Hierarchical Structure Formation of Cylindrical Brush Polymer

Mar 30, 2009 - The complex formation of cylindrical brush polymers with poly(l-lysine) side chains (PLL) and sodium dodecyl sulfate (SDS) can induce a...
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Hierarchical Structure Formation of Cylindrical Brush Polymer-Surfactant Complexes Yang Cong, Nikhil Gunari,† Bin Zhang, Andreas Janshoff,*,‡ and Manfred Schmidt* :: :: Institut fur Physikalische Chemie, Johannes Gutenberg-Universitat Mainz, Jakob-Welder-Weg 11, 55128 † Mainz, Germany. Present address: Department of Chemistry, Lash Miller Laboratories, 80 St. George St., :: Toronto, ON M5S 3H6 Canada. ‡ Present address: Institut fur Physikalische Chemie, Georg August :: :: :: Universitat Gottingen, Tammanstr. 6, 37077 Gottingen Received December 29, 2008. Revised Manuscript Received February 7, 2009 The complex formation of cylindrical brush polymers with poly(L-lysine) side chains (PLL) and sodium dodecyl sulfate (SDS) can induce a helical conformation of the cylindrical brush polymer in aqueous solution (Gunari, N.; Cong, Y.; Zhang, B.; Fischer, K.; Janshoff, A.; Schmidt, M. Macromol. Rapid Commun. 2008, 29, 821-825). Herein, we have systematically investigated the influence of surfactant, salt, and pH on the supramolecular structure formation. The cylindrical brush polymers and their complexes with surfactants were directly visualized by atomic force microscopy in air and in aqueous solution. The alkyl chain length (measured by the carbon number, n) of the surfactant plays a key role. While helical structures were formed with n = 10, 11, and 12, no helices were observed with n < 10 and n > 13. Addition of salt destroys the helical structures as do pH conditions below 4 and above 6, most probably because the polymersurfactant complexes start to disintegrate. Circular dichroism was utilized to monitor the PLL side chain conformation and clearly revealed that β-sheet formation of the side chains induces the helical conformation of the atactic main chain.

Introduction Lysine is an amino acid frequently found in proteins which is highly involved in the subtle interactions that determine protein structure and function. Therefore, synthetic polypeptides comprising lysine are appealing model systems for studying protein aggregation, complex formation of polypeptides and DNA, and fundamental problems of polyelectrolyte solutions. Accordingly, polypeptide based synthetic-biological hybrid molecules have attracted increasing attention. Such hybrid molecules are known to form defined intermolecular aggregates such as ribbons, fibrils, spirals, or fibers.1-8 Cylindrical polymers such as dendronized polymers and “bottlebrushes” constitute an interesting class of polymers. They have a linear flexible main chain with densely grafted linear or branched side chains, which force the main chain into an extended cylindrical conformation provided that the *To whom correspondence should be addressed. (M.S.) Telephone: +49-6131-3923769. Fax: +49-6131-3922970. E-mail: mschmidt@ uni-mainz.de. (A.J.) Telephone: +49-551-393226. Fax: +49-551-393228. E-mail: [email protected]. (1) Collier, J. H.; Messersmith, P. B. Adv. Mater. 2004, 16, 907–910. (2) Klok, H.-A.; Schlaad, H. Adv. Polym. Sci. 2006, 202, 1–160. :: (3) Borner, H. G.; Schlaad, H. Soft Matter 2007, 3, 394–408. :: (4) Lutz, J.-F.; Borner, H. G. Prog. Polym. Sci. 2008, 33, 1–39. :: (5) Borner, H. G.; Smarsly, B.; Hentschel, J.; Rank, A.; Schubert, R.; Geng, Y.; Discher, D. E.; Hellweg, T.; Brandt, A. Macromolecules 2008, 41, 1430–1437. (6) Frauenrath, H.; Jahnke, E. Chem.;Eur. J. 2008, 14, 2942–2955. (7) Jahnke, E.; Kreutzkamp, P.; Nikolai, J.; Rabe, J.; Frauenrath, H. Adv. Mater. 2008, 20, 409–414. (8) Jahnke, E.; Lieberwirth, I.; Severin, N.; Rabe, J. P.; Frauenrath, H. Angew. Chem., Int. Ed. 2006, 45, 5383–5386. :: (9) Sheiko, S. S.; Moller, M. Chem. Rev. 2001, 101, 4099–4123. :: (10) Schluter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864–883. :: (11) Shu, L.; Schluter, A. D.; Ecker, C.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2001, 40, 4666–4669. :: :: (12) Zhang, A.; Barner, J.; Goessl, I.; Rabe, J. P.; Schluter, D. Angew. Chem., Int. Ed. 2004, 43, 5185–5188. :: (13) Al-Hellani, R.; Barner, J.; Rabe, J. P.; Schluter, A. D. Chem.;Eur. J. 2006, 12, 6542–6551.

