Langmuir 1996, 12, 2169-2172
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Structure and Properties of Stoichiometric Complexes Formed by Sodium Poly(r,L-glutamate) and Oppositely Charged Surfactants Ekaterina A. Ponomarenko, Alan J. Waddon, David A. Tirrell,* and William J. MacKnight Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received November 7, 1995. In Final Form: January 31, 1996X The conformational and structural properties of the stoichiometric complexes formed by poly(R,Lglutamate) anions and alkyltrimethylammonium cations with chain lengths of twelve, sixteen, and eighteen carbon atoms were examined by circular dichroism, infrared, differential scanning calorimetry, and X-ray diffraction techniques. The polypeptide chains complexed with the surfactants adopt R-helical conformations. The shorter surfactant alkyl chains, consisting of twelve and sixteen carbon atoms, are disordered in the complexes, while the longer surfactant chains of eighteen carbon atoms crystallize on a hexagonal lattice. All of the complexes are organized in lamellar structures consisting of alternating layers of poly(R,Lglutamate) chains separated by bimolecular layers of the surfactants. The surfactant alkyl chains are interdigitated and perpendicular to the lamellar surfaces.
The material properties of the stoichiometric complexes consisting of polyelectrolytes and oppositely charged surfactants have recently attracted considerable interest.1-16 Such complexes assemble in aqueous solutions through electrostatic attraction of polyion chain units and oppositely charged surfactant ions and are stabilized by hydrophobic interactions of the surfactant alkyl chains in water (see, for example, refs 17 and 18). Stoichiometric polyelectrolyte-surfactant complexes are in general insoluble in water but soluble in organic solvents of low polarity.3-5,14 In the solid state, they adopt lamellar structures,6,8-11 similar to those of comblike polymers.19 Owing to the amphiphilic nature of polyelectrolytesurfactant complexes and the easy variability of their components, such compounds are promising as materials for molecular composites, separation membranes, solubilization, and compatibilization. X
Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) Higashi, N.; Kunitake, T. Chem. Lett. 1986, 105. (2) Niwa, M.; Mukai, A.; Higashi, N. Macromolecules 1991, 24, 3314. (3) Seki, M.; Morishima, Y.; Kamachi, M. Macromolecules 1992, 25, 6540. (4) Bakeev, K. N.; Yang, M. S.; Zezin, A. B.; Kabanov, V. A. Dokl. Akad. Nauk 1993, 332, 450. (5) Bakeev, K. N.; Yang, M. S.; MacKnight, W. J.; Zezin, A. B.; Kabanov, V. A. Macromolecules 1994, 27, 300. (6) Khandurina, Yu. V.; Dembo, A. T.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Polymer Sci. Russ. 1994, 36, 189. (7) Lee, B.; Kunitake, T. Chem. Lett. 1994, 1085. (8) Harada, A.; Mozakura, S. Polym. Bull. 1994, 11, 175. (9) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380. (10) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (11) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (12) Antonietti, M.; Fo¨rster, S.; Zisenis, M.; Conrad, J. Macromolecules 1995, 28, 2270. (13) Antonietti, M.; Kaul, A.; Thu¨nemann, A. Langmuir 1995, 11, 2633. (14) Kabanov, A. V.; Sergeev, V. G.; Foster, M.; Kasaikin, V. A.; Levashov, A. V.; Kabanov, V. A. Macromolecules 1995, 28, 3657. (15) Higashi, N.; Sunada, M.; Niwa, M. Langmuir 1995, 11, 1864. (16) Ponomarenko, E. A.; Waddon, A. J.; Bakeev, K. N.; Tirrell, D. A.; MacKnight, W. J. Submitted for publication. (17) Goddard, E. D. Colloids Surf. 1986, 19, 301. (18) Ibragimova, Z. Kh.; Kasaikin, V. A.; Zezin, A. B.; Kabanov, V. A. Vysokomol. Soedin. 1986, A28, 1640 (translated in Polym. Sci. USSR 1986, 28, 826). (19) Plate`, N. A.; Shibaev, V. P.; J. Polym. Sci., Macromol. Rev. 1974, 8, 117.
