Liquid-crystal imaging of molecular-tilt ordering in self-assembled lipid tubules

Abstract

Self-assembled cylindrical tubules of chiral phospholipids are interesting supramolecular structures. Understanding the moleculartilt order is a key step in controlling the size and shape of the tubules and designing new functional materials. The current theories based on the chiral interactions, coupled with molecular tilt, have predicted that the tubules could have both uniform and modulated tilt states. Here, we image the molecular-tilt order in the self-assembled tubules of a chiral phospholipid by using liquid crystals as an optical amplification probe. We demonstrate that the organization of the molecular-tilt azimuth in the lipid tubules can induce an azimuthal orientation in the liquid crystals. Both uniform and modulated tilt states of the lipid tubules are observed after liquid-crystal optical amplification.liquid-crystal amplification ͉ molecular ordering ͉ self-assembly ͉ optical amplification ͉ supramolecular structure T he self-assembled cylindrical tubules have attracted considerable attention because of their interesting supramolecular structures and technological applications (1, 2). It has been found that a variety of amphiphilies, including alkylaldonamides (3), diacetylenic phospholipids (4, 5), amino acid-based surfactants (6, 7), and multicomponent mixtures in bile (8, 9), selfassemble into cylindrical tubules in solutions. Recently, advances have been made in controlling the sizes and shapes of tubules by adjusting the chemical compositions and the conditions under which self-assembly occurs (10-16).Theoretical models based on the chiral interactions, coupled with molecular tilt, have been used to explain the formation of the cylindrical tubules (17)(18)(19). In these models, the chiral molecules do not pack parallel to their neighbors but, rather, at a nonzero angle with respect to their neighbors. This chiral packing causes the twisting of a bilayer stripe, which leads to the formation of cylindrical tubules. The model by Selinger et al. (19) suggests that the tubules can have both uniform and modulated tilt states. In the uniform tilt state, the tubule has a constant orientation of the molecule tilt with respect to the equator of the cylinder (Fig. 1a). In the modulated tilt state, the tubule has a periodic modulated structure by helical stripes in the molecular tilted direction winding around the cylinder. The direction of the molecular tilt smoothly varies across the stripes and jumps at the edges of the stripes (Fig. 1b). These stripes in the tubules are suggested to be analogous to the tilted stripes seen in the Langmuir monolayers (20) but in a cylindrical rather than a planar geometry. To our knowledge, there has been no experimental test of the theoretical predictions because of the difficulty in probing the local direction of the molecular tilt in the tubules.In recent years, there has been an increased interest in developing an imaging technique based on liquid-crystal optical amplification for studying the structures and physical properties of organic and biological interfac...

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