Alkylglycosides with an Isoprenoid-Type Hydrophobic Chain Can

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Alkylglycosides with an Isoprenoid-Type Hydrophobic Chain Can Afford Greater Control of Aqueous Phase Structures at Low Temperatures Masakatsu Hato,* Hiroyuki Minamikawa,† Rajesh A. Salkar, and Sanae Matsutani Bionanomaterial and Surface Interactions Group, Nanotechnology Research Institute, AIST, Tsukuba Central-5, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8565, Japan Received October 30, 2001. In Final Form: March 5, 2002 We have proposed novel alkylglycosides, AGs, that consist of an isoprenoid-type hydrophobic chain, the 3,7,11,15-tetramethylhexadecyl (phytanyl) group. One notable feature of the proposed AGs lies in the fact that their Krafft eutectic temperatures, TK, are well below room temperature although the total number of carbon atoms in the hydrophobic chain is as large as 20. As compared to the conventional AGs with straight and saturated alkyl chains, their low TK values significantly expand our freedom to control their aqueous phase structures at low temperatures. The aqueous phase structures are controlled by modification of the headgroup: an HII phase for the glycerol headgroup, a cubic phase with crystallographic space group Pn3m/Pn3 at low temperatures and an HII phase at higher temperatures for a xylose headgroup, and an LR phase for a glucose and a maltose headgroup.

Introduction In recent years, alkylglycosides are becoming increasingly important from the ecological and industrial viewpoints.1,2 The rationale for the interest in these surfactants lies in the fact that they can be synthesized from renewable resources and are generally nontoxic and biodegradable. Moreover, biological functions of the sugar groups such as molecular recognition3 or stabilizing effects of protein functions and structures in aqueous solution4 make them attractive as a new material for biotechnology.5-9 Each surfactant/water system displays a specific temperature, TK (Krafft eutectic temperature10), above which a hydrated solid surfactant transforms into fluid molecular assemblies such as micelles and a variety of liquid crystalline phases. These fluid phases are crucial to the functions of surfactant/water systems.11 At temperatures below TK, on the other hand, a surfactant precipitates as a hydrated solid and is difficult to use. Thus, for a usable surfactant, TK must be lower than working temperatures, most preferably well below room temperature.12 * Corresponding author. E-mail: [email protected]. Tel: +81298-61-9324. Fax: +81-298-61-6243. † Present address: Nanoarchitectonics Research Center, AIST, Tsukuba Central-5, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8565, Japan. (1) Hill, K.; von Rybinski, W.; Stoll, G. Alkyl Polyglycosides; VCH: Weinheim, 1997. (2) So¨derman, O.; Johansson, I. Curr. Opin. Colloid Interface Sci. 2000, 4, 391. (3) Hakomori, S. Pure Appl. Chem. 1991, 63, 473. (4) Franks, F.; Hatley, R. H. M. Stability and Stabilization of Enzymes; van den Tweel, W. J. J., Harder, A., Buitelaar, R. M., Eds.; Elsevier Science Publ.: New York, 1993. (5) Helenius, A.; McCaslin, D. R.; Fries, E.; Tanford, C. Methods Enzymol. 1979, 56, 734. (6) Ku¨hlbrandt, W. Q. Rev. Biophys. 1992, 25, 1. (7) Rigaud, J.-L.; Pitard, B.; Levy, D. Biochim. Biophys. Acta 1995, 1231, 223. (8) Rummel, G.; Hardmeyer, A.; Widmer, C.; Chiu, M. L.; Nollert, P.; Locher, K. P.; Pedruzzi, I.; Landau, E. M.; Rosenbush, J. J. Struct. Biol. 1998, 121, 82. (9) Baba, T.; Minamikawa, H.; Hato, M.; Motoki, A.; Hirano, M.; Zhou, D.; Kawasaki, K. Biochem. Biophys. Res. Commun. 1999, 265, 734. (10) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994; Chapter 5. (11) Larsson, K. J. Phys. Chem. 1989, 93, 7304.