S0743-7463(95)01012-2 CCC: $12.00
Most work in this area has been related to the complexes formed by conventional synthetic polyelectrolytes and low molecular weight surfactants. However, biopolymers, which can adopt highly ordered structures with a great variety of architectures, may offer special advantages in the development of new polymer-surfactant complexes with desired properties. We have recently reported the solid state properties of the stoichiometric complexes formed by the synthetic polypeptide sodium poly(R,L-glutamate) and the oppositely charged surfactants dodecyl and cetyltrimethylammonium bromide.16 We refer to these complexes as PGD and PGC, respectively. We have shown that the polymer chains in the PGD and PGC complexes adopt an R-helical conformation in the solid state, similar to that of the alkyl esters of poly(R,L-glutamic acid) (PALGs).20 These complexes are organized in lamellar structures, similar to those of comblike polymers19 and the complexes of conventional synthetic polyelectrolytes and oppositely charged surfactants.6, 8-11 However, the surfactant chains, attached to the R-helical poly(R,L-glutamate) backbone electrostatically, do not form alkane-type crystallites, unlike the PALGs with side chain lengths of ten or more carbon atoms.16 This raises the question of whether crystallization of “side chains” attached to an R-helical polymer backbone electrostatically is at all possible. To answer this question, we carried out an investigation of the properties of the stoichiometric complex formed by sodium poly(R,L-glutamate) and octadecyltrimethylammonium bromide. We refer to this complex as PGO. Experimental Section Materials. Sodium poly(R,L-glutamate) (PGNa) with a weightaverage degree of polymerization (provided by the supplier) of about 600 and the cationic surfactants dodecyl-, cetyl-, and octadecyltrimethylammonium bromide (DDTAB, CTAB, and ODTAB, respectively) were used as received from Sigma Chemical Co. The synthesis and composition determination of the PGD and PGC complexes were reported previously.16 The PGO complex was prepared by mixing equimolar quantities of 0.01 M PGNa (the molarity of the polymer solutions is based on the repeating unit equivalent weight) and 2 × 10-3 M ODTAB (20) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A.; Macromolecules, 1985, 18, 2141.
© 1996 American Chemical Society
2170 Langmuir, Vol. 12, No. 9, 1996 solutions in distilled and deionized water. The resulting white precipitate was isolated by centrifugation, washed twice with water, and dried in vacuum at 40 °C for at least 24 h. The composition of the PGO complex was estimated by elemental analysis and by high-resolution 1H NMR spectroscopy in deuterated chloroform. Elemental analysis showed good correlation between the experimental and calculated contents of C, N, and H, corresponding to an equimolar ratio of poly(R,L-glutamate) chain units and surfactant ions: (C/N)calcd ) 11.56, (C/N)found ) 11.67, Nafound < 0.1%, Brfound < 0.1%. From the ratio of the integrated NMR peak intensities of the R-CH protons (δ 4.0 ppm) of the polypeptide and the protons of the surfactant alkyl chains at δ 1.3-1.4 (CH2, 28 H) and δ 1.7 (CH2, 2 H) it was estimated that essentially 100% of the polymer chain units were paired with surfactant ions (the value of uncertainty was about 4%). The complexes were analyzed immediately or stored in vacuum at room temperature. Upon storage, irreversible changes in properties were observed, indicating formation of small amounts of unbound crystalline surfactants. Formation of surfactant molecules free of poly(R,L-glutamate) chains was promoted by film casting from chloroform solutions and/or by heating of the samples above room temperature. Powder samples for X-ray analysis were sealed in thin glass capillaries. Films of the complex for X-ray and differential scanning calorimetry (DSC) analyses were prepared by evaporation of chloroform solutions on Teflon plates at room temperature. Films were stretched by hand to prepare oriented samples. The draw ratio did not exceed two. For circular dichroism (CD) and Fourier transform infrared (FTIR) measurements films were cast from chloroform solutions on quartz and KBr windows, respectively. Measurements. 1H NMR spectra were obtained on a Bruker AMX 500 MHz instrument. CD spectra were recorded with an Aviv 62DC spectrometer. FTIR spectra were obtained using a Nicolet IR 44 spectrometer. X-ray diffraction patterns of powder samples were recorded using a Siemens D 500 diffractometer in transmission mode with a scintillation counter scanning in the desired 2Θ range (where Θ is the Bragg angle). Oriented films of the complexes were analyzed with an evacuated X-ray Statton camera with subsequent digitization of the X-ray films using an Optronics C4500 microdensitometer. In both types of X-ray experiments, Ni-filtered Cu KR radiation was used with wavelength λ ) 1.5418 Å. The scattering vector s range was 1.1 × 10-2 to 3.4 × 10-1 Å-1 (s ) 2 sin Θ/λ). DSC experiments were performed on a Perkin-Elmer DSC 7 system at a scanning rate of 10 °C/min. Heating and cooling scans were performed three times for each sample, and in all cases melting temperatures were essentially identical.
