Self-assembled lipid nanotubes by rational design
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Five lipids were self-assembled in aqueous dispersions into high axial ratio nanostructures. Thermal analysis was conducted on a glycolipid self-assembled into nanotubes, previously developed by Kamiya et al. [S. Kamiya, H. Minamikawa, J-H. Jung, B. Yang, M. Masuda, and T. Shimizu, Langmuir 21, 743 (2005)], showing a dry melting onset of 148.2 °C and evidence of a highly ordered supramolecular structure. A novel hybrid structure of the glycolipid nanotubes decorated with silver nanoparticles was created. The self-assembly of four new amphiphiles, with serine and glutamic acid head groups attached to vaccenic acid and diacetylenic hydrophobic tails, was also investigated. The morphologies of these aggregates included high axial ratio nanostructures, such as nanotubes; and flat, twisted, and helical ribbons. The supramolecular aggregates of the five lipids reflect aspects of the molecular structure, such as chirality, providing evidence that such organized aggregates can be created by a rational approach to molecular design.
I. INTRODUCTION
The self-assembly of amphiphilic molecules in aqueous environments is a bottom-up approach to the construction of nanoscale artifacts observed in nature that can be readily translated into workable synthetic nanostructures.1–7 In nature it can be seen that the shape of lipid-based assemblies is driven by the molecular structure of the lipid, and the shape of the assembly has an effect on the function of the aggregate.3,5,6,8–11 Likewise, by making rational alterations to the structure of a precursor amphiphile, the shape and function of synthetic aggregates can be tuned.12 One such useful aggregated structure is straight, high axial ratio nanotubes, which form by the tight, helical turning of a bilayer ribbon.3,6,12–15 In order to design a precursor amphiphile capable of assembling in this manner, several molecular characteristics have to be exploited. At the most basic level, the molecular geometry of the hydrophilic head and hydrophobic tail can be used to predict the likelihood of either the bilayers forming helically based tubes or forming a micellar assembly.10 Molecular motifs specific to ribbon and tube assembly include a rigid segment, often provided by unsaturation, within an otherwise flexible hydrophobic tail. This rigidity encourages bilayer assembly, and if the rigid segment causes angular displacement between neighboring hydrophobic tails, then the formation of elongated ribbon structures are possible.8 In order to facilitate the twisting of long ribbons into helices, groups that provide strong, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.3 322
J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
http://journals.cambridge.org
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directional interactions between neighboring molecules, such as amide and aromatic groups, are desirable.3,6,12 These groups are best positioned at the interface between a polar head group and a nonpolar tail, and close to a chiral center.8,16,17 Chirality imparts a specific d
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