TK values of conventional AGs are unusually high for “nonionic” surfactants.13,14 For example, the TK values of C12-glucosides are already 55 °C (n-dodecyl-R-D-glucoside) and 36 °C (n-dodecyl-β-D-glucoside).15 As further extension of the alkyl chain will result in AGs with TK significantly higher than room temperature, the conventional AGs have relatively short alkyl chains mainly in a range C8-C12. This fact should not be underestimated. Microemulsions based on the short-chain AGs, for example, require high surfactant concentrations that are not acceptable in many technical applications.16 Furthermore, the aqueous phases that appear in their phase diagrams are of the normal type,15,17,18 and a biologically important lamellar phase (that can form stable vesicles) or inverted liquid crystalline phases11,19 can appear only at temperatures significantly higher than room temperature. This seriously limits the usefulness of the AGs in many technical applications. There is clearly a need for developing AGs that have sufficiently large hydrophobic chain volumes and at the same time have low enough TK values. In this communication, we propose novel AGs that meet the above requirement. They consist of isoprenoid-type hydrophobic chains derived from natural plant products and display TK values that are well below room temperature, although the total number of carbon atoms in the hydrophobic chain is as large as 20. Taking phytanylchained AGs as examples, we demonstrate that the proposed AGs can afford greater control of the aqueous phase structures at low temperatures. Experimental Section Chemical structures of AGs examined are shown in Figure 1. Each AG was synthesized by similar procedures previously (12) Shinoda, K. Solution and Solubility, 3rd ed.; Maruzenn: Tokyo, 1991; Chapter 1. (13) Hato, M.; Minamikawa, H.; Tamada, K.; Baba, T.; Tanabe, Y. Adv. Colloid Interface Sci. 1999, 80, 233. (14) Hato, M. Curr. Opin. Colloid Interface Sci. 2001, 6, 268. (15) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359. (16) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160. (17) Sakya, P.; Seddon, J. M.; Vill, V. Liq. Cryst. 1997, 23, 409. (18) Nilsson, F.; So¨derman, O.; Hansson, P. Langmuir 1998, 14, 4050. (19) Lindblom, G.; Rilfors, L. Adv. Colloid Interface Sci. 1992, 41, 101.

10.1021/la0116185 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002

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Figure 1. Chemical structures of phytanyl-chained alkyl glycosides. Phyt: 3,7,11,15-tetramethylhexadecyl group (phytanyl group). Gly(Phyt): 1-glyceryl phytanyl ether. β-Xyl(Phyt): 1-O-phytanyl-β-D-xyloside. β-Glc(Phyt): 1-O-phytanylβ-D-glucoside. β-Mal2(Phyt): 1-O-phytanyl-β-D-maltoside. MalN: maltooligosaccharide that consists of N glucose residues that are linked by R-1,4-O-glycosidic bonds. reported20,21 and obtained as the single β-O-glycoside product. The purity was checked by 1H NMR and thin-layer chromatography and was at least 98%. The hydrophobic part is a phytanyl group (Phyt) that contains 16 backbone carbon atoms and 4 methyl groups at the 3, 7, 11, and 15 positions (a total of 20 carbon atoms). The hydrophilic headgroups are systematically altered, that is, maltose (Mal2) where 2 glucose residues are linked by R-1,4-O-glycosidic bonds, glucose (Glc), xylose (Xyl), and glycerol (Gly) in order of the decreasing number of hydroxyl groups of the headgroup. All of the AG/water systems exhibited a two-phase region of a liquid crystalline phase plus excess water in a dilute surfactant regime (see below). Thus, the TK value for each AG/water system was estimated by differential scanning calorimetry for a singlephase region of liquid crystalline phase with a concentration close to the water-liquid crystalline phase boundary. We first incubated the specimen at -80 to -100 °C for 2 h followed by a heating scan at a rate of 0.1-0.5 °C/min. The instrument used was a Seiko SSC/560U differential scanning calorimeter (DSC). The phase sequence of each AG/water system was examined by a water penetration scan.22 The different liquid crystalline phases were identified optically by the polarizing microscope23 and by small-angle X-ray scattering (SAXS) measurements. SAXS measurements were performed with Ni-filtered Cu KR radiation (wavelength ) 0.154 nm) generated by a Rigaku RU-200 X-ray generator (40 kV, 100 mA) with a double pinhole collimator (0.5 mm φ × 0.3 mm φ). The hydrated surfactant was flame-sealed into a quartz capillary (Glas, Berlin, 1.5 mm φ in outer diameter) after repeated cycles of freezing-thawing and centrifugation back and forth. The sample-loaded capillaries were then incubated at 4 °C for at least 3 days. The measurements were carried out at 4, 15, 25, 35, 45, 55, 65, 75, 85, and 95 °C in this sequence. At each temperature, we allowed the sample to equilibrate for 1-2 h. The sample temperature was controlled with a Mettler FP82HT hotstage within an accuracy of (0.5 °C. Exposure time was 30 min to 1 h at a sample to film distance of 195 mm. (20) Minamikawa, H.; Murakami, T.; Hato, M. Chem. Phys. Lipids 1994, 72, 111. (21) Minamikawa, H.; Hato, M. Langmuir 1997, 13, 2564. (22) Laughlin, R. G. Adv. Colloid Interface Sci. 1992, 41, 57. (23) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628.

