36
Langmuir 1993,9, 36-38
Configurational Changes Accompanying Vesiculation of Mixed Single-Chain Amphiphiles Michael Ambiih1,t Felix Bangerter,t Pier L. Luisi,t Peter Skrabal,$ and Heribert J. Watzke'st Inatitut fiir Polymere and Laboratorium fiir Technische Chemie, ETH Ziirich, CH-8092 Ziirich, Switzerland Received November 10, 1992
A new and versatile self-vesiculatingamphiphile system based on amino acid surfactanta is described which allows the investigation of changes in surfactant organization. Vesicle formation from catanionic micellar mixtures was confirmed by freeze fracture electron microscopy and quasi-elastic light scattering measurements. Molecular reorganization of the N-methylated amino acid headgroups upon vesiculation (E- end 2-configuration equilibrium) was revealed by using sodium N-lauroylsarcosinate 1137-16-61 as anioniccomponent. The fraction of E-configurationincreasessignificantly upon vesicle formation. Further evidence for a bilayer structure of the aggregates was provided by NMR relaxation measurements and chemical shift behavior.
Introduction1 A small number of cationic-anionic single-chain surfactant systems have been reported to form ve~icles.~-5 Here we report on a new and versatile class of vesicleforming cationic-anionic amphiphiles based on singlechain amino acid surfactants. This system allowsthe study of the molecular interactions and their dependence on composition. We investigated the vesiculation of sodium N-lauroylearcosinate (SLS) with cationic single-chain amphiphiles and compared it with that of related N-acyl amino acid anionics. Emphasis was directed toward the changes of the configuration equilibrium in mixtures of SLS with N-dodecylpyridinium chloride (DPC). The population of E- and Z-configurations of SLS was found to be sensitive to variations in aggregate structures (micelles versus vesicles). Furthermore, the SLS headgroups experience strong immobilization upon vesiculation. Experimental Methods Sodiumlauroylglycinate (SLG)and sodium lauroyl-L-alaninate (SLA) were synthesized by following the procedure of Jungermann et al.6 Samples were prepared by mixing parent micellar solutionsat 25 "C. Sampleswere vitrified by propane jet freezing 'Institut ftir Polymere. Laboratorium ftir Technische Chemie. (1) Abbreviations and author-suppliedCAS Registry numbers: sodium
N-lauroyl-L-alaninate (SLA), 55535-58-5; sodium N-lauroylglycinate (SLG), 18777-32-7; sodium N-lauroylsarcoeinate (SLS), 137-16-6; Ndodecylppidinium chloride (DPC), 104-74-5; N-hexadecylpyridinium chloride (HPC), 6004-24-6; N-dodecyltrimethylammonium bromide (DTAB), 1119-94-4;N-heradecyltrimethylammoniumbromide (CTAB), 57-09-0. (2) (a) Kaler, E. W.; Kamalakara Murthy, A.; Rodriguez, B. E.; Zaaadzineki,J.A.N.Science 1989,245,1371. (b)Kaler,E.W.;Kamnlalrara Murthy, A.; Rodriguez, B. E.; Zaeadzinski, J. A. N. J. Phys. Chem. 1992, 96,6698. (3) (a)Szdnyi,S.;Cambon,A.;Watzke,H. J.;Schurtenberger,P.;Wehrli,
E.InStructureand Conformationof Amphiphilic Membranes; Lipowski, R., Richter, D., Kremer, K., Eds.; Springer Proceedings of Physics; Springer: Berlin, 1992; Vol. 6, p 198. (b) SzBnyi, S.; Watzke, H. J. R o g . ColloidPolym. Sci., in press. (c) Ambiihl, M.; Bangerter, F.; Luisi, P. L.; Skrabal,P.; Watzke, H. J. Prog. Colloid Polym. Sci, in press. (d)Watzke, H. J. Prog. Colloid Polym. Sci., in press. (4) (a) Safran,S. A,; Pincus, P.; Andelman, D. Science 1990,248,354. (b) Safran,S. A.; Pincus, P.; Andelman, D.; MacKintoeh, F. C. Phys. Rev.
1991,43, 1071. (5) Wakita, M.; Edwards, K. A.; Regen, S. L.; Turner, D.; Gruner, S. M. J. Am. Chem. SOC.1988, 110, 5221. (b) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. J. Am. Chem. SOC.1990,112, 1635. (6) Jungermann, E.; Gerecht, J. F.; Krems, I. J. J. Am. Chem. SOC. 1966, 78, 172.
for freeze-fracture electron microscopy inspecting them in a Philips EM 301 electronmicro~cope.7 Hydrodynamic radii (R(h))were determined by quasi-elastic light scattering (Malvern 4700 PS/MW spectrometer) under angles of 90°, 60°, and 75O, using a Coherent Inova 200 argon ion laser as light source (A = 488 nm). The lH NMR spectra of the samples (in 0.1 m01/dm-~ deuterated phosphate/DzO buffer) were recorded on Bruker spectrometers. The fraction of E-configurationwas determined by f~ = ZE/ (ZE+ Iz),where ZEand ZZare the integralsof respective lH resonancelines (assignedby 2D-NOESY NMR). Changea of chemical shift of the N-CHz protons were expressed as AA6 (definedas PA6 = ( 6 ~ -06 ~~-)( 6 ~ -06 ~~in) ppm). Spin-lattice relaxation times (TI) and spin-spin relaxation times (2'2) were determined using the inversion recovery and the Carr-PurcellMeiboom-Gill spin-echosmethod, respectively.
