pubs.acs.org/Langmuir © 2009 American Chemical Society
Ordered Nanostructured Amphiphile Self-Assembly Materials from Endogenous Nonionic Unsaturated Monoethanolamide Lipids in Water Sharon M. Sagnella,† Charlotte E. Conn,‡ Irena Krodkiewska,‡ Minoo Moghaddam,† John M. Seddon,^ and Calum J. Drummond*,‡,§ †
CSIRO Molecular and Health Technologies, Bag 184, North Ryde, NSW, 1670 Australia, ‡CSIRO Molecular and Health Technologies, Bag 10, Clayton South, VIC, 3169, Australia, §CSIRO Materials Science and Engineering, Bag 33, Clayton South, VIC, 3169, Australia, and ^Chemistry Department, Imperial College London, London SW7 2AZ, U.K. Received August 12, 2009. Revised Manuscript Received October 22, 2009
The self-assembly, solid state and lyotropic liquid crystalline phase behavior of a series of endogenous n-acylethanolamides (NAEs) with differing degrees of unsaturation, viz., oleoyl monoethanolamide, linoleoyl monoethanolamide, and linolenoyl monoethanolamide, have been examined. The studied molecules are known to possess inherent biological function. Both the monoethanolamide headgroup and the unsaturated hydrophobe are found to be important in dictating the self-assembly behavior of these molecules. In addition, all three molecules form lyotropic liquid crystalline phases in water, including the inverse bicontinuous cubic diamond (QIID) and gyroid (QIIG) phases. The ability of the NAE’s to form inverse cubic phases and to be dispersed into ordered nanostructured colloidal particles, cubosomes, in excess water, combined with their endogenous nature and natural medicinal properties, makes this new class of soft mesoporous amphiphile self-assembly materials suitable candidates for investigation in a variety of advanced multifunctional applications, including encapsulation and controlled release of therapeutic agents and incorporation of medical imaging agents.
Introduction In this study, we examine the solid state and lyotropic liquid crystalline phase behavior of a series of endogenous unsaturated n-acylethanolamides (NAEs), and report a new class of soft mesoporous amphiphile self-assembly materials in aqueous solution. Amphiphiles self-assemble in the presence of a polar solvent with low volume fraction phase behavior governed by local constraints imposed by the effective shape of the molecules and strongly influenced by the competing interactions of the polar head-groups and the alkyl chains. The type of phase formed can be estimated by utilizing the critical packing parameter (CPP = ν/(lca0)), where lc is the effective length of an amphiphile chain, a0 is the effective amphiphile headgroup area, and ν is the average volume occupied by the amphiphile chain.1 Molecules with a packing parameter less than one will preferentially form normal phases, while those with a packing parameter greater than one, i.e., molecules with an effective reverse wedge shape, will preferentially form inverse phases. The fluid lamellar (LR) phase consists of a one-dimensional stack of flat amphiphilic bilayers separated by water layers and forms the basic building block of biological membranes. As the reverse wedge-shape of the molecule increases, there is an increased desire for curvature of the interface resulting in the formation of a mesophase of inverse curvature. The inverse bicontinuous cubic phases (QII) form at relatively low curvatures and have a structure based around the concept of a mathematical surface of constant zero mean curvature known as a triply *Corresponding author. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. II 1976, 72, 1525-1568.
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periodic minimal surface (TPMS). They consist of a single, continuous bilayer draped over such a surface and subdividing space into two interpenetrating, but not connected, water networks. Three types of inverse cubic phase have been identified in lipid systems.2 These are based on the Schwarz diamond (D) and primitive (P) and on the Schoen gyroid (G) minimal surfaces and have crystallographic space groups Pn3m, Im3m and Ia3d, respectively. As the wedge-shape of the molecule increases, an inverse hexagonal phase (HII) may form consisting of infinitely long cylinders of polar head-groups with hydrocarbon chains radiating outward packed onto a two-dimensional hexagonal lattice.3 An extremely high desire for curvature may result in the formation of inverse micelles where the hydrophilic headgroups are arranged toward water cores with hydrophobic chains radiating outward. These micelles may potentially pack into a cubic array of spacegroup Fd3m consisting of two different sizes of quasi-spherical micelles arranged on a face-centered cubic lattice. They may also form an entirely disordered packing resulting in a fluid inverse micellar phase, the L2 phase. Intermediate and swollen sponge-like phases have also been identified in some nonlamellar forming lipid systems.4-6 Dispersions of inverse lyotropic liquid crystalline phases such as hexagonal and cubic phases formed from aqueous amphiphile systems have been reported as an alternative to liposomes for (2) Luzzati, V.; Tardieu, A.; Gulikkrz.T.; Rivas, E.; Reisshus.F. Nature 1968, 220 (5166), 485. (3) Kaasgaard, T.; Drummond, C. J. Phys. Chem. Chem. Phys. 2006, 8, 4957-4975. (4) Conn, C. E.; Ces, O.; Mulet, X.; Finet, S.; Winter, R.; Seddon, J. M.; Templer, R. H. Phys. Rev. Lett. 2006, 96 (10), 108102. (5) Mulet, X.; Gong, X.; Waddington, L. J.; Drummond, C. J. ACS Nano 2009, 3, 2789-2797. (6) Yang, L.; Huang, H. W. Science 2002, 297 (5588), 1877-1879.