Letters

Figure 2. DSC thermograms for β-Xyl(Phyt)/water (heating rate, 0.1 °C/min) and β-Mal2(Phyt)/water (heating rate, 0.5 °C/ min) systems: (a) 55 wt % β-Xyl(Phyt), a two-phase region of W + Pn3m cubic phase; (b) 65 wt % β-Xyl(Phyt), a single-phase region of a Pn3m cubic phase; (c) 75 wt % β-Xyl(Phyt), a singlephase region of an Ia3d cubic phase; (d) 95 wt % β-Xyl(Phyt); (e) 83 wt % Mal2(Phyt), a single-phase region of an LR phase.

Results The type of headgroup significantly affected the thermal properties of the neat surfactants. The physical state at room temperature was a viscous liquid for Gly(Phyt), soft waxlike substance for β-Xyl(Phyt) and β-Glc(Phyt), and a white solid for β-Mal2(Phyt). Moreover, the headgroups significantly influenced the ease with which the aqueous AGs formed a hydrated solid phase, Lc. For example, 1 h of incubation at -10 °C induced an Lc phase of β-Xyl(Phyt), while 2 h of incubation at -100 °C did not appear sufficient for Gly(Phyt), β-Glc(Phyt), and β-Mal2(Phyt) to adopt an Lc phase. These effects could be more clearly demonstrated by the DSC measurements. DSC thermograms for β-Xyl(Phyt) are shown in Figure 2a (55 wt % β-Xyl(Phyt), a two-phase region of water plus Pn3m cubic phase), Figure 2b (65 wt % β-Xyl(Phyt), a single-phase region of a Pn3m cubic phase), and Figure 2c (75 wt % β-Xyl(Phyt), a single-phase region of an Ia3d cubic phase). All of the thermograms could be characterized by two endothermic peaks: the isothermal transition located at 0 °C and the second broader transition that completed at about 10 °C. As the water content further decreased, the first transition (0 °C) disappeared and the endotherm completion temperature for the second transition shifted to higher temperatures, for example, 16 °C for 95 wt % (Figure 2d) and 43 °C for anhydrous β-Xyl(Phyt) (data not shown). Thus, the TK value for β-Xyl(Phyt) was estimated to be about 10 °C. The first transition was most probably due to transition of water. Gly(Phyt), β-Glc(Phyt), and β-Mal2(Phyt) gave very different thermograms. The most representative DSC thermogram for this surfactant group is shown in Figure 2e (83 wt % β-Mal2(Phyt), a single-phase region of an LR phase). The thermogram obtained from -80 to 80 °C exhibited only a shallow endotherm with a peak position located at about -5 °C (indicated by an arrow). No thermal event was evidenced above 0 °C. Although we could not detect a clear transition and undercooling of the aqueous surfactants could not be completely excluded, the present data strongly suggest that the TK values for these surfactants were below 0 °C. A higher TK value for an alkyl-xyloside as compared to alkyl-glucoside was also observed for straight-chained