Results and Discussion Upon mixing micellar solutions of N-lauroyl amino acid surfactants with cationic single-chain surfactanta, we observed vesiculation at compositions close to equimolar. As a typical example, Figure 1 shows the freeze fracture electron micrograph of the vesiculated mixture of SLS with DPC at composition \k = 0.4 (\k = [SLSl/([SLSl + [DPCl)).g The parent micellar solutions of SLS and DPC form micelles at 0.013 and 0.015 mol dm-3, respectively.10JWa The hydrodynamic radii (R(h)) of the aggregates formed from catanionic mixtures were determined by quasi-elastic light scattering. Table I compares the hydrodynamic radii found in mixtures of SLG, SLA, and SLS with DPC. SLS formssmallvesicular aggregates (R(h) * 10nm). However, both SLG and SLA seem to favor larger aggregates (at \k = 0.4 and 0.6). While for SLG and SLA large aggregates appear in cationic excess (\k < 0.51,SLS gives maximum (7) MUer, M.; Meieter, N.; Moor, H. Mikroskopie 1980, 36, 129. (8)(a) Cam,H. Y .;Purcell,E. M. Phys. Reu. 1964,%, 630. (b) Meiboom, S.;Gill, D. Rev. Sci. Imtrum. 1958, 29,688. (9) Recently, sodium N-lauroylsarcoeinatewas used to prepare pauci-
lamellar vesicles in mixtures with cholesteroland under half protonation: Wallach, D. F. H.; Mathur, R.; Redziniak G. J. M.; Tranchant J. F. J.SOC. Cosmet. Chem. 1992,43,113. (10) (a) Miyagiehi, S.; Nishida, M. J. Colloid Interface Sci. 1978,65, 380. (b) Miyagiehi, S.; Ishibai, Y.;Asakawa, T.; Niehida; M. J. Colloid Interface Sci. 1986,103,164. (c) Miyagishi, S.; Asakawa,T.; Nishida, M. J. Colloid Interface Sci. 1989, 131, 68. (11) Lindman, B.; WennerstrBm, H. Top. Curr. Chem. 1980,87,3-83. (12) (a) Gebicki, J. M.; Hicks, M. Nature (London) 1973,243,232. (b) Gebicki,J. M.; Hicks, M. Chem.Phys. Lipids 1976,16,142. (c) Hargreavea, W. R.; Deamer, D. W. Biochemistry 1978, 17, 3759. (d) Haines, T. A. R o c . Natl. Acad. Sci. U.S.A. 1983,80, 160.
0743-7463/9312409-0036%04.00/0 Q 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No. 1, 1993 37
Z-configuration
E-configuration
Figure 2. E- and 2-configurations with respect to the amide bond of sodium N-lauroylsarcosinate (SLS).
Figure 1. Freeze fracture electron micrograph of SLS/DPC (@ = 0.40) at 25 O C (bar length is 100 nm). Total surfactant concentration was 0.05 mol dm-3. Table I. Hydrodynamic Radii R(h) of Various Amino Acid Surfactants with DPC in the Vesicular Composition Ranges
SLS
0.4 0.6 0.4 0.6 0.4 0.6
2.6 3.1 2.0 3.1
8.7 11.0 SLG 7.9 and 21.7 9.9 SLA 7.7 and 22.5 2.6 7.7 a Total surfactantconcentrationwas 0.05mol dm-3. b Determined at 9 0 O scattering angle; the first and second columns summarize micellar and vesicular radii, respectively. I
Table 11. Hydrodynamic Radii R(h) and f E Values of SLS-DPC Mixtures R(h)/nmb
JIa
fE
~
0.0 0.1 0.25 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.90 l.W
1.8 2.1 2.5
0.39 0.38 0.38 2.6 8.7 0.36 0.38 0.43 22.5 0.38 7.0 11.0 0.37 3.1 6.5 0.33 2.1 3.0 0.32 1.9 0.32 2.3 0.30 2.2 0.28 a Total surfactantconcentrationwas 0.05mol dm-3. Determined at 9 0 O scattering angle; the first and second columns summarize micellar and vesicular radii, respectively. Micellar SLS solution of 0.05 mol dm-3.