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providing matrices for sustained drug release.7-13 The properties and nanostructure of these particles make them attractive candidates for pharmaceutical and cosmetic delivery systems.7,8,10-14 Namely, these colloidal dispersions are stable against dilution, have a high internal surface area consisting of hydrophilic water channels, and are amphiphilic in nature such that they can be used to deliver both hydrophilic and hydrophobic payloads. In this study, we have attempted to achieve an inverse phase conformation through unsaturation of the hydrophobe. It has been well established that cell membranes contain a number of “non-lamellar forming lipids” capable of forming inverse hexagonal and bicontinuous cubic phases, and that chain unsaturation plays a role in the phase behavior of some of these lipids.15-17 Our group has previously demonstrated the ability to promote the formation of the inverse hexagonal phases in C18 urea amphiphiles where olefinic bonds were included (oleyl and linoleyl urea), and how small modifications to the molecular structure of the urea headgroup can have a large effect on phase behavior.18-20 Studies involving ethylene oxide amphiphiles containing a cis double bond demonstrate that these amphiphiles are able to form inverse bicontinuous cubic phases and an L3 sponge phase.21,22 Furthermore, a variety of unsaturated monoacylglycerols have been examined extensively and shown to form nonlamellar inverse lyotropic liquid crystalline phases.23-30 These studies have demonstrated the effect of degree of unsaturation, unsaturated chain length, and double bond position on phase behavior. The three monoethanolamide amphiphiles examined in this study possess similar characteristics to those previously studied, and belong to a family of naturally occurring amphiphilic molecules known as the n-acylethanolamides (NAEs). These amphiphiles make an interesting new self-assembly study group because they have been shown in a number of studies to exhibit (7) Boyd, B. J.; Whittaker, D. V.; Khoo, S. M.; Davey, G. Int. J. Pharm. 2006, 318 (1-2), 154-162. (8) Boyd, B. J.; Whittaker, D. V.; Khoo, S. M.; Davey, G. Int. J. Pharm. 2006, 309 (1-2), 218-226. (9) Drummond, C. J.; Fong, C. Curr. Opin. Colloid Interface Sci. 1999, 4, 449-456. (10) Larsson, K. J. Phys. Chem. 1989, 93, 7304-7314. (11) Larsson, K. J. Dispers. Sci. Technol. 1999, 20 (1-2), 27-34. (12) Spicer, P. T. Curr. Opin. Colloid Interface Sci. 2005, 10, 274-279. (13) Spicer, P. T.; Hayden, K. L.; Lynch, M. L.; Ofori-Boateng, A.; Burns, J. L. Langmuir 2001, 17, 5748-5756. (14) Fong, C.; Krodkiewska, I.; Wells, D.; Boyd, B. J.; Booth, J.; Bhargava, S.; McDowall, A.; Hartley, P. G. Aust. J. Chem. 2005, 58, 683-687. (15) Brenner, R. R. Prog. Lipid Res. 1984, 23 (2), 69-96. (16) Nakano, M.; Karno, T.; Sugita, A.; Handa, T. J. Phys. Chem. B 2005, 109, 4754-4760. (17) Seddon, J. M.; Squires, A. M.; Conn, C. E.; Ces, O.; Heron, A. J.; Mulet, X.; Shearman, G. C.; Templer, R. H. Philos. Trans. R. Soc. A: Math. Phys. Eng. Sci. 2006, 364 (1847), 2635-2655. (18) Fong, C.; Wells, D.; Krodkiewska, I.; Hartley, P. G.; Drummond, C. J. Chem. Mater. 2006, 18, 594-597. (19) Fong, C.; Wells, D.; Krodkiewska, I.; Weerawardeena, A.; Booth, J.; Hartley, P. G.; Drummond, C. J. J Phys Chem B 2007, 111, 10713-10722. (20) Wells, D.; Fong, C.; Drummond, C. J. J Phys Chem B 2006, 110, 12660-12665. (21) Funari, S. S.; Holmes, M. C.; Tiddy, G. J. T. J. Phys. Chem. 1992, 96, 11029-11038. (22) Walsh, J. M.; Tiddy, G. J. T. Langmuir 2003, 19, 5586-5594. (23) Briggs, J.; Caffrey, M. Biophys. J. 1994, 66, 1263. (24) Clogston, J.; Rathman, J.; Tomasko, D.; Walker, H.; Caffrey, M. Chem. Phys. Lipids 2000, 107 (2), 191-220. (25) de Campo, L.; Yaghmur, A.; Sagalowicz, L.; Leser, M. E.; Watzke, H.; Glatter, O. Langmuir 2004, 20, 5254-5261. (26) Misquitta, Y.; Caffrey, M. Biophys. J. 2001, 81, 1047-1058. (27) Qiu, H.; Caffrey, M. J. Phys. Chem. B 1998, 102, 4819-4829. (28) Qiu, H.; Caffrey, M. Chem. Phys. Lipids 1999, 100 (1-2), 55-79. (29) Qiu, H.; Caffrey, M. Biomaterials 2000, 21 (3), 223-234. (30) Takahashi, H.; Matsuo, A.; Hatta, I. Phys. Chem. Chem. Phys. 2002, 4, 2365-2370. (31) Axelrod, J.; Felder, C. C. Neurochem. Res. 1998, 23, 575-581. (32) Felder, C. C.; Glass, M. Annu. Rev. Pharmacol. Toxicol. 1998, 38, 179-200. (33) Porter, A. C.; Felder, C. C. Pharmacol. Therapeut. 2001, 90 (1), 45-60.