Letters

Langmuir, Vol. 18, No. 9, 2002 3427

Table 1. Krafft Eutectic Temperature, TK, Phase Sequence, and SAXS Diffraction Lines of Phytanyl-Chained AG/Water Systems surfactant Gly(Phyt)

TKa (°C)

phase sequenceb

e0

W - HII W - isotropic (J66 °C) W - QII (Pn3m/Pn3) - QII (Ia3d) - LR W - ΗΙΙ (J76 °C) W - LR W - LR

β-Xyl(Phyt)d

∼10

β-Glc(Phyt)e β-Mal2(Phyt)e

e0 e0

diffraction linesc d1, d2, d3 (nm)

diffraction line ratio d1/d2/d3

3.95, 2.28, 1.99

1:0.577:0.504

(see Figure 4) 5.02, 2.90, 2.53 (85 °C) 4.38, 2.18, 1.46 4.94, 2.46, 1.62

1:0.578:0.504 1:0.498:0.333 1:0.498:0.328

a A value obtained on heating runs after incubating an aqueous surfactant at -80 to -100 °C for 2 h (see text). b Phase sequence as surfactant concentration increases. c Values for the mesophase that is in equilibrium with water at 25 °C unless otherwise noted. d Phase sequence up to 85 wt % surfactants. e Phase sequence up to 97-98 wt % surfactants (one hydrated water/surfactant). Gly: glycerol. Xyl: xylose. Glc: glucose () Mal1). Mal2: maltose. W: an excess water phase where a very small amount of surfactant molecules exist. HII: an inverted hexagonal phase. QII: an inverted cubic phase. LR: a lamellar phase.

AGs, that is, 34 °C for n-heptyl-β-D-xyloside24 but below 0 °C for n-heptyl-β-D-glucoside.25 Note, however, that the TK value of β-Xyl(Phyt) was significantly lower than that for the shorter chain compound, n-heptyl-β-D-xyloside. Figure 3 shows the water penetration scan of the phytanyl-chained AGs at 25 °C. Table 1 summarizes the phase sequences observed at 25 °C and higher temperatures together with the relevant SAXS diffraction data. The aqueous phases for Gly(Phyt) and β-Xyl(Phyt) are sensitive to temperature, whereas those for β-Glc(Phyt) and β-Mal2(Phyt) are almost temperature-independent over a temperature range from 4 to 95 °C (the maximum temperature examined). To simplify the following discussion, we here mainly focus on a liquid crystalline phase that is in equilibrium with excess water, W. Figure 3a gives the result for β-Glc(Phyt). Once in contact with water, myelin figures immediately formed and grew into elongated tubes as water penetration continued. The inset is a birefringent texture (oily streak)23 observed for the 85 wt % β-Glc(Phyt), indicating that the phase in equilibrium with excess water, W, is a lamellar phase, LR. This conclusion is consistent with SAXS diffractions as shown in Table 1. β-Mal2(Phyt) exhibited behavior very similar to that of β-Glc(Phyt) (data not shown), indicating that an addition of one more glucose residue to the headgroup does not appreciably affect the phase behavior. Thus, the different regions found for the β-Glc(Phyt)/water and β-Mal2(Phyt)/water systems were W and an LR phase. The LR phases of both systems appeared to continue up to about 97-98 wt % (one hydrated water/surfactant) and were stable at least over a temperature range from 4 to 95 °C. The major difference lies in a maximum number of hydration of the LR phase; about 8 water molecules per β-Glc(Phyt) and about 13 water molecules per β-Mal2(Phyt), values which are significantly lower than that for the lecithin LR phase. The lower hydration in LR phases has also been found for n-octyl-β-D-glucoside26 and a variety of nonionic glycolipids, both synthetic13 and biological27-29 in origin. Thus, the low hydration appears to be a common feature that characterizes the LR phase of nonionic AGs and glycolipids. This may in part arise from strong inter-sugar-headgroup attractions in the LR phase30 and in part from the smaller (24) Shinoyama, H.; Gama, Y.; Nakahara, H.; Ishigami, Y.; Yasui, T. Bull. Chem. Soc. Jpn. 1991, 64, 291. (25) Do¨rfler, H.-D.; Go¨pfert, A. J. Dispersion Sci. Technol. 1999, 20, 35. (26) Nilsson, F.; So¨derman, O.; Johansson, I. Langmuir 1996, 12, 902. (27) Wieslander, A.; Ulmius, J.; Lindblom, G.; Fontell, K. Biochim. Biophys. Acta 1978, 512, 241. (28) McDaniel, R. V. Biochim. Biophys. Acta 1988, 940, 158. (29) Shipley, G. G.; Green, J. P.; Nichols, B. W. Biochim. Biophys. Acta 1973, 311, 531. (30) Korchowiec, B. M.; Baba, T.; Minamikawa, H.; Hato, M. Langmuir 2001, 17, 1853.