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main chain is much longer than the side chains.9-22 By controlling the molecular architecture such as the grafting density, polymerization degrees of the main chain and side chains, and the chemical structure of the main chain and side chains, cylindrical brushes with responsive properties may be designed23-25 and applied to various fields ranging from nanotechnology26-28 to biology.29,30 Recently, cylindrical :: (14) Zhang, M.; Muller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461–3481. (15) Wintermantel, M.; Schmidt, M.; Tsukahara, Y.; Kajiwara, K.; Kohjiya, S. Makromol. Chem., Rapid Commun. 1994, 15, 279–284. (16) Wintermantel, M.; Gerle, M.; Fischer, K.; Schmidt, M.; Wataoka, I.; Urakawa, H.; Kajiwara, K.; Tsukahara, Y. Macromolecules 1996, 29, 978– 983. :: (17) Sheiko, S.; Gerle, M.; Fischer, K.; Schmidt, M.; Moller, M. Langmuir 1997, 13, 5368–5372. :: (18) Dziezok, P.; Sheiko, S.; Fischer, K.; Schmidt, M.; Moller, M. Angew. Chem., Int. Ed. 1997, 36, 2812–2815. :: (19) Gerle, M.; Fischer, K.; Muller, A. H. E.; Schmidt, M.; Sheiko, S. S.; :: Prokhorova, S.; Moller, M. Macromolecules 1999, 32, 2629–2637. (20) Fischer, K.; Schmidt, M. Macromol. Rapid Commun. 2001, 22, 787– 791. (21) Zhang, B.; Zhang, S.; Okrasa, L.; Pakula, T.; Stephan, T.; Schmidt, M. Polymer 2004, 45, 4009–4015. (22) Neiser, M. W.; Okuda, J.; Schmidt, M. Macromolecules 2003, 36, 5437–5439. (23) Li, C.; Gunari, N.; Fischer, K.; Janshoff, A.; Schmidt, M. Angew. Chem., Int. Ed. 2004, 43, 1101–1104. :: (24) Sun, F.; Sheiko, S. S.; Moller, M.; Beers, K.; Matyjaszewski, K. J. Phys. Chem. A 2004, 108, 9682–9686. (25) Xu, Y.; Bolisetty, S.; Drechsler, M.; Fang, B.; Yuan, J.; Ballauff, M.; :: Muller, A. H. E. Polymer 2008, 49, 3957–3964. (26) Djalali, R.; Li, S. Y.; Schmidt, M. Macromolecules 2002, 35, 4282– 4288. :: (27) Zhang, M.; Drechsler, M.; Muller, A. H. E. Chem. Mater. 2004, 16, 537–543. (28) Yuan, J.; Xu, Y.; Walther, A.; Bolisetty, S.; Schuhmacher, M.; :: Schmalz, H.; Ballauff, M.; Muller, A. H. E. Nat. Mater. 2008, 7, 718– 724. :: :: (29) Gossl, I.; Shu, L.; Schluter, A. D.; Rabe, J. P. J. Am. Chem. Soc. 2002, 124, 6860–6865. :: (30) Storkle, D.; Duschner, S.; Heimann, N.; Maskos, M.; Schmidt, M. Macromolecules 2007, 40, 7998–8006.