Results and Discussion As we have shown previously,16 stoichiometric complexes formed by sodium poly(R,L-glutamate) and alkyltrimethylammonium bromides with chain lengths of twelve and sixteen carbon atoms are soluble in chloroform as well as in solvents of higher polarity, such as benzyl alcohol, methanol, dimethylformamide, and dimethyl sulfoxide. The PGO complex is soluble in the same range of solvents. In all complexes, the R-CH protons of the polypeptide backbone are observed at δ ) 4.0 ppm in dilute solutions in deuterated chloroform, as indicated by 1H NMR spectra (not shown), suggesting the presence of hydrogen-bonded secondary structures.21 No proton resonances characteristic of chain units free of hydrogen bonding were observed, suggesting that the poly(R,Lglutamate) chains complexed with surfactants ions are most likely to be either in R-helical or β-sheet conformations. Our preliminary studies of dilute chloroform solutions of the poly(R,L-glutamate)-surfactant complexes by dynamic light scattering indicate that no intermolecular aggregates are present in the concentration range 1-2 wt %. This observation combined with our 1H NMR data is strongly suggestive of the R-helical conformation, given (21) Block, H. Poly(γ-benzyl-L-glutamate) and Other Glutamic Acid Containing Polymers; Gordon and Breach: New York, 1983.
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Figure 1. Wide angle X-ray diffractograms of PGO (a) and PGD (b) powders.
the known propensity of β-sheet polypeptides to aggregate in solution. CD spectra of chloroform-cast films of the PGO complex exhibit a positive band at about 190 nm and two negative bands at about 210 and 220 nm, consistent with an R-helical conformation of the polypeptide backbone in the solid complex.22 This behavior is similar to that of the alkyl esters of poly(R,L-glutamic acid) and to that of the PGD and PGC complexes described previously.16 This conclusion is further supported by the FTIR spectra of the solid PGO complex, in which the amide I band is observed at 1653 cm-1.21 The wide-angle X-ray diffractogram (WAXD) of the PGO complex (Figure 1, curve a) exhibits a relatively sharp peak, corresponding to a Bragg spacing of 4.2 Å, superimposed on a broad halo centered at a spacing of 4.6 Å. In contrast, the PGD and PGC complexes exhibit only the broad halo (compare curves a and b in Figure 1). The sharp peak on the WAXD pattern of the PGO complex indicates that a portion of the surfactant chains is organized on a well-defined crystal lattice; a second fraction of the chains remains noncrystalline, as shown by the shoulder with the Bragg spacing of 4.6 Å. In the PGD and PGC complexes, on the other hand, the packing of the “side chains” is characterized by only short range order. A spacing of 4.2 Å has been observed previously in comblike poly(alkyl methacrylate)s with twelve or more carbon atoms in the side chains,19 as well as in complexes consisting of cross-linked sodium polyacrylate and cetyltrimethylammonium bromide,6 and is believed to indicate hexagonal packing of the alkyl chains. A small peak on the WAXD pattern of the PGO complex corresponding to the Bragg spacing of 3.6 Å (2Θ ) 24.5°) is characteristic of free surfactant and, therefore, is attributed to the presence of trace amounts of uncomplexed crystalline surfactant. We do not believe that this peak arises from orthorhombic crystals of the “side chains” in the PGO complex, owing to its extremely low intensity. Thus, the “side chain” crystals in the PGO complex differ from crystals formed by alkyltrimethylammonium bromides, as well as from those of the side chains of poly(R,Lglutamate)s: the alkyltrimethylammonium bromides crystallize in a monoclinic lattice,23 and PALGs with side chain lengths of 10 or more carbon atoms form triclinic crystallites.20 We estimated the degree of crystallinity of the PGO complex on the basis of the WAXD data (see, for example, ref 24). Assuming that the broad reflection with the Bragg spacing of about 4.6 Å results from noncrystal(22) Stevens, L.; Townend R.; Timasheff, S. N.; Fasman, G. D.; Potter, J.; Biochemistry 1968, 7, 3717. (23) Szulzewsky, K.; Schulz, B.; Vollardt, D. Cryst. Res. Technol. 1983, 18, 1003. (24) Ruland, W. Polymer 1964, 5, 89.
