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Nanotubes and bilayers in a model peptide system

Seminar Group: 


Professor Ulf Olsson


Division of Physical Chemistry
Lund University, Sweden


Monday, November 14, 2011 - 10:30am


MRL Room 2053

The trifluoroacetate (tfa) salt of the hepta-peptide (ala)6lys self-assembles in water into very long, hollow nanotubes with a monodisperse radius R=26 nm, above a critical aggregation concentration (volume fraction), φcac=0.10. The peptides carry a net positive charge that ensures colloidal stability of the self-assembly structures through a long-range electrostatic repulsion. There is only a weak temperature dependence of φcac from which an enthalpy of aggregation of ≈ -kBT per molecule is estimated. SAXS data show that the thickness of the nanotube wall, δ, is less than 1 nm indicating that the peptides form a monolayer in the nanotube wall. XRD indicate a crystalline ordering, but only a few reflections are obtained. The nanotubes have a very large aspect ratio and form an ordered nematic or hexagonal phase. Because of the low δ/R ratio, the nanotube volume fraction grows very rapidly with increasing peptide concentration, φ, and reaches close packing already at φ = 0.15. Upon further increase of the concentration, there is a phase transition to a novel lamellar phase where the peptide molecules form bilayers consisting of two, presumably oppositely oriented, monolayers.