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brushes with poly(L-lysine) (PLL) side chains were successfully prepared.31,32 Interactions between polyelectrolytes and oppositely charged surfactants have attracted a great deal of interest due to their importance both in fundamental polymer physics as well as in biological and industrial applications.33-35 When polyelectrolytes and oppositely charged surfactant solutions are mixed, the formation of polyelectrolyte-surfactant complexes is driven by both electrostatic attraction and hydrophobic interactions. While linear polyelectrolyte/surfactant complexes have been widely investigated,33-40 only a limited amount of studies are available utilizing branched and dendronized polymers such as star polymers or dendrimers.41-47 The results suggest that the molecular architecture of the polymer has a crucial influence on the interaction of surfactants with polymers. It is well-known that some special synthetic polymers may form helices, particularly if they contain chiral groups in either the main or the side chains.48-51 Recently, some achiral polyphenylacetylenes were observed to adopt a helical conformation upon complexation with chiral amines or dendrons.52 Double helix formation of polymers with dendritic side chains was reported which do not contain any chiral groups53,54 as well as helix and double helix formation of anionically charged cylindrical micelles upon complexation with oligoamines.55 Very recently, also the complexation of cylindrical polyelectrolyte brushes with trivalent counterions (31) Zhang, B.; Fischer, K.; Schmidt, M. Macromol. Chem. Phys. 2005, 206, 157–162. (32) Gunari, N.; Cong, Y.; Zhang, B.; Fischer, K.; Janshoff, A.; Schmidt, M. Macromol. Rapid Commun. 2008, 29, 821–825. (33) Zhou, S.; Chu, B. Adv. Mater. 2000, 12, 545–556. (34) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17–31. :: (35) Antonietti, M.; Thunemann, A. Curr. Opin. Colloid Interface Sci. 1996, 1, 667–671. (36) Chu, D. Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270–6276. (37) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950–953. (38) Xia, J.; Zhang, H.; Rigsbee, D. R.; Dubin, P. L.; Shaikh, T. Macromolecules 1993, 26, 2759–2766. (39) Ikkala, O.; Brinke, G. Chem. Commun. 2004, 19, 2131–2137. (40) General, S.; Antonietti, M. Angew. Chem., Int. Ed. 2002, 41, 2957– 2960. (41) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515–8522. (42) Gopidas, K. R.; Leheny, A. R.; Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1991, 113, 7335–7342. :: (43) Canilho, N.; Kasemi, E.; Schluter, A. D.; Mezzenga, R. Macromolecules 2007, 40, 2822–2830. :: (44) Canilho, N.; Kasemi, E.; Schluter, A. D.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2007, 40, 7609–7616. (45) Li, Y.; Mcmillan, C. A.; Bloor, D. M.; Penfold, J.; Warr, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 7999–8004. (46) Mesz aros, R.; Thompson, L.; Bos, M.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 609–615. (47) Wang, H.; Wang, Y.; Yan, H.; Zhang, J.; Thomas, R. K. Langmuir 2006, 22, 1526–1533. (48) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. Rev. 2001, 101, 4039–4070. (49) Green, M. M.; Cheon, K. S.; Yang, S. Y.; Park, J. W.; Swansburg, S.; Liu, W. Acc. Chem. Res. 2001, 34, 672–680. (50) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013–4038. (51) Zhang, W.; Shiotsuki, M.; Masuda, T. Macromol. Rapid Commun. 2007, 28, 1115–1121. (52) Kamikawa, Y.; Kato, T.; Onouchi, H.; Kashiwagi, D.; Maeda, K.; Yashima, E. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4580–4586. :: :: (53) Bottcher, C.; Schade, B.; Ecker, C.; Rabe, J. P.; Shu, L.; Schluter, A. D. Chem.;Eur. J. :: 2005, 11, 2923–2928. :: :: (54) Ding, Y.; Ottinger, H. C.; Schluter, A. D.; Kroger, M. J. Chem. Phys. 2007, 127, 094904. (55) Zhong, S.; Cui, H.; Chen, Z.; Wooley, K. L.; Pochan, D. J. Soft Matter 2008, 4, 90–93. (56) Xu, Y.; Bolisetty, S.; Drechsler, M.; Fang, B.; Yuan, J.; Harnau, L.; :: Ballauff, M.; Muller, A. H. E. Soft Matter 2009, 5, 379–384.

