Langmuir 2003, 19, 9981-9983
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Synthesis and Gelling Properties of N-Palmitoyl-L-phenylalanine Sucrose Esters Juliette Fitremann,*,† Alain Bouchu, and Yves Queneau‡ Laboratoire de Sucrochimie CNRS-Be´ ghin-Say, UMR 143, c/o Be´ ghin-Say, C.E.I., 66 boulevard Niels Bohr, B.P. 2132, 69603 Villeurbanne Cedex, France Received July 30, 2003. In Final Form: September 5, 2003
Introduction In the context of the use of sucrose as an organic raw material, sucrose fatty acid esters are today among the most important derivatives, useful as nontoxic and nonionic surfactants, in food or cosmetic products. While increasing and recent work1-4 is devoted to sucrose esters with saturated linear chains, such as palmitic, stearic, and lauric acid derivatives, very little work has been achieved on sucrose esters bearing other kinds of fatty chains. However, in the field of low molecular weight gelling agents, it has been shown that the introduction of structural groups such as aromatics and amide bonds, in addition to the fatty chain, could lead to compounds displaying gelling properties in organic solvents, while the introduction of a carbohydrate head opened the way to gelling properties in water.5,6 The gelling behavior is due to the formation of networks of self-assembled fibers entrapping the molecules of solvent. The self-assembly is driven by hydrophobic interactions between the chains, as is the case in micelles, but with additional interactions by π-π stacking of the aromatic rings and by hydrogen bonding between amide groups, allowing the assembly to be upheld on a longer scale. Helical supramolecular assemblies have been obtained too with similar sugar derivatives.7-9 Simple fatty acid sugar esters such as sorbitan esters displayed gelling properties, but in organic solvents.10 On the other hand, some phenylalanine derivatives are already known for their ability to give gels in organic solvents,11 but N-dodecyl-L-phenylalanine itself did not display such properties.12 While N-acyl amino * To whom correspondence should be addressed. E-mail:
[email protected]. † New address: J. Fitremann, IMRCP, UMR CNRS 5623, bat. 2r1, Universite´ Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse cedex 4, France. Tel: 00 33 5 61 55 86 96. Fax: 00 33 5 61 55 81 55. ‡ New address: Y. Queneau, Laboratoire de Chimie Organique, UMR CNRS 5181 INSA, Bat. J. Verne, 20 av. A. Einstein, F-69621 Villeurbanne cedex, France. Tel: 00 33 4 72 43 82 21. Fax: 00 33 4 72 43 88 96. E-mail:
[email protected]. (1) Ferrer, M.; Comelles, F.; Plou, F. J.; Cruces, M. A.; Fuentes, G.; Parra, J. L.; Ballesteros, A. Langmuir 2002, 18, 667-673. (2) Garofalakis, G.; Murray, B. S. Langmuir 2002, 18, 4764-4774. (3) Kabir, M. H.; Ishitobi, M.; Kunieda, H. Colloid Polym. Sci. 2002, 280 (9), 841-847. (4) Muller, A.-S.; Gagnaire, J.; Queneau, Y.; Karaoglanian, M.; Maitre, J.-P.; Bouchu, A. Colloids Surf., A 2002, 203, 55-66. (5) Jung, J. H.; John, G.; Masuda, M.; Yoshida, K.; Shinkai, S.; Shimizu, T. Langmuir 2001, 17, 7229-7232. (6) Kiyonaka, S.; Shinkai, S.; Hamachi, I. Chem.sEur. J. 2003, 9, 976-983. (7) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1991, 113, 74367437. (8) Fuhrhop, J.-H.; Blumtritt, P.; Lehmann, C.; Luger, P. J. Am. Chem. Soc. 1991, 113, 7437-7439. (9) Blanzat, M.; Massip, S.; Spe´ziale, V.; Perez, E.; Rico-Lattes, I. Langmuir 2001, 17, 3512-3514. (10) Murdan, S.; Gregoriadis, G.; Florence, A. T. J. Pharm. Sci. 1999, 88 (6), 608-614.