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Figure 1. Molecular models of unsaturated fatty acid monoethanolamide amphiphiles.
active biological and medicinal properties.31-33 Among these, oleoyl monoethanolamide has been shown to play a role in modulating feeding behavior.34,35 Linoleoyl monoethanolamide inhibits fatty acid amide hydrolase (FAAH) hydrolysis of anandamide, and undergoes fatty acid amide lipoxygenase metabolism in vivo.36 The prolonged lifetime of anandamide by linoleoyl monoethanolamide could have a considerable effect on cellular signaling. The final molecule examined, linolenoyl monoethanolamide, has been detected in the porcine brain; however, its functional role has not yet been elucidated.37 Despite many studies involving the biological properties of this fatty acid monoethanolamide class of molecules, very little is known about their self-assembly materials-related properties. As previously mentioned, the unsaturated fatty acid monoethanolamides are ideal candidates for forming inverse phases in that they consist of a small monoethanolamide headgroup and highly cis-kinked hydrophobes potentially giving them the desired reverse wedge shape (Figure 1). Hence, the amphiphilic structure of the fatty acid monoethanolamides combined with their pharmacological properties makes it interesting to investigate their self-assembly behavior in water. To our knowledge, this is the first report comparing the lyotropic phase behavior of these three unsaturated C18 NAE’s.
Experimental Section Materials. All reagents were obtained from Sigma-Aldrich. Organic solvents were either of analytical or spectroscopic grade and used as received. A Milli-Q Plus Ultrapure water system (Millipore, Australia) was used to filter deionized tap water to obtain high purity water. P407, the polymer used for stabilization of the cubosomes, was also purchased from Sigma-Aldrich. (34) Lo Verme, J.; Gaetani, S.; Fu, J.; Oveisi, F.; Burton, K.; Piomelli, D. Cell. Mol. Life Sci. 2005, 62, 708-716. (35) Oveisi, F.; Gaetani, S.; Eng, K. T. P.; Piomelli, D. Pharmacol. Res. 2004, 49, 461-466. (36) vanderStelt, M.; Paoletti, A. M.; Maccarrone, M.; Nieuwenhuizen, W. F.; Bagetta, G.; Veldink, G. A.; Agro, A. F.; Vliegenthart, J. F. G. FEBS Lett. 1997, 415, 313-316. (37) Hanus, L.; Gopher, A.; Almog, S.; Mechoulam, R. J. Med. Chem. 1993, 36, 3032-3034.
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prepared by weighing out an amount of the monoethanolamide amphiphile in glass ampules, and then adding the necessary quantity of water by weight. Samples ranging from 5 wt % amphiphile in water to 95 wt % amphiphile in water were prepared. The glass ampules were sealed, and the amphiphile was initially dissolved (as much as possible) by heating the samples followed by rapid cooling. Samples were then submerged into a glass water bath with cross polarizing film attached to the outside of the water bath. The water temperature was controlled and monitored by a polystat cc1 water bath heater (Crown Scientific Pty. Ltd., Sydney, Australia) equipped with an internal digital thermometer. The temperature was incrementally increased and then left to allow the samples to equilibrate properly. Samples were illuminated and examined through the cross polarizing filters to determine phase transitions.
Small Angle X-ray Scattering (SAXS). Small angle X-ray diffraction measurements were used for definitive phase assignment and to obtain lattice parameters for all samples. Samples were made up to the required water content by adding a known volume of HPLC-grade water to the preweighed dry lipid. To ensure homogeneity samples were allowed to equilibrate for a period of not less than 1 week. Preliminary 2-D SAXS measurements were carried out at Imperial College, London and the Advanced Photon Source (APS), Argonne, Illinois. For oleoyl monoethanolamide and linoleoyl monoethanolamide samples some scattering images were obtained using a custom-built SAXS beamline at Imperial College London. A copper target Bede microsource X-ray generator (Durham, U.K.) with integrated glass polycapillary optics produced a beam, 300 μm in diameter, at the sample. Diffraction images were acquired on an image-intensified CCD (chargecoupled device) Photonic Science Gemstar detector (East Sussex, U.K.). The sample holder has computer-controlled Peltier-based temperature control over a range of -30-120 C with an accuracy of ( 0.5 C, calibrated with samples of known transition points. For linolenoyl monoethanolamide preliminary measurements were carried out at the 15-ID-D (ChemMatCARS) beamline of the Advanced Photon Source, Argonne, Illinois.39 The experiments used a beam of wavelength λ = 1.50 A˚ (8.27 keV) with dimensions 200 μm x 100 μm and a typical flux of 1 1012 photons/s. 2-D diffraction images were recorded on a Bruker 6000 CCD detector with an active area of 94 94 mm2 and a pixel size of 92 μm. The CCD detector was offset from the main beam allowing analysis in the q-range 0.0187-0.807 A˚-1 at a sample to detector distance of 0.6 m. Temperature control was in the range 7-90 C. Temperature scan experiments were then carried out on all samples using the HUBER and SOL beamlines at Imperial College London. These have a motorized film carriage and controllable heating element allowing the generation of continuous SAXS/WAXS patterns as a function of temperature. For both beamlines X-rays were produced using a Phillips PW2213/20 generator operating at 40 kV and 30 mA through an AEG type 50/ 21 X-ray tube. The X-rays were monochromated using a quartz crystal monochromator which isolates the Cu radiation and a line focus beam is generated. The diffraction pattern therefore does not consist of a set of concentric circles but instead a series of symmetrical lines that are detected using X-ray sensitive photographic negative (Kodak, MR). Line X-ray sources present problems for the analysis of aligned samples. Here a rotation device attached to the SOL beamline allowed for rotation of the sample holder about the beam direction and was used to produce a pseudopowder pattern for the linoleoyl monoethanolamide and linolenoyl monoethanolamide samples which form partially aligned cubic phases. The spacing range of the beamlines was from 105 to 2 A˚ enabling both wide and small-angle X-ray scattering to be observed. Samples were scanned between 0 and 75 C at a rate of either 2 or 4 C/h, while the carriage was moved at a rate of either 0.06 or 0.03 mm/min. Liquid crystalline mesophases give rise to distinct diffraction patterns which may be used as an unambiguous identification for each phase, provided an adequate number of reflections are observed. All images were analyzed using AXcess, a custom-built SAXS analysis program written by Dr. Andrew Heron: details on the use of AXcess for SAXS analysis are reported in a review article.17 Production and Characterization of Dispersions. Dispersions of linoleoyl monoethanolamide were prepared by a modification of the methods of Fong et al.14 P407 (0.75% w/w) was first dissolved in water and then 10% w/w amphiphile was added to the solution. The amphiphile/polymer solution was mixed at elevated temperature under shear using an Ultraturrax homogenizer for 5 min. The coarse dispersion was rapidly transferred to
(38) Ramakrishnan, M.; Sheeba, V.; Komath, S. S.; Swamy, M. J. Biochim. Biophys. Acta: Biomembr. 1997, 1329 (2), 302-310.