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was observed to induce a main chain helix.56 A statistical copolymacromonomer with incompatible poly(methyl methacrylate) and poly(vinyl pyridinium) side chains was reported to exhibit a “meanderlike” shape on a mica surface57 which might be reinterpreted as the projection of a helical structure. Chiral peptide based side chains of dendronized polymers were demonstrated to also exhibit a helical main chain conformation.58-60 Helix I to helix II transitions typically observed for linear polyproline upon changing the solvent from alphatic alcohols to water could not be detected in cylindrical brushes with polyproline side chains, probably due to side chain overcrowding.59 In a previous publication,32 we reported that sodium dodecyl sulfate (SDS) induces a coil to β-sheet conformational transition of the PLL side chains of a cylindrical brush polymer, which induces a helical conformation of the cylinder with a pitch of approximately 20 nm. Circular dichroism (CD) measurements suggested that the helix formation of the cylindrical brush polymers is driven by the β-sheets formed by the PLL side chain-SDS complexes. Though it is known that ionic surfactants are able to induce helical or β-sheet structures when interacting with oppositely charged coiled polypeptides,61-65 the system presented here constitutes an intriguing example of hierarchical structure formation. In the present work, we systematically investigated the effects of surfactants, salt concentration, and pH on the helical structure formation of the complex.

Experimental Section Polymer Brush Synthesis and Characterization. The cylindrical brush polymers with poly(L-lysine) side chains were synthesized by grafting of the protected monomer Zε-lysine from the Scheme 1. Chemical Structure of the Cylindrical Poly(L-lysine) Brush Polymer

(57) Zhang, A. Macromol. Rapid Commun. 2008, 29, 839–845. (58) Zhang, A.; Rodriguez-Ropero, F.; Zanuy, D.; Aleman, C.; Meijer, E. :: W.; Schluter, A. D. Chem.;Eur. J. 2008, 14, 6924–6934. (59) Zhang, A.; Guo, Y. Chem.;Eur. J. 2008, 14, 8939–8946. (60) Zhao, H.; Sanda, F.; Masuda, T. Macromol. Chem. Phys. 2006, 207, 1921–1926. (61) Satake, I.; Yang, J. T. Biochem. Biophys. Res. Commun. 1973, 54, 930– 936. (62) Igou, D. K.; Lo, J. T.; Clark, D. S.; Mattice, W. L.; Younathan, E. S. Biochem. Biophys. Res. Commun. 1974, 60, 140–145. :: :: :: (63) Thunemann, A. F.; Schutt, D.; Sachse, R.; Schlaad, H.; Mohwald, H. Langmuir 2006, 22, 2323–2328. (64) Liu, J.; Takisawa, N.; Kodama, H.; Shirahama, K. Langmuir 1998, 14, 4489–4494. (65) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860–1866.

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Figure 1. AFM (a) height image taken in air and (b) deflection image taken in water of the cylindrical poly(L-lysine) brushes; AFM (c) height image taken in air and (d) deflection image taken in water of the poly(L-lysine) brush-SDS complexes (SDS/lysine molar ratio: 0.87). Arrows indicate clearly resolvable helical conformations. Corresponding height profiles are shown below the images.

Figure 2. 3D representation of a cylindrical brush (a) and of a helical brush (b).