Figure 1. Synthesis of N-palmitoyl-L-phenylalanine sucrose esters.
acids are already known as specialist surfactants displaying biological activity,13 new surfactants made of a carbohydrate polar head, an amino acid bringing new properties, and a fatty chain have not been investigated a lot to date.14,15 In this mindset, we are describing our results on the synthesis and gelling properties of a sucrosebased surfactant, obtained by esterification of sucrose with N-palmitoyl-L-phenylalanine. This new family of sucrose esters, fully biocompatible, displaying extra rheological properties in water, could be interesting for cosmetic, biomedical, and food formulations, where nonpolymeric and nontoxic gelling agents could be useful. Results and Discussion Synthesis and Characterization. When using sucrose as an organic raw material, functionalization has to be made directly on unprotected sucrose, avoiding long synthetic pathways, which is more suitable for possible large scale preparations. In our case, esterification was achieved with N-palmitoyl-L-phenylalanine in the presence of dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP) in dimethylformamide (DMF) (Figure 1). A complex mixture of regioisomers and different substitution degrees was obtained. Its purification and characterization were made according to methods developed in our laboratory4,16 and are summarized in the following. The monoester fraction (65% yield) was purified by flash chromatography as a mixture of regioisomers and characterized by NMR, mass spectroscopy, elemental analysis, thin-layer chromatography (TLC), and highperformance liquid chromatography (HPLC). Purification of this fraction by semipreparative HPLC on a NH2-grafted column allowed us to separate the main regioisomers, characterized by 1H, 13C, and correlation spectroscopy (COSY) NMR. The following distribution was observed: (11) Bhattacharya, S.; Ghanashyam Acharya, S. N. Chem. Mater. 1999, 11, 3121-3132. (12) Luo, X.; Liu, B.; Liang, Y. Chem. Commun. 2001, 17, 15561557. (13) Ma, C.-Y.; Paquet, A.; McKellar, R. C. J. Agric. Food Chem. 1993, 41, 1182-1186. (14) Bellahouel, S.; Rolland, V.; Roumestant, M. L.; Viallefont, P.; Martinez, J. Prep. Biochem. Biotechnol. 2001, 31 (1), 71-80. (15) (a) Ishii, H.; Funakubo, T. JP 2000229921, Chem. Abstr. 133: 165449 AN 2000:585418. (b) Kretzschmar, G.; Hoppe, H.-U.; Brandstetter, T. WO 9948901, Chem. Abstr. 131:259136 AN 1999:626202. (16) Queneau, Y.; Gagnaire, J.; West, J. J.; Mackenzie, G.; Goodby, J. W. J. Mater. Chem. 2001, 11, 2839-2844.
10.1021/la035392c CCC: $25.00 © 2003 American Chemical Society Published on Web 10/10/2003
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Langmuir, Vol. 19, No. 23, 2003
Notes
Figure 4. Dynamic rheology measurements of an aqueous solution of monoesters (2 wt %) at fixed strain (2%) and (20.0 ( 0.1) °C. G′ ) storage modulus; G′′ ) loss modulus. Figure 2. TEM of an aqueous solution of monoesters, with negative staining with uranyl acetate. The fiber thickness is about 11 nm.
Figure 3. TEM of an aqueous solution of monoesters, with negative staining with sodium silicotungstate. The fiber thickness is about 5-6 nm.