(39) Cookson, D.; Kirby, N.; Knott, R.; Lee, M.; Schultz, D. J. Synchrotron Radiat. 2006, 13, 440-444.
Monoethanolamide Amphiphile Synthesis. Monoethanolamide amphiphiles were prepared as described previously.38 Briefly, the desired fatty acid (C18:1, C18:2, C18:3) was placed in a round-bottom flask and dissolved in dichloromethane (DCM). The flask was placed in an ice bath and 2 mol equiv of oxalyl chloride was added with vigorous stirring. The reaction was left in the ice bath for 10 min with stirring. The flask was then sealed and the atmosphere was evacuated with a vacuum. Nitrogen was added to the flask, and the reaction was stirred for 2 h at room temperature under nitrogen. At the end of 2 h, the excess oxalyl chloride was evaporated, and the resulting fatty acid chloride was dissolved in DCM. The fatty acid chloride was slowly added dropwise to ethanolamine (2 mol equiv ) in DCM in an ice bath with rapid stirring. Again, the reaction was left for 10 min in the ice bath. It was then sealed, the atmosphere was evacuated, and nitrogen was added. The flask was returned to the ice bath and left to react with stirring for 2 h. The resulting product was filtered twice using Whatman 542 filter paper. The filtered solution was then sequentially rinsed with 4% citric acid, 4% sodium bicarbonate solution, and Milli-Q water. Purity of the monoethanolamide amphiphile was determined by HPLC, NMR, and LC/MS. Differential Scanning Calorimetry (DSC). DSC was performed using a Mettler DSC822e system with a Mettler TSO 801RO sample robot (Mettler Toledo, Melbourne, Australia). Samples were run at scan rates of 10 and 2.5 C/min and data collected using the STARe software package (Mettler Toledo, Melbourne, Australia). Certain samples were run at 0.1 C/min to resolve overlapping peaks. Temperature calibration ((0.3 C) of the ceramic sensor was performed using octane, water, indium, and zinc. Integration of a standard indium peak was used for thermal calibration. The energies and peak temperatures of the endotherms of the monoethanolamide amphiphiles were determined using the STARe software package. Water Penetration into Monoethanolamide Amphiphiles. A small amount of crystalline monoethanolamide amphiphile was placed onto a microscope slide and heated to melting. A coverslip was placed on top of the melted amphiphile and then cooled to room temperature prior to addition of water. Water placed at the edges of the coverslip was drawn between the two glass surfaces to surround the solidified material by capillary action. The microscope slide was placed into a Linkam PE94 hot stage (Linkam Scientific Instruments Ltd., Surry, England) and heated at 1 C/ min or less. The interaction of water and the monoethanolamide amphiphile was observed with an Olympus GX51 inverted optical microscope (Olympus Australia Pty. Ltd., Melbourne, Australia) via polarizing optical microscopy (POM) in the presence and absence of cross polarizing lenses. Images were captured with an Olympus c-5060 digital camera (Olympus Australia Pty. Ltd., Melbourne, Australia).
Binary Phase Behavior of Monoethanolamide Amphiphiles. Monoethanolamide amphiphile-water mixtures were
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Article Table 1. Thermal Transitions for Neat Monoethanolamide Lipids Determined from DSC Scanned at 2.5C/mina
amphiphile
transition temperatures (C)
transition enthalpy (kJ/mol)
melting point (C)
, 22.4 , -31.1 22.6 ( 0.4 γ-linolenoyl monoethanolamide (C18:3) linoleoyl monoethanolamide (C18:2) (18.41), 39.24 (-2.3), -30.2 39.0 ( 0.5 oleoyl monoethanolamide (C18:1) (21.0), (36.2), 62.1 (-0.6), (-3.2), -30.8 63.4 ( 0.6 (84), (94), 103 (-2.8), (-6.8), -54.2 102.8 ( 0.5 stearoyl monoethanolamide (C18:0)b a Values in parentheses refer to a transition that occurs before the crystal-isotropic liquid transition. Melting point determined by visual inspection. b Data added for comparison. c Unable to resolve pretransition even at slow scan rates. c
Figure 2. Differential scanning calorimetry of oleoyl monoethanolamide, linoleoyl monoethanolamide, and linolenoyl monoethanolamide. Scan Rate 2.5 C/min. a high pressure homogenizer (Avestin C5) and subjected to recirculation at 170 MPa (25000 psi) for 30 min. Once the dispersion cooled to room temperature, the particle size (1:10 dilution) was measured using a Malvern Zetasizer Nano ZS (Malvern Instruments, Sutherland, Australia). Cryo-Transmission Electron Microscopy (TEM) was employed to visualize the nanostructure of the dispersed mesophases. A drop of the lyotropic mesophase dispersion was placed on a perforated carbon coated TEM grid and gently blotted with filter paper to obtain a thin liquid film (20-400 nm). The grid was then quickly plunged into liquid ethane at -180 C and transferred into liquid nitrogen (-195 C). The sample was then transferred to a cryo-TEM (TECNAI 30 or TECNAI 12). The working temperature was kept below -180 C, and images were acquired digitally with a CCD camera. Molecular Modeling. Chem 3D pro v10 (Cambridgesoft Corporation) was used to obtain the energy minimum conformation using its MM2 energy minimization routine. This software package ceases optimization at local minimums. Therefore, when a change from linearity occurred, bonds were rotated manually, and minimization was continued. Thus, an optimum energy minimum conformation was obtained.
