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Langmuir 2007, 23, 10772-10778
Reversible Helix Sense Inversion in Surface-Grafted Poly(β-phenethyl-L-aspartate) Films Jeroen Luijten, Eltjo J. Vorenkamp, and Arend J. Schouten* Department of Polymer Chemistry, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ReceiVed April 17, 2007. In Final Form: July 11, 2007 The reversible manipulation of the helix screw sense in surface-grafted poly(β-phenethyl-L-aspartate) (PPELA) films by means of external stimuli was investigated. Ringopening polymerization of β-phenethyl-L-aspartate N-carboxyanhydride initiated from primary amino-functionalized silicon and quartz substrates results in surfacegrafted PPELA films in which the end-grafted polypeptide chains have a right-handed R-helical conformation. Upon annealing of the film at 150 °C for 30 min, a helix screw sense inversion takes place and the grafted chains adopt a left-handed π-helical conformation. In the solid state, this left-handed π-helical form is completely stable and cannot be changed by reheating and/or cooling. Upon immersion of the annealed grafted film in chloroform or other helicogenic solvents, the grafted polypeptide chains completely revert to their original right-handed R-helical form. Successive annealing and solvent treatment steps show that this helix sense inversion cycle can be repeated many times.
Introduction Polyaspartates based on the β-esters of L-aspartic acid (see Figure 1) form a unique class of helical polypeptides because their helix screw sense (i.e., the right- or left-handedness) depends on the structure of the ester side chain as well as external factors such as temperature and solvent.1-6 For example, poly(β-methylL-aspartate) and poly(β-benzyl-L-aspartate) have a left-handed (LH) R-helical conformation in chloroform at 30 °C and in the solid state, whereas poly(β-ethyl-L-aspartate) and poly(β-phenethyl-L-aspartate) adopt a right-handed (RH) R-helix under the same conditions. In addition to the varying helix sense, some polyaspartates also exhibit a thermally induced R-ω7-9 or R-π10 transition. In this case, the R-helices (with 3.6 monomer residues per helical turn) are converted to so-called ω-helices (with 4 residues per turn) or π-helices (with 4.4 residues per turn). This conformational variation is caused by the fact that the peptide bonds in polyaspartates cannot form perfectly stable hydrogen bonds along the main chain due to competitive hydrogen bonding between NH groups in the polymer backbone and CdO groups in the ester side chain.11-16 So, the intramolecular hydrogen-bonding * Corresponding author. E-mail:
[email protected]. (1) Bradbury, E. M.; Downie, A. R.; Elliot, A.; Hanby, W. E. Proc. R. Soc. London, Ser. A 1960, 259, 110. (2) Karlson, R. H.; Norland, K. S.; Fasman, G. D.; Blout, E. R. J. Am. Chem. Soc. 1960, 82, 2268. (3) Goodman, M.; Boardman, F.; Litowsky, L. J. Am. Chem. Soc. 1963, 85, 2491. (4) Bradbury, E. M.; Carpenter, B. G.; Goldman, H. Biopolymers 1968, 6, 837. (5) Toriumi, H.; Saso, N.; Yasumoto, Y.; Sasaki, S.; Uematsu, I. Polym. J. 1979, 12, 977. (6) Watanabe, J.; Okamoto, S.; Satoh, K.; Sakajiri, K.; Furuya, H. Macromolecules 1996, 29, 7084. (7) Sasaki, S.; Ogawa, H.; Kimura, S. Polym. J. 1991, 23, 1325. (8) Tsujita. Y.; Fukagawa, M.; Uematsu, I. Polym. J. 1982, 14, 773. (9) Tsujita, Y.; Fukagawa, M.; Uematsu, I. Polym. J. 1982, 14, 781. (10) Sasaki, S.; Yasumoto, Y.; Uematsu, I. Macromolecules 1981, 14, 1797. (11) Yang, J. T. In Protein Models for Conformational Studies; Fasmann, G. D., Ed.; Marcel Dekker: New York, 1967; Chapter 6. (12) Tsujita, Y.; Satomi, N.; Takizawa, A. J. Macromol. Sci. Phys. 19831984, B22, 619. (13) Watanabe, T.; Tsujita, Y.; Takizawa, A.; Kinoshita, T. Polymer 1987, 28, 1809. (14) Tsujita, Y. Biophys. Chem. 1988, 31, 11. (15) Tsujita, Y.; Watanabe, T.; Takizawa, A.; Kinoshita, T. Polymer 1991, 32, 569.
