pubs.acs.org/Langmuir Peptide Nanotube Nematic Phase S. Bucak,* ,† C. Cenker, ‡ I. Nasir, †,‡ U. Olsson, ‡ and M. Zackrisson ‡,§ † Department of Chemical Engineering, Yeditepe University, Istanbul, Turkey and ‡ Physical Chemistry 1, Lund University, Box 124, SE-221 00 Lund, Sweden. § Present address: The Adolphe Merkle Institute, Universit e de Fribourg Rte de l’Ancienne Papeterie, Box 209, CH-1723 Marly, Switzerland Received December 18, 2008. Revised Manuscript Received February 7, 2009 The self-assembly of the trifluoroacetate salt of the short peptide (ala) 6 -lys (A 6 K) in water has been investigated by cryo-transmission electron microscopy and small-angle X-ray scattering. For concentrations below ca. 12%, the peptide does not self-assemble but forms a molecularly dispersed solution. Above this critical concentration, however, A 6 K self-assembles into several-micrometer-long hollow nanotubes with a monodisperse cross- sectional radius of 26 nm. Because the peptides carry a positive charge, the nanotubes are charge-stabilized. Because of the very large aspect ratio, the tubes form an ordered phase that presumably is nematic. Introduction The development of modern peptide chemistry 1,2 has opened the possibility of custom peptide synthesis that allows for systematically investigating the relationship between a specific oligopeptide molecular structure and the macroscopic phases and structures formed in such systems. 3-5 Under- standing the assembly behavior of peptides is important not only in designing nanomaterials for a desired functionality 6,7 but also in combating neurodegenerative diseases such as Alzheimer’s and Parkinson’s, which are strongly associated with an accumulation of amyloid-forming peptides in the brain. 8,9 Several peptides have been found to undergo self-assembly into various morphologies and structural length scales. Much work has naturally focused on the amyloid-forming peptides, or selected fragments of these, because of the close connection to neurodegenerative diseases. However, several synthetic oligopeptides have also been shown to self-assemble. Gener- ally, β-sheet formation through hydrogen bonding is the basis of peptide self-assembly, resulting in ribbons, tapes, and sometimes nanotubes. These structures may further aggregate or precipitate as fibrils, as in amyloid formation. If properly stabilized, however, stable self-assemblies may be obtained where ribbons or tapes may entangle, forming a viscoelastic solution of “living polymers”, and more rigid nanotubes may form ordered nematic or hexagonal phases. It has also been suggested that certain amphiphilic or surf- actant-like peptides may self-assemble because of hydrophobic interactions. Depending on the peptide, one observes micelle formation 10 or peptide bilayers forming spherical or tubular vesicles. 11,12 It is difficult to determine the local peptide organization in these structures. Val  ery et al. were able to obtain detailed structural information on nanotubes of the lanreotide peptide on the basis of high-resolution fiber diffraction data. 13 In analyzing the ordered structure, they found that the tube wall was made up of parallel helical β-sheet ribbons in two layers, giving a bilayer structure. A similar helical ribbon structure has been proposed in another system, although with a slight polydispersity in the number of layers. 14 In the case of rigid nanotubes, one expects the formation of ordered nematic or hexagonal phases driven by excluded volume interactions. Such ordering is also of interest from an application point of view because it allows for aligning the nanotubes using an external field, such as shear. Ordered phases, however, have until now been identified in only a few systems. 13-15 In this letter, we report on the self-assembly structures formed by short synthetic oligopeptide A 6 K in water, includ- ing the formation of liquid-crystalline, presumably nematic, ordering. The self-assembly structures are investigated using cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Cryo-TEM has an advantage over ordinary TEM or SEM in that it allows the imaging of the real solution structure at the given composi- tion, not only its solid content after drying. Although there are some reports on peptide nanotubes, to our knowledge this is the first time that cryo electron microscopy data has been available for these kinds of systems, and the data presented in this letter explicitly show the 3D nanotubes of a certain size in great detail. *To whom correspondence should be addressed. E-mail: seyda@ yeditepe.edu.tr. (1) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–2154. (2) Howl, J. Peptide Synthesis and Applications; Humana Press: Totawa, NJ, 2005. (3) Gazit, E. Chem. Soc. Rev. 2007, 36, 1263–1269. (4) Ulijn, R. V.; Smith, A. M. Chem. Soc. Rev. 2008, 37, 664–675. (5) Zhao, X. A.; Zhang, S. G. Macromol. Biosci. 2007, 7, 13–22. (6) Pouget, E.; Dujardin, E.; Cavalier, A.; Moreac, A.; Valery, C.; Marchi- Artzner, V.; Weiss, T.; Renault, A.; Paternostre, M.; Artzner, F. Nat. Mater. 2007, 6, 434–439. (7) Reches, M.; Gazit, E. Science 2003, 300, 625–627. (8) Harper, J. D. BMC Chem. Biol. 1997, 4, 119–125. (9) Walsh, D. M.; Lomakin, A.; Benedek, G. B.; Condron, M. M.; Teplow, D. B. J. Biol. Chem. 1997, 272, 22364–22372. (10) Dong, H.; Paramonov, S. E.; Aulisa, L.; Bakota, E. L.; Hartgerink, J. D. J. Am. Chem. Soc. 2007, 129, 12468–12472. (11) Soto, P.; Griffin, M. A.; Shea, J. E. Biophys. J. 2007, 93, 3015–3025. (12) Zhang, S. G.; Holmes, T.; Lockshin, C.; Rich, A. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 3334–3338. (13) Valery, C.; Paternostre, M.; Robert, B.; Gulik-Krzywicki, T.; Nar- ayanan, T.; Dedieu, J. C.; Keller, G.; Torres, M. L.; Cherif-Cheikh, R.; Calvo, P.; Artzner, F. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10258–10262. (14) Krysmann, M. J.; Castelletto, V.; McKendrick, J. E.; Clifton, L. A.; Hamley, I. W.; Harris, P. J. F.; King, S. M. Langmuir 2008, 24, 8158. (15) Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11857–11862. Published on Web 3/10/2009 © 2009 American Chemical Society DOI: 10.1021/la804175h Langmuir 2009, 25(8), 4262–4265 4262 Downloaded by BIBSAM CONSORTIA SWEDEN on August 5, 2009 Published on March 10, 2009 on http://pubs.acs.org | doi: 10.1021/la804175h