Results and Discussion Differential Scanning Calorimetry. DSC scans run at 2.5 C/min for each of the monoethanolamide amphiphiles are shown in Figure 2. DSC scans run at 10 C/min (not shown) were started at -130 C, and no additional peaks were observed at temperatures below those shown in Figure 2. Table 1 displays the melting point transition temperatures and enthalpies for the transitions, along with melting points obtained from visual observations. Transition temperatures were obtained from the peak maxima of the endotherms. Enthalpies were obtained by integration of the transition peaks. The melting points obtained by DSC for the monoethanolamide amphiphiles were dependent on the degree of unsaturation in the hydrocarbon chain. The crystal-isotropic transition temLangmuir 2010, 26(5), 3084–3094
c
peratures obtained by visual inspection were similar to those recorded by DSC. As the degree of unsaturation increases, the crystal-isotropic transition temperature decreases. The relationship between the melting transition temperature and the degree of unsaturation for the C18 monoethanolamide amphiphiles as well as, the literature values for the corresponding fatty acids, ureas, amides, alcohols, and monoglycerides are plotted on the same graph and can be viewed in the electronic Supporting Information (Figure S1).20,40-44 The melting behavior of C18 amphiphiles with different polar head groups follows the general trend: ureas > amides > monoethanolamides > monoglycerides > fatty acids > alcohols. The melting points of monoethanolamide amphiphiles, similar to the monoglycerides, display a more linear dependence on the degree of unsaturation than the other C18 amphiphiles. Previous studies by our group have indicated that the strong bifurcated hydrogen bonding between urea moieties leads to the extremely high melting points for the urea amphiphiles.18,20,40,45,46 Because of the similarity in molecular structure, monoethanolamide amphiphiles would be capable of H-bonding in the polar moiety, although to a weaker extent than for the urea headgroup, thus accounting for lower melting temperatures than those of the urea counterparts but higher melting over their analogous fatty acids and alcohols. In addition, the stiff kink introduced via cis unsaturation effectively disrupts the close packing of the crystal structure thereby leading to a decrease in melting point with an increase in unsaturation. The resulting combination of the monoethanolamide headgroup with the unsaturated alkyl chains results in a more uniform, monotonic change in melting transition at higher temperatures than for any of the other analogous nonionic amphiphiles suggesting that a balance exists between the role of hydrogen bonding and unsaturation in dictating melting behavior. Endothermic pretransitions (shown in Table 1) appear before the melting transition for all three unsaturated monoethanolamide amphiphiles, similar to what has been seen with long saturated chain fatty acids.38,47 Oleoyl monoethanolamide shows two transitions while linoleoyl monoethanolamide only displays one. Linolenoyl monoethanolamide displays a broad melting transition with a very small transition immediately before the melting transition, which is difficult to resolve even at a scan rate of 0.1 C/min. Pure oleic acid displays a single pretransition which has been ascribed to a well characterized polymorphic transition from the γ- to R-form. The difference between the two forms has to do with the conformation of the section of the hydrocarbon (40) Gong, X. J.; Sagnella, S.; Drummond, C. J. Int. J. Nanotechnol. 2008, 5 (2-3), 370-392. (41) Linstrom, P. J.; Mallard, W. G. E. NIST Chemistry WebBook; NIST Standard Reference Database Number 69. (42) Skau, E. L.; Mod, R. R.; Harris, J. A.; Mitcham, D. J. Chem. Eng. Data 1980, 25, 88-89. (43) Turpeinen, O. J. Am. Chem. Soc. 1938, 60, 56-57. (44) Lutton, E. S. J. Am. Oil Chem. Soc. 1965, 42, 1068. (45) Wells, D.; Drummond, C. J. Langmuir 1999, 15, 4713-4721. (46) Wells, D.; Fong, C.; Krodkiewska, I.; Drummond, C. J. J. Phys. Chem. B 2006, 110, 5112-5119. (47) Wouters, J.; Vandevoorde, S.; Culot, C.; Docquir, F.; Lambert, D. M. Chem. Phys. Lipids 2002, 119 (1-2), 13-21.
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Figure 3. Optical microscope image of water penetration into oleoyl monoethanolamide (A) with cross polarizers at 45 C and (B) without cross polarizers at 55 C. Between 45 and 55 C, an anisotropic phase (seen in part A) forms in addition to 3 isotropic phases. Above 55 C, the anisotropic phase disappears and only the three isotropic phases remain (seen in part B). (C) Water penetration into linoleoyl monoethanolamide at 30 C. Three isotropic phases are present at the water/amphiphile interface. (D) Optical microscope image of water penetration into γ-linolenoyl monoethanolamide at 25 C. Three isotropic phases are present at the water/amphiphile interface.