Figure 3. Water penetration scans of the phytanyl-chained AGs at 25 °C. (a) Water penetration scan for β-Glc(Phyt) and birefringent texture (oily streak) for 85% β-Glc(Phyt) (×100). The far left region is a neat surfactant phase. (b) Water penetration scan for β-Xyl(Phyt) (×100). The far left region is a neat surfactant phase. (c) Water penetration scan for Gly(Phyt) (×100).

dipole of the sugar headgroups as compared to the phosphocholine headgroup.31 The β-Xyl(Phyt)/water system exhibited richer phase behavior. The water penetration scan exhibited four different regions, excess water, W, the isotropic region, a (31) Marrink, S.-J.; Tieleman, D. P.; van Buuren, A. R.; Berendsen, H. J. C. Faraday Discuss. 1996, 103, 191.

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Letters

monoolein/water32,33 and alkyl-β-D-glucopyranosyl-racglycerol/water systems.34 When the number of hydroxyl groups in the headgroup is further reduced to two, that is, Gly(Phyt), a phase in equilibrium with W was an inverted hexagonal phase, HII (Figure 3c), as confirmed from a ratio of the SAXS diffraction lines of 1:1/x3:1/2 (Table 1). The HII phase was stable from at least 4 to about 65 °C. Above 65 °C, the HII phase transforms into a fluid isotropic phase whose structure is not yet identified. Discussion

Figure 4. (a) SAXS diffractograms of β-Xyl(Phyt)/water [50 wt % β-Xyl(Phyt) at 25 °C]. q ) (4π sin θ)/λ is the scattering vector, where 2θ is the scattering angle. A linear fit, intersecting the origin, is obtained from a plot of q versus (h2 + k2 + l2)1/2 for a crystallographic space group Pn3m/Pn3 with a lattice parameter of 8.8 nm. The numbers indicate the indexing of the peaks in the Pn3m/Pn3 space group. (b) SAXS diffractograms of β-Xyl(Phyt)/water [80 wt % β-Xyl(Phyt) at 55 °C]. A linear fit, intersecting the origin, is obtained from a plot of q versus (h2 + k2 + l2)1/2 for a crystallographic space group Ia3d with a lattice parameter of 10.6 nm. The numbers indicate the indexing of the peaks in the Ia3d space group.

birefringent region, and a “dry” surfactant region as β-Xyl(Phyt) concentration increases (Figure 3b). The SAXS measurements indicated that the isotropic phase in equilibrium with W was a cubic phase with a crystallographic space group Pn3m/Pn3 (Figure 4a), which was stable at least from 4 to 75 °C (up to ∼65 wt % surfactant). Above 76 °C, the Pn3m/Pn3 cubic phase transformed into an HII phase that persisted at least up to 95 °C. With increasing the surfactant concentration, a second cubic phase with a crystallographic space group Ia3d appeared (70-85 wt % surfactant) (Figure 4b). Thus, the isotropic region sandwiched between W and the birefringent region (an LR phase) in Figure 3b consisted of cubic phases with space groups Pn3m/Pn3 and Ia3d. We at present were not able to fully determine phase structures at higher concentrations (>85%) due to prolonged time for equilibrium and extra peaks that could be ascribed to the presence of unknown phases. Nevertheless, the data so far obtained indicate that the general phase behavior of the β-Xyl(Phyt)/water system is very similar to that of

