Aqueous Cholesteric Liquid Crystals Using Uncharged Rodlike Polypeptides Enrico G. Bellomo, † Patrick Davidson, ‡ Marianne Impe ´ ror-Clerc, ‡ and Timothy J. Deming* ,† Contribution from the Departments of Materials and Chemistry, Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, and Laboratoire de Physique des Solides, UMR 8502 CNRS, Ba ˆ t. 510, UniVersite ´ d’Orsay, F-91405 Orsay, France Received April 9, 2004; E-mail: tdeming@mrl.ucsb.edu Abstract: The aqueous, lyotropic liquid-crystalline phase behavior of the R-helical polypeptide, poly(Nǫ- 2-[2-(2-methoxyethoxy)ethoxy]acetyl-lysine) (1), has been studied using optical microscopy and X-ray scattering. Solutions of optically pure 1 were found to form cholesteric liquid crystals at volume fractions that decreased with increasing average chain length. At very high volume fractions, the formation of a hexagonal mesophase was observed. The pitch of the cholesteric phase could be varied by a mixture of enantiomeric samples L-1 and D-1, where the pitch increased as the mixture approached equimolar. The cholesteric phases could be untwisted, using either magnetic field or shear flow, into nematic phases, which relaxed into cholesterics upon removal of field or shear. We have found that the phase diagram of 1 in aqueous solution parallels that of poly(γ-benzyl glutamate) in organic solvents, thus providing a useful system for liquid-crystal applications requiring water as solvent. Introduction Aqueous, lyotropic liquid crystals have great potential for a wide range of applications. These include use as templates for biomimetic inorganic materials synthesis 1 and as electric-field responsive materials in actuators and devices. 2 Furthermore, cholesteric aqueous liquid crystals would be valuable as chiral NMR solvents for enantiomer resolution, 3 and as a chiral medium for asymmetric transformations. 4 In the area of polypeptide liquid crystals, poly(γ-benzyl-L-glutamate) (PBLG) is perhaps the best studied example. 5 This rodlike, R-helical polymer forms cholesteric liquid crystals in a number of organic solvents at concentrations above ca. 20 wt %, depending on chain length and temperature. 6 Unfortunately, there is no aqueous phase polypeptide analogue of PBLG, even though the organic system has been known for over 40 years. A good reason for this has been the lack of a water-soluble, R-helical homopolypeptide. The repulsion of abundant-like charges on most water-soluble homopolypeptides (e.g., polylysine‚HBr or polyglutamate‚Na salt) is sufficient to prevent R-helix formation in aqueous solutions of these samples. 7 The most studied water-soluble, nonionic polypeptides, poly(ω-hydroxyalkyl)glutamines (e.g., poly(3-hydroxypropyl)glutamine), 8 are only weakly R-helical, which is likely a consequence of the low hydrophobicity of the glutamine side-chains. Our solution to this problem was preparation of polypeptides using ethyleneglycol-modified lysine residues. 9 Short, uniform diethyleneglycol segments provided excellent water solubility, and the increased hydrophobicity of a lysine relative to glutamine backbone resulted in increased helix stability in water. These homopolymers, poly(N ǫ -2-[2-(2- methoxyethoxy)ethoxy]acetyl-lysine), 1 (Figure 1), were found to be miscible with water in all proportions at ambient temperature, completely R-helical in aqueous solution, and able to form birefringent liquid-crystalline solutions at high weight fractions. 9 Here, we describe the nature of these aqueous polypeptide liquid crystals in detail. Results Optical Observations. Figure 2 shows test tubes filled with increasing volume fractions of polymer L-1 in water, viewed † University of California, Santa Barbara. ‡ Universite ´ d’Orsay. (1) Dujardin, E.; Blaseby, M.; Mann, S. J. Mater. Chem. 2003, 13, 696-699. (2) Thomsen, D. L., III; Keller, P.; Naciri, J.; Pink, R.; Jeon, H.; Shenoy, D.; Ratna, B. R. Macromolecules 2001, 34, 5868-5875. (3) (a) Canet, I.; Meddour, A.; Courtieu, J.; Canet, J. L.; Salau ¨n, J. J. Am. Chem. Soc. 1994, 116, 2155-2156. (b) Merlet, D.; Ancian, B.; Courtieu, J.; Lesot, P. J. Am. Chem. Soc. 1999, 121, 5249-5258. (4) Hatano, M.; Nozawa, N. Prog. Polym. Sci. Jpn. 1972, 4, 223-271. (5) Block, H. Poly(γ-benzyl-L-glutamate) and other Glutamic Acid Containing Polymers; Gordon and Breach: New York, 1983. (6) (a) Robinson, C.; Ward, J. C. Nature 1957, 180, 1183-1184. (b) Robinson, C. Tetrahedron 1961, 13, 219-234. (7) Katchalski, E.; Sela, M. AdV. Protein Chem. 1958, 13, 243-492. (8) (a) Lupu-Lotan, N.; Yaron, A.; Berger, A.; Sela, M. Biopolymers 1965, 3, 625-655. (b) Lupu-Lotan, N.; Yaron, A.; Berger, A. Biopolymers 1966, 4, 365-368. (c) Okita, K.; Teramoto, A.; Fujita, H. Biopolymers 1970, 9, 717-738. (9) Yu, M.; Nowak, A. P.; Pochan, D. J.; Deming, T. J. J. Am. Chem. Soc. 1999, 121, 12210-12211. Figure 1. Structure and schematic representation of polymer L-1. Published on Web 07/02/2004 10.1021/ja047932d CCC: $27.50 © 2004 American Chemical Society J. AM. CHEM. SOC. 2004, 126, 9101-9105 9 9101