linear macroinitiator poly(N-tert-butoxycarbonyl-N0 -(2-methylacryloyl)-1,3-diaminopropane) (PMA-N6NH-C) via anionic ring opening polymerization of N-carboxyanhydride (NCA). The protected cylindrical poly(L-lysine) brush PMA-N6NH-CLys (ε-Z) was hydrolyzed in CF3COOH and a 4-fold molar excess of a 33 wt % solution of HBr in acetic acid. The chemical structure of the cylindrical poly(L-lysine) brush polymer is schematically shown in Scheme 1. The synthesis and characterization are described in refs 31 and 32. In short, the molar mass was determined by light scattering to be Mw = 334 000 g/mol. The average side chain length of poly (L-lysine) brush was determined by NMR to be 26 repeat units assuming 50% grafting density (for details, see ref 32). Sample Preparation. The poly(L-lysine) brush aqueous solution was diluted to c = 0.012 g/L for AFM imaging in air. The poly(L-lysine) brush-SDS complex was prepared by adding 1 mM SDS solution 100 μL dropwise into 2 mL of an aqueous poly(L-lysine) brush solution (c = 0.012 g/L) at constant stirring (with a SDS/lysine molar ratio of 0.87). The other poly(L-lysine) brush-surfactant complexes were prepared in the same way as for SDS described above. Substrates are mainly freshly cleaved mica. We also used freshly cleaved graphite and mica coated with poly-L-ornithine hydrobromide (molecular weight = 30 000-70 000 g/mol, Sigma) by resting a 20 μL droplet of a 0.1 mg/mL poly-Lornithine hydrobromide solution on the freshly cleaved mica for 4 min, rinsing with ∼300 μL of Milli-Q water, and then blowing with dry nitrogen before spin-coating for AFM imaging in air. Atomic Force Microscopy (AFM). AFM on the dry samples was performed with a Multimode atomic force microscope (NanoScope IIIa controller, Veeco, Santa Barbara, CA) in the intermittent contact mode in ambient atmosphere. Silicon cantilevers (spring constant of ∼42 N/m and resonant frequency of ∼290 kHz) with etched conical tips (radius of curvature of ∼15 nm) from Nanosensors (Neuchatel, Switzerland) were used for scanning in tapping mode (air). A drop of the sample solution was placed for 5 s on a substrate surface and spincoated at 3000 rpm. Imaging in solution was carried out by placing 300 μL of the polymer solution (c ≈ 0.05 g/L for pure polymer brush, and c ≈ 0.012 g/L for polymer brush-SDS complex) on a freshly cleaved mica sheet mounted in a homemade AFM poly(tetrafluoroethylene) (PTFE) fluid cell and incubating for 5 min. Afterward, the mica surface was rinsed with Milli-Q water to remove excess unbound polymer molecules and blown dry with a stream of nitrogen. The sample was imaged in an aqueous solution with a MFP 3D microscope (Asylum Research, Santa Barbara, CA) in contact mode at room temperature (20 °C). V-shaped silicon nitride cantilevers purchased from Veeco with a nominal spring constant of ∼0.01 N/m were used for the measurement. Circular Dichroism (CD). CD spectra were recorded at 20 °C using a JASCO J-810 recording spectropolarimeter with 2 mm path length cuvettes in the region of the characteristic absorption bands of the amide groups between 190 and 250 nm. The CD data are reported in terms of the molar ellipticity [θ] in

Table 1. Surfactants Used in This Study surfactant

molecular structure

number of carbons in alkyl tail

sodium 1-hexanesulfonate sodium 1-heptanesulfonate sodium 1-octanesulfonate sodium 1-nonanesulfonate sodium 1-decanesulfonate sodium 1-undecanesulfonate sodium 1-dodecanesulfonate sodium 1-dodecylsulfate sodium 1-tetradecanesulfonate sodium dodecanoate

CH3(CH2)5SO3Na CH3(CH2)6SO3Na CH3(CH2)7SO3Na CH3(CH2)8SO3Na CH3(CH2)9SO3Na CH3(CH2)10SO3Na CH3(CH2)11SO3Na CH3(CH2)11OSO3Na CH3(CH2)13SO3Na CH3(CH2)11COONa

6 7 8 9 10 11 12 12 14 12

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hydrophilic head

conformation

-SO3 -SO3 -SO3 -SO3 -SO3 -SO3 -SO3 -OSO3 -SO3 -COO-

wormlike wormlike wormlike wormlike helix helix helix helix wormlike wormlike

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Cong et al. degree 3 cm2/(decimole  residue) based on the molecular weight of the lysine repeat unit.