Complexes of Poly(R,L-Glutamate) and Surfactants
Figure 2. DSC thermograms of PGO film (a) and ODTAB powder (b) on heating.
line “side chains” and that the surfactant chains in the PGD complex are fully amorphous, fitting the amorphous halo of the PGO complex to the semicrystalline curve of the PGO complex indicates that about 30% of the “side chains” are crystalline. Figure 2 (curve a) shows a DSC thermogram of the PGO complex. The PGO complex undergoes an endothermic first-order transition upon heating at 48 °C. The transition is reproducible on heating and cooling. This transition correlates with considerable broadening of the WAXD peak of the complex and probably corresponds to the melting of the “side chain” crystallites. This conclusion is supported by the fact that no thermal transitions were observed for the PGD and PGC complexes in the temperature range 0-170 °C, consistent with the observation of only broad halos in the WAXD patterns of the complexes. The melting transition of the PGO crystallites is broader than that of the corresponding surfactant (Figure 2, curve b) and occurs at a lower temperature. Depression of the melting temperature of surfactants upon complexation with oppositely charged polyelectrolytes has also been observed in complexes formed by cross-linked polyacrylate anions and cetyltrimethylammonium cations6 and by poly(N-ethyl-4-vinylpyridinium) cations and cetyl sulfate anions.25,26 The melting enthalpy ∆Hm of the PGO crystallites, estimated from the first heating scan, is about 2.1 kcal/mol of surfactant ions. On the basis of the known value of about 1 kcal/mol of CH2 units reported for the triclinic-to-liquid transition of n-alkanes27 and for melting of the side chain crystallites of PALGs with ten or more carbon atoms,20 we estimated that only about 10% of the “side chains” are crystalline in the PGO complex. The low value of the melting entalpy is consistent with the prominent halo on the WAXD pattern of the PGO complex. However, for hexagonal phases, like the one observed in the case of the PGO complex, the melting enthalpy per mole of CH2 is known to be lower than that for triclinic crystals28 and may account for the difference in the degree of crystallinity estimated from the WAXD and DSC data. The PGO complex does not undergo any thermal transitions in the temperature range 10-170 °C, other than melting of the “side chain” crystallites. Unlike the PALGs with crystalline side chains, the PGO complex does not flow upon heating before decomposition. No ordered melts are observed. The small angle diffractogram (SAXD) of the PGO complex (Figure 3a) exhibits two narrow peaks with Bragg (25) Shu, Y. M. Ph.D. Thesis. Moscow State University, 1995. (26) However, complexation of bilayer-forming lipids with oppositely charged polyelectrolytes leads to an increase in the phase transition temperature of the former.13 (27) Broadhurst, M. G. J. Res. Natl. Bur. Stand. 1962, 66A, 241. (28) Lo´pez-Carrasquero, F.; Montserrat, S.; Ilarduya, A. M.; MunozGuerra, S. Macromolecules 1995, 28, 5535.
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Figure 3. Small angle X-ray diffractograms of PGO (a), PGC (b), and PGD (c) powders.