chain between the double bond and the terminal methyl group which retains an all trans structure in the γ-form, while possessing a disordered liquid-like conformation in the R-form.48-50 Similar results were seen for DSC scans of pure linoleic and R-linolenic acid, which displayed two and three pretransitions respectively corresponding to different polymorphic forms of these molecules. These transitions are most likely attributed to conformation changes to the fatty acid chains similar to what occurs with oleic acid.50 In fact, a variety of polymorphic forms have been identified for different cis-monounsaturated fatty acids, and due to the similarities seen for linoleic and R-linolenic acid, it is easy to attribute similar polymorphic transitions to other cis-polyunsaturated fatty acids.51 Inspection of neat oleoyl monoethanolamide by polarized optical microscopy displayed polymorphic changes in the crystal structure at temperatures similar to the pretransitions seen via DSC (Supporting Information, Figure S2). Lyotropic Phase Behavior of Monoethanolamide Amphiphiles. Water Penetration into Monoethanolamide Amphiphiles. Water penetration scans involving the direct observation of the birefringence of lyotropic phases via cross-polarized microscopy were initially used to provide a simple and rapid assessment of the lyotropic phase behavior of novel amphiphile systems. Under cross polarizers, cubic and micellar phases appear as dark bands while anisotropic phases such as lamellar and hexagonal phases are birefringent with well characterized textures.52 In addition, examination of isotropic phases optically without cross polarizers allows for the determination of the number of isotropic phases present based on refractive index discontinuities. The sequence of phases formed from the water/ amphiphile interface along with their evident viscosity and texture (48) Inoue, T.; Hisatsugu, Y.; Ishikawa, R.; Suzuki, M. Chem. Phys. Lipids 2004, 127 (2), 161-173. (49) Inoue, T.; Hisatsugu, Y.; Yamamoto, R.; Suzuki, M. Chem. Phys. Lipids 2004, 127 (2), 143-152. (50) Ueno, S.; Miyazaki, A.; Yano, J.; Furukawa, Y.; Suzuki, M.; Sato, K. Chem. Phys. Lipids 2000, 107 (2), 169-178. (51) Kaneko, F.; Yano, J.; Sato, K. Curr. Opin. Struct. Biol. 1998, 8, 417-425. (52) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628-639.
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formation provide circumstantial evidence for phase identification. Amphiphile/water mixtures ranging from 5 wt % water to 95 wt % water were then made in sealed ampules and examined over a range of temperatures to construct a preliminary partial binary phase diagram. SAXS experiments were run at various concentrations sampling the region of interest determined from the partial binary phase diagram in order to provide definitive phase assignment and lattice parameters. POM measurements indicated that all three unsaturated NAE’s form self-assembly phases in water. The phase behavior observed for all three compounds was quite similar to each forming three isotropic phases (Figure 3). A lamellar phase was also visualized for oleoyl monoethanolamide (Figure 3A). Initial water penetration scans using cross-POM indicated that oleoyl monoethanolamide, which has the highest melting point, forms three isotropic phases at 45 C, all of which are present up to 65 C, Figure 3B. An anisotropic band is also observed between 45 and 55 C at low water content, Figure 3A, which is attributed to the presence of a fluid lamellar phase over a narrow concentration range. Above 65 C, the amphiphile/water mixture becomes an isotropic melt. In the case of linoleoyl monoethanolamide, three isotropic phases form between 20 and 30 C (Figure 3C), two between 30 and 37 C and the system becomes an isotropic melt above 37 C. Water penetration scans of linolenoyl monoethanolamide are quite similar to those of linoleoyl monoethanolamide, with the exception of the temperatures at which the isotropic phases begin to form. In the case of linolenoyl monoethanolamide, the three isotropic phases that form at 30 C and below are stable down to ∼0 C (Figure 3D), two isotropic phase are present from 30 to 35 C, and above 35 C, it becomes an isotropic melt. Binary Phase Behavior. Partial binary phase diagrams of oleoyl, linoleoyl, and linolenoyl monoethanolamide were constructed from samples with a controlled water content ranging from 5 wt % water to 95 wt % water sealed in glass ampules allowing us to define approximate phase boundaries in both composition and temperature. These are shown as “hashed” areas in Figure 4. Langmuir 2010, 26(5), 3084–3094
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Figure 4. (A) Partial phase diagram for the oleoyl monoethanolamide - water system showing results from SAXS experiments and from
polarizing microscopy. SAXS results show an Lc (9), a putative QIIG (]), a QIID (O) and a fluid isotropic phase (2). Polarizing microscopy results are shown as hashed areas and include a crystalline phase, a fluid isotropic phase and a viscous isotropic phase. The excess water point is marked by a dashed line at approximately 29 wt % H2O. B.) Partial phase diagram for the linoleoyl monoethanolamide - water system showing results from SAXS experiments and from polarizing microscopy. SAXS results show two Lc phases, Lc(1) (9) and Lc(2) (), a QIIG cubic phase (]), a QIID cubic phase (O) and a fluid isotropic phase (2). Polarizing microscopy results are shown as hashed areas and include a crystalline phase, a fluid isotropic phase and a viscous isotropic phase. The excess water point is marked by a dashed line at approximately 33 wt % H2O. C.) Partial phase diagram for the linolenoyl monoethanolamide- water system showing results from SAXS experiments and from polarizing microscopy. SAXS results show a QIIG cubic phase (]), a QIID cubic phase (O) and a fluid isotropic phase (2). Polarizing microscopy results are shown as hashed areas and include a fluid isotropic phase and a viscous isotropic phase. The excess water point is marked by a dashed line at approximately 55 wt % H2O. Langmuir 2010, 26(5), 3084–3094