In addition to intensive variables, phase structures in surfactant/water systems are correlated with surfactant molecular parameters such as headgroup area, hydrophobic chain volume, and length.35 Though an actual system may not give all the phases, modulation of the phase structures such as L1 (HI) f LR f HII f L2 (an inverted micelle) will, in principle, be obtained by changing the molecular parameters, for example, by increasing the alkyl chain length. The cubic phases are interspersed between these main phases. This however is a situation expected at temperatures above TK. If TK is higher than the working temperature, only a hydrated solid surfactant will prevail. To understand the correlation of molecular structures with the available phases of AG/water systems at room temperature, it is worth reviewing the n-alkyl-β-Dglucoside/water system. The shorter chain compounds, from n-heptyl-β-D-glucoside to n-dodecyl-β-D-glucoside, form normal micelles in a dilute solution regime,15,17,18,25,36 whereas a longer chain compound, n-octadecyl-β-D-glucoside, can no longer form normal micelles but forms an LR phase that is in equilibrium with W.37 The values of TK rise as the alkyl chain length increases: below 0, 36, and 55 °C for C8-, C12-, and C18-glucoside, respectively.15,37 This implies that n-alkyl-β-D-glucosides with C12 or longer chains precipitate at room temperature and the aqueous phases available at room temperature are only of the normal type. The question naturally arises as to whether it is possible to depress the TK by modifying the glucose headgroup. The use of other monosaccharide headgroups, however, does not appear promising, because glucose generally gives lower TK values as compared to other monosaccharides such as galactose, xylose, and fucose.14 One effective way to depress TK is to increase the number of sugar residues, N, in the headgroup, for example, to increase the number of glucose residues in the maltooligosaccharide headgroup (MalN).38,39 This is best exemplified by surfactants with double dodecyl chains, 1,3-di-O-dodecyl-2-O-(β-glycosyl)glycerols bearing the maltooligosaccharide headgroup, (32) Hyde, S. T.; Andersson, S.; Ericsson, B.; Larsson, K. Z. Kristallogr. 1984, 168, 213. (33) Qui, H.; Caffrey, M. Biomaterials 2000, 21, 223. (34) Turner, D. C.; Wang, Z.-G.; Gruner, S. M.; Mannock, D. A.; McElhaney, N. J. Phys. II France 1992, 2, 2039. (35) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525. (36) Sakya, P.; Seddon, J. M.; Templer, R. H. J. Phys. II France 1994, 4, 1311. (37) Vill, V.; von Minden, H. M.; Koch, M. H. J.; Seydel, U.; Brandenburg, K. Chem. Phys. Lipids 2000, 104, 75. (38) Hato, M.; Minamikawa, H. Langmuir 1996, 12, 1658. (39) An increased number of glucose residues in the cellooligosaccharide headgroup results in a steep rise in TK (ref 38). In the cellooligosaccharide, all of the N glucose residues are linked via β-1,4O-glycosidic bonds, whereas all of the N glucose residues are linked via R-1,4-O-glycosidic bonds in maltooligosaccharide. This demonstrates the dominating influence of the headgroup stereochemistry on the values of TK.