3 (18)/2 (3)/6 (47)/1′ (12)/6′ (20) (/position of the chain on sucrose (%)/), showing that the grafting occurred at about 80% on the primary positions. Analytical HPLC on a C8grafted column allowed the quantification of the residual N-palmitoyl-L-phenylalanine (0-1% depending on the sample), palmitic acid (0%), and diesters (0%) in the samples, since these byproducts could affect the physicochemical behavior.4 Macroscopic and Microscopic Observations of the Gels. Gelling properties have been investigated for solutions of monoesters, as a mixture of regioisomers, at 2 wt % in water. By visual inspection, they gave loose transparent jellies that were flowing by successive cohesive lumps when transferred from one recipe to another, like an “egg white”. Observation of the jelly by transmission electron microscopy (TEM) showed the presence of a network of fibers made of self-aggregated molecules, supporting the macroscopic behavior. Depending on the method of staining used, the aspect of these fibers was different. Negative staining with uranyl acetate gave fibers having a thickness of about 11 nm and displaying “pseudocrystalline” junctions (Figure 2), while staining with sodium silicotungstate gave fibers that were about 5-6 nm thick and were less organized (Figure 3). It is known that wet samples can be affected by the conditions of preparation and of the drying in situ, in the electronic
Table 1. Viscosity of Aqueous Solutions of Monoesters (2 wt %, (25 ( 0.2) °C) shear rate (s-1)
η (mPa s)
shear rate (s-1)
η (mPa s)
30 50
210 160
100 200
100 60
microscope. An hypothesis could be that the complexation abilities of sugar with uranyl acetate would promote dimerization of the self-assemblies, which did not occur with sodium silicotungstate, less prone to complexation.18-20 On the other hand, to check the specificity of the sucroseheaded derivative, the behavior of N-palmitoyl-L-phenylalanine itself in water was checked. After neutralization by potassium carbonate, a highly hydrated mesophase of soap, at 2 wt % in water, congealed and turbid, was obtained. It did not flow when the vessel was turned upside down but was very easily broken by shaking, thus not displaying the same elasticity as the solutions of Npalmitoyl-L-phenylalanine sucrose esters did, insofar as it can be assessed by a simple visual observation. By contrast, neutralization by sodium hydroxide led to solutions displaying neither gelled nor viscous aspect, as was reported for N-dodecyl-L-phenylalanine.12 The interaction with carbonate may be an explanation for this difference, recalling similar effects described in some CO2-containing systems.17 Rheology. The viscosity of these jellies was measured at different shear rates, giving the magnitude of viscosifying power of these compounds. At low shear rate, an increase of the viscosity by a factor of 200 was observed, compared to pure water (Table 1). Stress-controlled flow rheograms have been made in order to get the yield stress. On a fresh jelly, a yield stress of about 12-15 Pa was observed, consistent with the visual observations showing that it did not flow upon a slight tilt of the vessel. Further cycles led increasingly to the destructuration of the jelly, and the yield stress became lower or disappeared completely. The recovery of a yield stress above 10 Pa after several hours of standing was observed for some samples, evidencing a partial thixotropy (see the Supporting Information). The viscoelastic behavior of a solution at 2 wt % was investigated with a dynamic rheometer (Figure 4) in order to quantify the elastic component. The storage modulus G′ remains about 10 times higher than G′′, over the whole range of frequency from 0.01 to 100 Hz, which (17) (a) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 1039310394. (b) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124-7135. (18) Terech, P. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 2 (11), 16301643. (19) Hayat, M. A.; Miller, S. E. In Negative staining; McGraw-Hill: New York, 1990; pp 13-17. (20) Gyurcsik, B.; Nagy, L. Coord. Chem. Rev. 2000, 203, 81-149.
Notes
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is distinctive of a gel, but both start to increase over 2 Hz, evidencing two areas of different behavior. The curves of G′ and G′′ may cross at very low frequencies, but this point has to be investigated further.18,21
more effective π-stacking interactions. Further work is in progress to investigate more precisely the effect of different parameters on the properties of this family of sucrose esters.
Conclusion
Acknowledgment. Be´ghin-Say and CNRS are greatly acknowledged for their financial support and for their day-to-day technical and scientific contributions. We also thank Dr. Annie Rivoire (CTM, UCBL) for the observations by TEM.
This work gives a new example of a low molecular weight gelling agent in water and, to our knowledge, is the first one based on sucrose as the starting material. The introduction of the phenylalanine group in the structure appeared to be very effective for inducing the formation of supramolecular aggregates, possibly thanks to the flexibility let to the phenyl group in the molecule, allowing (21) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 122 (47), 11679-11691.
Supporting Information Available: Synthesis and characterization of sucrose esters, HPLC methods, preparation of the gels, and rheology measurements. This material is available free of charge via the Internet at http://pubs.acs.org. LA035392C