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Both oleoyl monoethanolamide and linoleoyl monoethanolamide display a fully crystalline phase. In the case of oleoyl monoethanolamide, this crystalline phase forms below 44 C, whereas for the linoleoyl monoethanolmide, this occurs at a much lower temperature (∼18 C). A crystalline phase was not observed for linolenoyl monoethanolamide, which can be attributed to the fact that crystalline phase formation occurs at sub zero temperatures in this system. At higher temperatures all three unsaturated monoethanolamides form a fluid isotropic phase at low water contents which is assigned to an L2 micellar phase. At mid range temperatures this L2 phase occurs at e10 wt % water in both the oleoyl and linoleoyl monoethanolamide systems, and at glycerate ≈ glycerol ether ≈ urea). We note that formation of the highly curved hexagonal phase is promoted by extremely small headgroup size and/or strong H-bonding (i.e., oleyl 2,3DHPU and OBU), while those systems forming inverse bicontinuous cubic phases at room temperature tend to have either slightly reduced H-bonding (monoacylglycerides, monoethanolamides, 1,3HEU) and/or a larger headgroup (ethylene oxides, 1,3HEU). The molecular structure of the monoethanolamide headgroup is quite similar to that of the monoglycerides, although just slightly smaller and with one less hydroxyl group, and the phase behavior of the unsaturated C18 monoethanolamides is quite similar to that of their monoglyceride analogues with a few exceptions. As described above, both systems form bicontinuous cubic, rather than hexagonal, phases at room temperature. However while most C18 monoglyceride amphiphiles will form an HII phase, albeit at higher temperatures, this phase is absent for those with a monoethanolamide headgroup. The absence of HII is also seen for 18:1c9(EO)4, but here can be accounted for by the much larger headgroup size. As the monoethanolamide amphiphiles have similar headgroup size to those of the monoacylglycerides, we explain its absence by a consideration of the strength of H-bonding within the system. As stated above, in the case of monoolein and monovaccenin, HII formation occurs only at high temperatures. This behavior can mostly be rationalized by the changes that occur to the hydrophobic chain at high temperatures. As the temperature of the system is increased, conformational fluctuations occur resulting in an effective decrease in average chain length resulting in an increased packing and spontaneous curvature leading to HII phase formation. However, by this rationale alone, the unsaturated C18 monoethanolamides would also be expected to exhibit an HII at high temperatures, which is not the case. H-bonding can be used to explain this difference. Stronger H-bonding in the monoethanolamide headgroup than in the monoglycerides results in a difference in the thermal coefficient of expansion for the hydrocarbon region thus preventing the effective hydrocarbon chain volume from expanding to the same degree as for the monoglycerides. Instead, the system effects a direct transition to an isotropic melt at high temperatures. An interesting feature of the unsaturated C18 monoethanolamide amphiphile series is their ability to systematically form two bicontinuous cubic phases as a function of degree of unsaturation. In fact, the temperature at which the bicontinuous cubic phase formation occurs decreases in an almost linear fashion with an increase in the degree of unsaturation. Moreover, the QIID cubic phase can be easily dispersed in excess water to form cubosomes for payload delivery purposes. This is demonstrated here with linoleoyl monoethanolamide which has been successfully dispersed in excess water to form cubosomes with an average size of ∼136 nm (Figure 7). Langmuir 2010, 26(5), 3084–3094
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Figure 7. Cryo-TEM image of a cubosome dispersion made from 10% w/w linoleoyl monoethanolamide stabilized with 0.75% w/w P407.
Similar to what has been reported for monoolein, at low temperatures, both oleoyl monoethanolamide and linoleoyl monoethanolamide form a crystalline packing of stacked bilayers for all water contents studied. Upon increasing temperature this crystalline packing is lost and, at higher water contents, the system forms an inverse bicontinuous cubic mesophase. The temperature at which mesophase behavior is accessed is highly dependent upon the degree of unsaturation of the hydrocarbon chain. This behavior is due to the fact that amphiphile chains must be fluid in order to adopt a cubic phase, and as the degree of unsaturation increases, the temperature at which the hydrophobe transforms from a crystalline chain to a fluid chain decreases. Hence, for oleoyl monoethanolamide, which is singly unsaturated, mesophase behavior is not accessed until ca. 44 C; for linoleoyl monoethanolamide, which is doubly unsaturated, mesophase behavior is accessed by 18 C while for linolenoyl monoethanolamide which contains three double bonds mesophase behavior is observed right down to 0 C, the lowest temperature studied for this system. This is similar to what is seen for the monoglycerides and may again be explained by changes in chain geometry with increasing unsaturation. An increase in the number of double bonds in the hydrocarbon chain results in an effective increase in volume occupied by that chain. This increased volume results in a more wedge-shaped molecule with an increased desire for curvature, and thus phase formation will occur at lower temperatures. This is apparent, not only when comparing the phase behavior of the three monoethanolamides examined in this study, but also when comparing monolinolein to monoolein. The phase diagrams for the two monoglcyerides appear almost identical, but the monolinolein phase diagram is shifted downward to lower transition temperatures.25,29 A similar trend is not observed for the urea amphiphiles.18-20,40 Here the temperature at which formation of the highly curved inverse hexagonal phase occurs is similar for oleoyl and linoleoyl urea, appearing independent of chain unsaturation.3,20 We explain this in terms of the strength of H-bonding. Unlike either the monoethanolamides or the monoglycerides, the urea amphiphiles form an extremely strong bifurcated H-bond network that dominates their physicochemical behavior with a concomitant reduction in the effect of chain geometry. This is also reflected in the melting points. For urea amphiphiles the melting points are independent of degree of unsaturation, while we have shown that the melting behavior of the unsaturated monoethanolamides is dependent on the degree of unsaturation (Figure S1). All of the NAEs form both a QIIG and a QIID cubic phase, with the QIIG phase consistently occurring to the low water side of the QIID phase region. Such behavior is in accord with the vast Langmuir 2010, 26(5), 3084–3094