Results and Discussion AFM measurements of the cylindrical poly(L-lysine) brushes in air (Figure 1a) reveal a typical wormlike chain conformation. Similar wormlike structures were also observed by AFM in solution (Figure 1b), suggesting that drying artifacts only marginally affect the images. The consistency between AFM imaging in air and aqueous solution holds for all polymer brush-surfactant complexes investigated. Upon addition of SDS, the cylinders adopt a helical conformation as revealed by the AFM pictures taken in air (Figure 1c) and in solution (Figure 1d).32 As briefly mentioned in the Introduction, a helical structure compressed to a surface might be difficult to distinguish from a meandering worm. There are two arguments in favor of the helix: (1) The helices are about 0.4 nm higher than the wormlike cylinders (1.3 nm versus 0.9 nm), even if the latter also consist of brush-surfactant complexes. Apparently, the complexation with surfactant alone does not alter the height of the cylinders within experimental error. (2) The 3D image for the wormlike cylinder in Figure 2a shows a smooth surface, whereas the surface of the helix in Figure 2b exhibits a kind of periodicity reminiscent of a helix. Clearly, a meandering structure can be safely excluded. Nature of Surfactant: Functionality and Chain Length. The hydrophilicity or hydrophobicity of the polymer-surfactant complexes depends inter alia on the head group and on the length of the alkyl tail of the surfactant. Therefore, various surfactants were chosen with different hydrophilic head groups and with different alkyl chain lengths. The details of the surfactants used are listed in Table 1. Typical AFM images of the polymer brush-surfactant complexes in air are displayed in Figure 3. For sodium 1-dodecanesulfonate, complexes with the polymer brush exhibit pronounced helical structures (Figure 3a), similar to those observed for SDS-polymer brush complexes (Figures 1c). However, no helical structures are found if sodium dodecanoate is used (Figure 3b). Both surfactants have the same alkyl chain length (measured by carbon number in alkyl tail, n = 12) and are believed to be virtually identical from the viewpoint of hydrophobicity. Therefore, the electrostatic interaction between the surfactant head group and the polylysine side chains may significantly influence the complex formation with the PLL side chains. Obviously, in the relevant pH range of 4-5, the partly protonated carboxyl groups do not as effectively complex the protonated amino groups of the PLL as the sulfate or sulfonate groups. In order to investigate the effect of the alkyl chain length of the surfactant, sodium alkane sulfonates with different carbon numbers (n = 6, 7, 8, 9, 10, 11, 12, and 14) in the alkyl chains were utilized in the complexation experiments. No helical structures were observed for n = 6, 7, 8, and 9. Merely, wormlike structures were obtained as shown in Figure 3c. However, helical structures were found for the sulfonate surfactants with n = 10 (Figure 3d), 11 (not shown), and 12 (not shown). Unfortunately, the sulfonate surfactant with n = 13 was not commercially available. For n = 14, no helical structures were detectable (Figure 3e). The sulfonate surfactants with longer alkyl chains were not used in this study because of their poor solubility in water. Langmuir 2009, 25(11), 6392–6397

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The alkyl chain length of the surfactant affects not only the interfacial curvature of the surfactant ions in the case of micelle formation, but also the hydrophobic interactions of surfactants and the polyelectrolyte chains.33 When the alkyl chain length of the sulfonate surfactant is less than 10, helical structures are absent, because the hydrophobic interaction is not strong enough for β-sheet formation, in agreement with

Figure 3. AFM height images of poly(L-lysine) brush-surfactant complexes on mica with various different surfactants: (a) sodium 1-dodecanesulfonate; (b) sodium dodecanoate; (c) sodium 1-nonanesulfonate; (d) sodium 1-decanesulfonate; and (e) sodium 1-tetradecanesulfonate. Corresponding height profiles are shown below the images.

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early studies on the complex formation with linear PLL chains.61 For longer alkyl chains, however, the relatively strong hydrophobic interaction and steric repulsion among the long alkyl tails may disfavor the formation of ordered β-sheet conformation. Impact of Salt. Since the interaction between polyelectrolyte and oppositely charged surfactant is primarily of electrostatic origin, the polymer charge density as well as the ionic strength should influence the complex formation. The addition of salt, for example, NaCl or NaHSO4, to a solution of PLL brush-SDS helices, effectively destroyed the helices, and only wormlike structures could be detected as shown in Figure 4a (air) and b (aqueous solution). This transition from helices to wormlike structures took place even if the salt concentration was as low as 0.02 mM and persisted up to 50 mM. Either the stability of the helices is very sensitive to the presence of salt or the β-sheet formation is effectively destabilized, for instance, by replacing the surfactant with the excess anions of the added salt. CD spectroscopy revealed the latter to be the main factor as shown in Figure 5: prior to salt addition, the spectrum shows the coexistence of random coil and β-sheet (40%) conformations32 of the PLL side chains in the polymer brush-SDS complex solution, which converts into 100% coil fraction upon addition of salt. Effect of pH Changes. The solution pH determines the degree of ionization of the PLL side chains. Typically, the pH of the bare PLL brush solution has a pH of 3.9 (Figure 1a) which increases to pH = 4.6 (Figure 1c) upon addition of SDS (SDS/lysine ratio = 0.87). If the pH is adjusted to pH = 1.3 by addition of HBr, the helical structures become destabilized and wormlike structures are formed as shown in the

AFM image (Figure 6a). No helical structures could be observed at pH values below 3.8. At pH = 4.0, helical structures started to form as the minor component (Figure 6b) and prevail at 4.6 < pH < 5 (Figures 1c and 6c), while at pH = 5.9 the some helical structures have already collapsed into globules, as shown in Figure 6d. At pH = 9.1, only globules and small aggregated structures were observed (Figure 6e). In a strong alkaline environment

Figure 4. AFM (a) height image taken in air and (b) deflection image taken in water of the poly(L-lysine) brush-SDS complex after addition of NaCl solution.

