carbohydr. Res. 1983, 115, 53. Barrall, E.; Grant, B.; Oxsen, M.; Samulski, E.; Moews, P.; Knox, J.; Gaskill, R.; Haberfield, J. Org. Coat. Plast. Chem. 1979, 40, 67. Sakya, P.; Seddon, J.; Vill, V. Liq. Cryst. 1997, 23, 409. Goodby, J. Mol. Cryst. Liq. Cryst. 1984, 110, 205. Jeffrey, G.; Wingert, L. Liq. Cryst. 1992, 12, 179. (7) Pfannemu¨ller, P.; Welte, W.; Chin, E.; Goodby, J. Liq. Cryst. 1986, 1, 357. (8) Nilsson, F.; Soderman, O.; Johansson, I. J. Colloid Interface Sci. 1998, 203, 131. (9) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Weerawardena, A.; Furlong, D. N.; Grieser, F. Langmuir 2001, 17, 6100.
10.1021/la0014858 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002
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The preparation of the surfactants used in this study essentially followed literature procedures for the preparation of alkyl β-D-glucoside surfactants, using the appropriate optically resolved, secondary dodecanols as the alkylation reagents.10-12 The procedure for optical resolution of (()-2-dodecanol and subsequent preparation and purification of the two surfactants is described below. Preparation of (+)-2-Dodecanol. (+)-2-Dodecanol was prepared by adapting a known method for the resolution of (()2-octanol.13 Racemic (()-2-dodecanol was converted to the mixed
(()-2-dodecyl hydrogen phthalate. This was then treated with the chiral L-brucine in warm acetone solution, to form a mixture of (+)-2-dodecyl L-brucine phthalate and (-)-2-dodecyl L-brucine phthalate. (+)-2-Dodecyl L-brucine phthalate precipitates on cooling and was separately cleaved from the brucine moiety to give the crude (+)-2-dodecyl hydrogen phthalate. Successive recrystallizations from 90% acetic acid afforded the pure (+)2-dodecyl hydrogen phthalate, which was then hydrolyzed with base, to obtain the pure (+)-2-dodecanol (quantities used were the same as in the literature preparation, allowing for 0.416 mole of alcohol reactant; yield ) 11.0 g, 14.2%). Unfortunately, the (-)-2-dodecanol residue was found to contain around 25% of the (+)-isomer, and hence pure (-)-2-dodecanol could not be obtained by this method. Preparation of (-)-2-Dodecanol. A method for the production of pure (-)-2-alkanols has been described by Hirata et al.,14 in which racemic (()-2-alkanols were selectively transesterified with tributyrin in hexane, by the lipase Porcine Pancreas (EC 3.1.1.3). The enzyme acts to selectively form the butyl ester of the (-)-enantiomer. Although the authors do not specifically use (()-2-dodecanol in their preparation, the method was effective for the homologous series (()-2-heptanol, (()-2-octanol, and (()2-nonanol and was therefore utilized in this work, for the (()2-dodecyl case. (()-2-Dodecanol (ChemSamp Co., U.S.A., 80 g) was dissolved in dry hexane (900 mL), and the lipase (Sigma, 168 g) and freshly distilled tributyrin (BDH, LR grade, 129.8 g) were added with magnetic stirring at 30 °C. The progress of the butyl ester formation was followed by gas chromatography (GC), and when the reaction appeared complete (132 h reaction time) the enzyme was filtered from the reaction mixture. (-)-2-Dodecyl butanoate was removed from the alcoholic brew by applying the reaction mixture to a silica column. Progress of the column separation was monitored by TLC using an ethyl acetate/petrol mixture (1:9 v/v) as the solvent system and confirmed by GC. (-)-2-Dodecyl butanoate eluted first with hexane as the eluent. Hydrolysis of the ester provided >99% pure (-)-2-dodecanol (16.0 g, 19.4%), the purity determined by the presence of only one anomeric doublet on 500 MHz 1H NMR (after coupling to the glucose unit to form the alkyl β-D-glucoside). The two optically pure dodecanols were then separately reacted via the Koenigs-Knorr synthesis described for linear β-C12G1,9 to produce the two diastereomerically pure (+)-2-β-C12G1 and (-)-2-β-C12G1 surfactants. After purification by normal phase chromatography on silica, no R-anomers were detected in the 500 MHz 1H NMR spectrum of either surfactant. Analytical data for the two pure diastereomers are presented below. (+)-2-β-C12G1. Yield: 23.2% from starting glucose. 