Figure 1. (A) L-Aspartic acid; (B) L-aspartic acid β-ester; (C) poly(L-aspartate).
pattern that leads to the formation of an R-helix is disturbed, resulting in a less stable conformation. In certain polyaspartates and copolyaspartate systems, the helix sense in solution and in the solid state varies with the temperature. For example, poly(β-n-propyl-L-aspartate), dissolved in chloroform, exhibits an inversion from a RH to a LH R-helix when the temperature of the solution is raised from 30 to 60 °C. After subsequent cooling to 30 °C, the polymer returns to the RH state again.4 Polyaspartates based on p-chlorobenzyl and p-methylbenzyl esters7,17-19 show a similar thermally induced helix inversion, although the transition appears to be irreversible in solid films. Copolymers of β-benzyl-L-aspartate and β-alkylL-aspartate (with R g C2)4,8,9,12,13,15,20-22 show a LH to RH transition both in solution and in the solid state as the degree of alkylation increases. In the case of an intermediate degree of alkylation, the reversal in helix sense is also observed when the temperature is increased.14 Solvent-induced helix inversion in polyaspartates was first demonstrated by Hashimoto.23 In chloroform, a 50/50 copolymer of β-benzyl-L-aspartate and β-p-methylbenzyl-L-aspartate adopts a LH helix, whereas in N,N-dimethylformamide (DMF) it takes the RH helical form, the transition occurring at 90% chloroform/ 10% DMF. (16) Oosterling, M. L. C. M.; Willems, E.; Schouten, A. J. Polymer 1995, 36, 4485. (17) Hashimoto, M.; Aritomi, A. Bull. Chem. Soc. Jpn. 1966, 39, 2707. (18) Hashimoto, M.; Arakawa, S. Bull. Chem. Soc. Jpn. 1967, 40, 1698. (19) Bradbury, E. M.; Carpenter, B. G.; Stephens, R. M. Biopolymers 1968, 6, 905. (20) Tsujita, Y.; Fujii, N.; Imoto, Y.; Takizawa, A.; Kinoshita, T. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 2533. (21) Kusakawa, M.; Tsujita, Y.; Takizawa, A. Polymer 1985, 26, 848. (22) Nishizima, R.; Tsujita, Y.; Takizawa, A. Polymer 1985, 26, 379. (23) Hashimoto, M. Bull. Chem. Soc. Jpn. 1966, 39, 2713.
10.1021/la7011217 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/15/2007
Surface-Grafted Poly(β-phenethyl-L-aspartate) Films
Langmuir, Vol. 23, No. 21, 2007 10773 Scheme 1. Synthesis of β-Phenethyl-L-aspartate and Its Corresponding NCA and Preparation of a Surface-Grafted Poly(β-phenethyl-L-aspartate) Film
Figure 2. Helical polypeptides with isomeric monomer units: (A) poly(γ-benzyl-L-glutamate) and (B) poly(β-phenethyl-L-aspartate).