Figure 4. Dependence of the long period of the lamellae of the complexes (a) and of the corresponding surfactants (b) on the number of carbon atoms (n) in the surfactant chains.
spacings of 39.3 ( 0.9 and 19.4 ( 0.2 Å, corresponding to the first and second orders of the lamellar structure for the complex. The second-order signal arising from the long period of the lamellae of the PGO complex is much more pronounced than in the patterns of the PGD and PGC complexes (compare curves a, b, and c in Figure 3). The increase in the intensity of this signal coincides with the onset of crystallization of the surfactant chains in the complex, consistent with formation of a structure in which the lamellar interface is more sharply defined.29 We have shown previously that PGD and PGC possess lamellar structures, consisting of layers of the R-helical poly(R,L-glutamate) chains separated by layers of surfactant ions, with the alkyl chains perpendicular to the lamellar surfaces and interdigitated.16 Figure 4 (curve a) presents the dependence of the long period of the lamellae on the number of carbon atoms in the surfactant chains in the complexes studied. The dependence can be represented as a straight line, indicating that the three complexes studied possess similar lamellar structures. The slope of the dependence is about 1.3 Å per CH2 group, suggesting that the surfactant chains in the complexes are nearly fully extended, interdigitated, and perpendicular to the lamellar surface.19 The dependence of the long period of the lamellae of alkyltrimethylammonium bromides on the number of carbon atoms in the chain is presented for comparison (Figure 4, curve b). In this case, the increment of the lamellar thickness is about 1.1 Å per CH2 group, indicating that the surfactant chains are tilted with respect to the lamellar surfaces,19 in agreement with the literature.30 Additional information about the orientation of the surfactant chains with respect to the lamellar surfaces is (29) Donald, A. M.; Windle, A. H. Liquid Crystalline Polymers; Cambridge University Press: Cambridge, 1992; p 156.
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spacing (Figure 5, curve b) exhibit orthogonal orientations, with the lamellae aligned parallel to the draw direction. The presence of only two peaks on the oriented WAXD pattern and the same azimuthal width of the peaks on the oriented WAXD and SAXD patterns suggest again perpendicular orientation of the “side chains” with respect to the lamellae.
Figure 5. Dependence of the X-ray intensity of the reflections corresponding to the Bragg spacings of 4.2 Å (a) and 19.4 Å (b) on the azimuthal angle Φ of the stretched PGO film.
provided by comparison of the WAXD and SAXD patterns of stretched films of the complexes. It should be noted that, while the PGD and PGC complexes are plastic and deformable, the PGO films are quite brittle, probably owing to the presence of the “side chain” crystallites. Figure 5 presents the dependence of the X-ray intensity of the reflections corresponding to the Bragg spacings of 4.2 Å (a) and 19.4 Å (b) on the azimuthal angle Φ of the stretched PGO film. The oriented X-ray patterns are essentially identical for all complexes studied. It is clear that the reflections corresponding to the lateral packing of the surfactant chains (Figure 5, curve a) and to the lamellar (30) The tilt angle of the surfactant chains with respect to the lamellae estimated from the SAXD data is about 60°, while the reported value is 67°.23
Concluding Remarks Stoichiometric complexes formed by poly(R,L-glutamate) anions and alkyltrimethylammonium cations with chain lengths of twelve to eighteen carbon atoms adopt R-helical conformations similar to those of the alkyl esters of poly(R,L-glutamic acid). In the complexes studied, two types of surfactant organization were observed: shorter chains consisting of twelve or sixteen carbon atoms are extended but positionally disordered, while the longer chains of the PGO complex crystallize in a hexagonal lattice. The “side chain” crystals, in the latter case, are different from those of the PALGs and the corresponding surfactants. The minimum number of carbon atoms in the side chain required for crystallization is ten in the case of the PALGs20 and sixteen for the known complexes of the conventional synthetic polyelectrolytes and oppositely charged surfactants.6,25 It is likely that, for the complexes studied herein, the R-helical polymer backbone combined with the bulky headgroups of the surfactants imposes additional restrictions on alkyl chain packing and increases the minimum crystallization chain length. In the solid state, the complexes are organized in lamellae consisting of alternating layers of the polypeptide chains separated by bimolecular layers of surfactant, with the surfactant chains aligned perpendicular to the lamellar surfaces and interdigitated. The lamellar structure is not subservient to the state of the surfactant chains, as it is in the alkyl esters of poly(R,L-glutamic acid), where hexagonal packing of the R-helices is observed if the side chains are crystalline and lamellar packing dominates only if the side chains are disordered.20 Acknowledgment. This work was supported by a grant from the National Science Foundation (DMR 9311658) and by the NSF Materials Science and Engineering Center at the University of Massachusetts. LA951012E