500 MHz 1H NMR (CD OD): H-1, δ ) 4.30 (d, 1H); O-CH, δ ) 3.85 (m, 3 1H); H-2,3,4,5,6,6′, δ ) 3.70-3.10 (m, 10H); CH2, δ ) 1.82-1.75 (m, 4H); -CH2-, δ ) 1.51-1.41 (14H); -CH3, δ ) 0.90-1.00 (m, 6H). TLC (CH3OH/CHCl3, 4:1): Rf ) 0.48, [R]20D ) -25.1 (c ) 1, MeOH). (-)-2-β-C12G1. Yield: 15.1% from starting glucose. 500 MHz 1H NMR (CD OD): H-1, δ ) 4.32 (d, 1H); O-CH, δ ) 3.85 (m, 3 1H); H-2,3,4,5,6,6′, δ ) 3.70-3.10 (m, 10H); CH2, δ ) 1.82-1.75 (m, 4H); -CH2-, δ ) 1.51-1.41 (14H); -CH3, δ ) 0.90-1.00 (m, 6H). TLC (CH3OH/CHCl3, 4:1): Rf ) 0.48, [R]20D ) -27.9 (c ) 1, MeOH). The surfactant was dispersed in water and lyophilized before storing in the freezer over a desiccant. Mixtures of the two surfactants were prepared in known quantities by weighing the appropriate amounts of the two components into a flask, dispersing in methanol to homogenize the mixture. The methanol was removed under vacuum, and the surfactant mixture was redispersed in water and lyophilized, before storage in the freezer over desiccant. Thermotropic liquid crystalline behavior was studied primarily by DSC (Mettler TA3000, 2.5 °C/min), and the nature of DSC transitions was confirmed by polarized light optical microscopy. Endotherm minima were reported as the transition temperatures in this study, due to overlap of some of the endotherms in these systems. The DSC instrument was calibrated with indium for
(10) Koenigs, W.; Knorr, E. Ber. Dtsch. Chem. Ges. 1901, 34, 957. (11) de Grip, W.; Bovee-Guerts, P. Chem. Phys. Lipids 1979, 23, 321. (12) Vanaken, T.; Foxall-Vanaken, S.; Castleman, S.; FergusonMiller, S. Methods Enzymol. 1986, 125, 27.
(13) Vogel, A. A Text-Book of Practical Organic Chemistry, 3rd ed.; Longmans: London, 1956; p 505. (14) Hirata, H.; Yamashina, T.; Higuchi, K.; Sakaki, K.; Iida, I. J. Jpn. Oil Chem. Soc. 1991, 40, 995.
Figure 1. (a) Structures of the two chiral diastereomers of (()-2-dodecyl β-D-glucopyranoside and (b) energy-minimized structures of the two chiral diastereomers of (()-2-dodecyl β-Dglucopyranoside. Green represents the carbon atoms, red represents the oxygen atoms, and gray represents the hydrogen atoms. The rationale for the assignment of the R-stereochemistry to the (-)-isomer and the S- to the (+)-isomer is described in the text.
investigated using differential scanning calorimetry (DSC) and polarized light optical microscopy (water penetration scans). In addition, the temperature of the transition from hydrated crystals + L1 phase to isotropic L2 + L1, Tlyo, was determined for 1 wt % mixtures of the surfactants in water. Changes in the thermotropic and lyotropic liquid crystalline behavior, with changes in the ratio of diastereomers in mixtures prepared from the two pure diastereomeric surfactants, have also been investigated in this study. Experimental Section
Chiral Glucose-Derived Surfactants temperature and enthalpy accuracy. The Mettler software was used to calculate the energy represented by any transition that was observed. The samples were prepared by weighing approximately 10 mg of the dry surfactant into preweighed aluminum pans, which were then placed under a vacuum of