Letters

MalN(C12)2.13,38,40 The values of TK are effectively depressed as N increases, that is, 52 °C (N ) 1), 45 °C (N ) 2), 15 °C (N ) 3), and below 0 °C (N ) 5, 6, 7). A liquid crystalline phase that forms at the lowest surfactant concentrations in the phase diagram is shifted from an HII to an HI phase as N increases from 1 to 7, that is, an HII (N ) 1), an LR (N ) 2, 3), and an HI (N ) 7), indicating that the “hydrophilicity” of the surfactant is enhanced as N increases. It is remarkable that with a maltoheptaosyl headgroup (N ) 7), even a surfactant with strongly “hydrophobic” double dodecyl chains can form normal micelles with a critical micelle concentration of less than 5 × 10-6 M.40 The maltooligosaccharide headgroup is also effective to depress TK for single-chained surfactants; the TK values of n-dodecyl-β-maltoside (e0 °C)41 and n-octadecyl-β-maltoside (∼40 °C)42 are lower than those of corresponding n-alkyl-β-D-glucoside.15,37 The hydrophilicity of the surfactant is again enhanced as N increases; n-octadecyl-β-glucoside forms an LR phase, while n-octadecyl-β-maltoside forms an HI phase.37,42 The above discussion teaches us an important lesson that such structural modification as to depress the TK entails the enhanced hydrophilicity of the resultant AG molecule. In other words, as far as the straight and saturated alkyl chains are employed, hydrophobic AGs capable of adopting an LR phase (in equilibrium with W) or inverted phases will exhibit TK values significantly higher than room temperature. Rational design of the hydrophobic part of AGs is, therefore, a key to overcoming these difficulties. As demonstrated in this communication, the phytanyl group is large enough to form the inverted phases and at the same time effective in keeping the TK values well below room temperature. Though other branched chains such as mono-, dimethyl iso-, and methyl- and ethyl-antesio branches consistently depress TK,43 their effectiveness in (40) Hato, M.; Seguer, J. B.; Minamikawa, H. J. Phys. Chem. B 1998, 102, 11035. (41) Auvray, X.; Petipas, C.; Anthore, R. Langmuir, 1995, 11, 433. (42) von Minden, H. M.; Brandenburg, K.; Seydel, U.; Koch, M. H. J.; Garamus, V.; Willumeit, R.; Vill, V. Chem. Phys. Lipids 2000, 106, 157.

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depressing TK is far less than that of the phytanyl group. Unsaturated alkyl chains do not appear to be as efficient in depressing TK of sugar-based surfactants.44 Thus, the phytanyl group (and other isoprenoid-type hydrophobic groups as well) appears best suited to prepare user-friendly AGs. Moreover, the phytanyl group can introduce many alternatives for the molecular architectures of sugar-based surfactants. For example, surfactants with double alkyl chains with low TK values are readily prepared; for example, 1,3-di-O-phytanyl-2-O-(β-glycosyl)glycerol compounds bearing a series of maltooligosaccharide headgroups, MalN(Phyt)2, have TK values that are well below 0 °C.21,45 The aqueous phase structures can also be controlled by changing N: an inverse micellar cubic phase of a crystallographic space group Fd3m for Glc(Phyt)2 (N ) 1), an HII phase for Mal2(Phyt)2, and an LR phase for Mal3(Phyt)2 and Mal5(Phyt)2.13,21,45 In conclusion, the AGs with isoprenoid-type hydrophobic chains allow us for the first time to control the aqueous phase structures to seek energy-efficient low-temperature applications such as in biotechnology fields. In relation to this, we have recently found that an LR phase of Mal3(Phyt)2 is useful in reconstitution of the cyanobacterial photosystem II complex, PS II, representing the first evidence that a well-designed sugar-based surfactant is useful for the reconstitution of complex and labile membrane proteins such as PS II.9 Acknowledgment. R.A.S. is grateful for the STA fellowship. The financial support of AIST is highly acknowledged (subject: Structures and Functions of Organized Solutions). This work was also performed as a part of the International Joint Research Program FY 2001 supported by NEDO. We also thank reviewers for valuable comments. LA0116185 (43) Mannock, D. A.; Lewis, R. N. A. H.; McElhaney, N. Chem. Phys. Lipids 1990, 55, 309. (44) Hato, M.; Seguer, J. B.; Minamikawa, H. Stud. Surf. Sci. Catal. 2001, 132, 725. (45) Minamikawa, H.; Hato, M. Langmuir 1998, 14, 4503.