Photoresponsive polyaspartates24-28 have been reported as well. In these systems, the helix sense inversion is triggered by photoisomerization of azobenzene moieties in the polymer. In terms of structure, the previously mentioned poly(βphenethyl-L-aspartate) (PPELA) is very similar to poly(γ-benzylL-glutamate) (PBLG), as shown in Figure 2. PBLG forms a stable RH R-helix in the solid state and in a variety of organic solvents.29 PPELA, however, shows interesting temperature- and solvent-induced helix sense inversion behavior, both in solution and in the solid state.30,31 In the solid state as well as in helicogenic solvents such as chloroform, 1,2dichloroethane, and 1,1,2,2-tetrachloroethane, PPELA adopts a RH R-helical structure at room temperature. Upon heating to 85-100 °C, the polymer forms a LH R-helix in 1,1,2,2tetrachloroethane,32 both in the isotropic solution and in the lyotropic liquid crystalline state.33 The addition of small amounts (1-2%) of dichloroacetic acid (DCA) to a chloroform solution of PPELA also induces helix inversion due to disruption of the intramolecular hydrogen bonds by DCA.5 In the case of solid PPELA films prepared by solution casting, heating to 130150 °C results in an irreversible helix sense inversion, accompanied by an R-π transition.10 The aim of this study is to investigate the possibility of reversibly manipulating the helix screw sense in surface-grafted PPELA films by means of external stimuli. In theory, such functional polymer films could be used for the construction of organic switches and (reversible) data storage systems.34-38 So, silicon- and quartz-grafted PPELA films were prepared (see Scheme 1), and the influence of temperature and solvent on the helix sense of the end-grafted polyaspartate chains was studied. Experimental Section Materials and Procedures. L-Aspartic acid (Janssen Chimica, 98+%), phenethyl alcohol (Aldrich, 99%), 3-aminopropyltriethoxy(24) Ueno, A.; Anzai, J.; Osa, T.; Kadoma, Y. J. Polym. Sci., Polym. Lett. Ed. 1977, 15, 407. (25) Ueno, A.; Takahashi, K.; Anzai, J.; Osa, T. Macromolecules 1980, 13, 459. (26) Ueno, A.; Takahashi, K.; Anzai, J.; Osa, T. J. Am. Chem. Soc. 1981, 103, 6410. (27) Ueno, A.; Takahashi, K.; Anzai, J.; Osa, T. Chem. Lett. 1981, 113. (28) Ueno, A.; Adachi, K.; Nakamura, J.; Osa, T. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 1161. (29) Block, H. Poly(γ-benzyl-L-glutamate) and other Glutamic Acid Containing Polymers; Gordon and Breach Publishers: New York, 1983. (30) Okamoto, S.; Furuya, H.; Watanabe, J.; Abe, A. Polym. J. 1996, 28, 41. (31) Okamoto, S.; Furuya, H.; Abe, A. Polym. J. 1995, 27, 746. (32) Yamamoto, T.; Honma, R.; Nishio, K.; Hirotsu, S.; Okamoto, S.; Furuya, H.; Watanabe, J.; Abe, A. J. Mol. Struct. 1996, 375, l. (33) Abe, A.; Furuya, H.; Okamoto, S. Biopolymers 1997, 43, 405. (34) Furukawa, K.; Ebata, K.; Fujiki, M. AdV. Mater. 2000, 12, 1033. (35) Saxena, A.; Guo, G.; Fujiki, M.; Yang, Y.; Ohira, A.; Okoshi, K.; Naito, M. Macromolecules 2004, 37, 3081. (36) Kim, S.-Y.; Fujiki, M.; Ohira, A.; Kwak, G.; Kawakami, Y. Macromolecules 2004, 37, 4321. (37) Guo, G.; Naito, M.; Fujiki, M.; Saxena, A.; Okoshi, K.; Yang, Y.; Ishikawa, M.; Hagihara, T. Chem. Commun. 2004, 276. (38) Ohira, A.; Okoshi, K.; Fujiki, M.; Kunitake, M.; Naito, M.; Hagihara, T. AdV. Mater. 2004, 16, 1645.
silane (APS; Acros, 99%), triphosgene (Acros, 99%), and chloroform (Acros, extra dry, water