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majority of experimental phase diagrams published for amphiphile systems.3 These universally show that the bicontinuous cubic phases appear with increasing hydration in the order QIIG < QIID < QIIP, although not all three bicontinuous cubic phases will necessarily occur for a given lyotropic system. Such behavior has been previously explained in terms of the minimum water content required to keep open the narrow necks of lipid bilayer for each cubic phase10,56 and by considering the “topological index” (a mathematical construct that is related to the porosity of a phase) which predicts the phase sequence shown above.57,58 The relative hydrations of the QIIG and QIID phases also allow us to explain the temperature dependence of the lattice parameter of these phases. For the QIID phase, we observe a decrease in lattice parameter with increasing temperature for all three NAEs. This reflects the increased desire for curvature of the system at higher temperatures (because of increased chain splay due to thermally activated trans-gauche isomerism) resulting in a reduced unit cell size. However for the QIIG phase either a small decrease (oleoyl monoethanolamide) or no decrease at all is observed (linoleoyl and linolenoyl monoethanolamide). Although the QIIG phase experiences the same temperaturedependent change in spontaneous curvature, the limited water available here constrains the lattice parameter damping the temperature effect. We consider also how chain unsaturation and headgroup size affect lattice parameter at the same water content. Figure S7 displays the change in lattice parameter with temperature of the QIID phase for a variety of amphiphile systems at 40 wt % water. This water content was chosen based on the fact that it corresponds to the excess water point for two of the monoethanolamides examined and is widely reported in the literature for the monoglycerides. Although not all of these systems form the QIID phase at the same temperature, the clear trends in lattice parameter admit extrapolation of the graphs allowing a direct comparison to be made between the different systems. We consider first the variation in chain unsaturation. At the same temperature we observe that lattice parameter decreases with increasing chain unsaturation in the order oleoyl > linoleoyl > linolenoyl. This is true for both the monoethanolamide and monoglyceride headgroups. Such behavior may be explained by a consideration of the change in preferred curvature of the different chain geometries. At the same temperature the desire for curvature of the system will increase with increasing chain unsaturation with a concomitant reduction in lattice parameter. Headgroup size is also observed to affect the lattice parameter of the system. Although the monoglyceride headgroup is only slightly larger than the monoethanolamide headgroup a clear increase in lattice parameter is observed between these two systems with monoolein having a larger lattice parameter than oleoyl monoethanolamide and monolinolein a larger lattice parameter than linoleoyl monoethanolamide.
Conclusions The current study has demonstrated the ability of a series of endogenous fatty acid monoethanolamides with an increasing degree of hydrocarbon unsaturation to form bicontinuous cubic phases over a broad range of temperatures. The introduction of a chain kink via the addition of cis unsaturated bonds to the (56) Hyde, S.; Andersson, S.; Larsson, K.; Blum, Z. The Language of shape: the role of curvature in condensed matter: physics, chemistry and biology; Elsevier: Amsterdam and New York, 1997; pp xii, 383. (57) Schwarz, U. S.; Gompper, G. Phys. Rev. Lett. 2000, 85, 1472-1475. (58) Schwarz, U. S.; Gompper, G. Langmuir 2001, 17, 2084-2096.
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hydrocarbon chain reduces the effective length of the chain thereby restricting the packing ability of the molecules. This conformation combined with a relatively small polar headgroup results in an effective wedge-shaped conformation that is ideal for forming inverse self-assembly phases. In this case, the proper balance of headgroup association and effective hydrocarbon chain volume results in the formation of both QIID and QIIG phases for all three molecules. We report a new class of nanostructured amphiphile selfassembly materials based on endogenous nonionic monoethanolamide lipids in water. In addition to the endogenous nature of the lipids which makes these materials potentially biocompatible, all three amphiphiles form inverse bicontinuous cubic phases that are stable in excess water, and they all possess biological/medicinal properties making them suitable candidates for further investigation as therapeutic matrices. Moreover, the inverse bicontinuous cubic phase of linoleoyl monoethanolamide has been successfully dispersed into cubosomes which are stable at both room temperature and physiological temperature, demonstrating the ability of this class of nanostructured self-assembly amphiphiles to be readily administered as colloidal dispersions. Furthermore, although these molecules possess inherent biological activity, their ability to self-assemble also makes them potentially useful vehicles for the encapsulation and controlled release of hydrophilic, hydrophobic, and amphiphilic drugs. Consequently, selfassembled unsaturated monoethanolamide lipids in water are prospective soft mesoporous materials for a range of drug delivery/medical applications. Acknowledgment. The authors would like to thank Dr. David Cookson and Dr. Robert Knott for their assistance with experiments carried out at the ChemMatCARS beamline, sector 15 at the Advanced Photon Source. Use of the ChemMatCARS sector 15 at the Advanced Photon Source
3094 DOI: 10.1021/la903005q
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was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. ChemMatCARS Sector 15 is principally supported by the National Science Foundation/Department of Energy under Grant No. CHE-0535644. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The work at Imperial College London was partially supported by EPSRC (U.K.) Platform Grants GR/S77721 and EP/G00465X. The authors thank Beatrice Gauthe and Nick Brooks for their kind assistance with SAXS experiments carried out at Imperial College London. C.J.D. is the recipient of an Australian Research Council Federation Fellowship. Supporting Information Available: Figures showing melting points for the monoethanolamides and other families of unsaturated C18 amphiphiles, optical microscopy images of neat oleoyl monoethanolamide, representative temperature scans of diffraction data for the oleoyl monoethanolamide, linoleoyl monoethanolamide, and linolenoyl monoethanolamide systems at various water contents, representative 2-D diffraction images for the oleoyl monoethanolamide, linoleoyl monoethanolamide, and linolenoyl monoethanolamide systems at various water contents, and lattice parameters of the QIID phase of oleoyl monoethanolamide, linoleoyl monoethanolamide, linolenoyl monoethanolamide, monoolein, and monolinolein at 40 wt % water and tabulated data of the lattice parameters of the oleoyl monoethanolamide, linoleoyl monoethanolamide, and linolenoyl monoethanolamide systems extracted as a function of water content and temperature. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(5), 3084–3094