Figure 6. AFM height images (air) of poly(L-lysine) brush-SDS Figure 5. CD spectra of poly(L-lysine) brush-SDS complex and of poly(L-lysine) brush-SDS complex after addition of NaCl.

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complex after adjusting the pH of the complex solution to (a) 1.3; (b) 4.0; (c) 5.0; (d) 5.9; (e) 9.1; and (d) 12.6. Corresponding height profiles are shown below the images.

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In aqueous solution, the positively charged poly(L-lysine) side chains adsorb readily onto the hydrophilic and negatively charged mica surface, which does not significantly disturb the helical structure of the complex (Figure 1c). On extremely hydrophobic graphite, the PLL side chains do not easily adsorb from solution and are eventually forced to the surface during the drying process where either capillary forces squeeze the helices into globules or the brushes adopt a globular shape in order to minimize contact with the surface (Figure 7a). When the mica surface is coated with positively charged poly-L-ornithine hydrobromide, these charges compete with the charges on the brushes for complexation with surfactant, resulting in a lower SDS/lysine ratio where no helices form but only cylindrical structures are present (Figure 7b).

Conclusion Figure 7. AFM height images of the poly(L-lysine) brush-SDS complexes spin-coated on (a) graphite and (b) poly-L-ornithine hydrobromide coated mica surfaces. Corresponding height profiles are shown below the images.

such as at pH = 12.6, only big aggregates with a height of 20-40 nm are seen in the AFM images (Figure 6f). It should be noted that some fractal surface structures of uniform height (approximately 4 nm) emerge if the solution is aged for about 3 days (see the Supporting Information). In the case of complexes formed at pH = 4-5, the amine groups in the PLL side chains are protonated, and complex formation is mainly attributed to the electrostatic attraction between the dodecyl sulfate ions and the protonated amine groups of the PLL side chains. If the pH of the complex solution is decreased to values below the pKa of SDS (∼4), too large a fraction of the surfactant is protonated and the electrostatic interaction between SDS and PLL side chains is reduced. At pH > 6, more and more of the protonated amine groups of the PLL side chains become deprotonated, and the surfactant complexes start to dissociate. Simultaneously, the solubility of the bare uncharged PLL decreases, which eventually causes its collapse into globules (Figure 6e) and larger aggregates (Figure 6f). Finally, we want to point out that the helical structures are not observable on all substrates. Helices were only observed on mica surfaces (Figure 1c), while globular structures were found on graphite (Figure 7a), and wormlike structures prevailed on poly-L-ornithine hydrobromide coated mica (Figure 7b).

Langmuir 2009, 25(11), 6392–6397

Structure formation of poly(L-lysine) brushes with negatively charged surfactant in aqueous solution was investigated with respect to surfactant head group, alkyl chain length, ionic strength, pH, and substrate. Helical structures were observed under conditions (in terms of pH, added salt, and surfactant) where the PLL side chain/surfactant or the respective linear PLL/surfactant complex does form a β-sheet. Thus, the previously postulated hypothesis is confirmed that the twist induced by the β-sheet formation of the side causes the main chain to adopt a helical conformation. Given this example of hierarchical structure formation, future work will focus on the influence of a helix-coil transition of the side chains on the conformation of the main chain which may open novel pathways to supramolecular structure control and to responsive polymer structures. Acknowledgment. We thank the German Science Foundation for financial support (SFB 625) and Dr. Holger Frauen:: rath, ETH Zurich, for fruitful discussions. The help of Dr. Stephan Hobe (Institute of Botanics, University of Mainz) with the CD measurements is gratefully acknowledged. Supporting Information Available: AFM image and height profile of complexes of cylindrical brushes with poly (L-lysine) side chains and SDS at pH = 12.6, and magnification, including height profile, of fractal structure shown in in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la804290r

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