*
vesicle sizesin anionicexcess (\k > 0.5). Vesiclesare formed in coexistence with mixed micelles. In the SLS/DPC system micelles form over the whole range of compositionin phosphate buffer (Table 11). The vesicles exhibit large polydispersities. The angle dependence of the scattered light intensity indicated that the
aggregates are spherical in all the mixtures. Coacervate droplets of the same size could be excluded by the narrow line shapes of 1H NMR. The numbers of vesicles were calculated from the scattered light intensities employing a hollow sphere model for the vesicular aggregates. Approximately 1%of the aggregates were found to be vesicles. Comparison of mixtures of SLS with other cationic amphiphiles (HPC, DTAB, and CTAB) shows that changing chain length or headgroup size does not strongly affect the hydrodynamic radii in the vesiculation range (R(h),i,lw = 2.3 nm and R(h)vwic1- = 8 nm). However, SLS itself forms quite large and very stable vesicles from micellar solutions upon protonation (R(h) = 25-lo00 nm) similar to fatty acid soaps.12 The E- and 2-configuration of the amide bond of SLS (Figure 2) can be identified by the respective 1Hresonances of N-CH2, N-CH3, and alkyl a-CH2 groups. The populations of E- and 2-configuration of surfactants containing tertiary amide bonds were found to be sensitive to micellization.l3 While the monomer solution of SLS (0.008 mol dm-3) shows f~ = 0.40, a strong decrease to f~ = 0.28 occurs upon micellization. In contrast, a 0.05 mol dm-3 solution of the SLS-DPC ion-pair complex (1:l)gave f~ = 0.51 in CDCls; dispersed in aqueous buffer the fraction decreased to f~ = 0.43. Increasing the ratio of DPC to SLS in the mixed micelles (\k = 0.9 to \k = 0.6) increases f~ from 0.30 to 0.37. Surprisingly, the formation of vesicles (\k = 0.6 to \k = 0.4) is accompanied by a significant increase in f~ approaching that found for the ion-pair complex (Table 11). This indicates that a substantial change of the headgroup configurations occurs due to vesiculation. Determination of f~in mixtures of SLS with other cationic amphiphiles (HPC, DTAB, and CTAB) also resulted in values between 0.39 and 0.43. In comparison, for vesicles in half neutralized SLS solution, f~ equals 0.36. The determined values off^ are sensitive to the distributions of SLS between vesicles, mixed micelles, and monomers. At 0.05 mol dm-3 the monomer contribution can be neglected; hence the f~ values will depend only on the distribution of SLS between mixed micelles and vesicles. However, SLS will be incorporated mainly in the bilayer due to the large aggregation numbers of vesicles despite their small number. (13)a) Takahashi,H.; Nakayama, Y.;Hori, H.;Kihara, K.; Okabayashi, H.; Okuyama, M. J. Colloid Interface Sci. 1976,54,102.(b) Okabayashi, H.;Takahashi, H.Chem. Scr. 1977,11,128. (c) Okabayashi,H.;Yoshida, T.; Terada, Y.; Matsushita, K. J. Colloid Interface Sci. 1982,87(21,527. (d) Gerig, J. T.; Peyton, D. H.; Nicoli, D. F. J. Am. Chem. SOC.1982,104, 5034. (e) Yahagi, K.; Tsujii. J. Colloid Interface Sci. 1987,117,415.
38 Langmuir, Vol. 9, No.1, 1993
Furthermore the chemical shift difference (AA6) between the E and 2 N-CH2 protons changes sign upon mixed aggregate formation (AA6 = -0.017 to 0.022 ppm for 9 = 1.00to 9 = 0.10, respectively). The chemical shift of the 2N-CH2 protons remains constant. This indicates that the E-configuration is strongly affected by the ionpair formation. Further agreement with vesicle formation from mixed micellar solutions is found from spin-lattice and spinspin relaxation measurementa. Spin-spin relaxation times for the E-configurations of the alkyl a-CH2 change considerably from T2 = 0.086 s (micelles) to T2 = 0.018 s (micelles/vesicles), while spin-lattice relaxation times remain almost the same (TI = 0.423 s versus 2'1 = 0.419 8 ) . This indicates strong local immobilization of the SLS headgroups in the vesicular solutions. T2* values of both SLG and SLA containing catanionic mixtures (assessed from the half width of the alkyl a-CH2 resonance lines)
Letters indicate a similar immobilization of the amphiphilic molecule8 in the micelles/vesicle range. The electron microscopic inspection of all veaicle samples ale0 reveals larger vesicular aggregates, which adds to the evidence that N-acylated amino acid surfactant aalta form closedshell bilayer structuree from micellar mixtures with singlechain cationic amphiphiles. We are currently extendingour studiea to functionalized amino acid single-chain amphiphilee, because these systems can be used to prepare stable vesicles with a variety of novel functions, e.g. catalytic peptide functions, molecular recognition sites, etc.
Acknowledgment. The authors expreaa their gratitude to Ernst Wehrli, Stefan Egelhaaf, and Peter Schurtenberger for support in electron microscopy and light scattering, respectively.
