Lipid tubules

Schnur, J.M. (1993) Lipid tubules a paradigm for molecularly engineered structures. Science, 262 (5140), 1669-1676. 

Goren, M., Qi, Z. and Lennox, R.B. (2000) Selective templated growth of polypyrrole strands on lipid tubule edges. Chemistry of Materials, 12, 1222-1228. 

Figure 5.2 Micrographs of metal-coated lipid tubules. Top panel shows scanning electron micrograph of copper-plated microtubules (bar = 2.0 (Jim), while bottom panel shows optical micrograph of iron-coated microtubules embedded in acrylic-urethane clear coating (bar = 25 p,m). Reprinted from Ref. 135 with permission of Wiley-VCH.

In experiments on diacetylenic lipid tubules formed from mixed l- and d-lipids, the tubule radius did not change as a function of enantiomeric excess. Rather, tubules formed from the mixed enantiomers have approximately the same diameter, length, and density as tubules of the pure chiral lipid.125 Electron microscopy shows that the mixed enantiomers form mixtures of tubules... 

The Helfrich-Prost model was extended in a pair of papers by Ou-Yang and Liu.181182 These authors draw an explicit analogy between tilted chiral lipid bilayers and cholesteric liquid crystals. The main significance of this analogy is that the two-dimensional membrane elastic constants of Eq. (5) can be interpreted in terms of the three-dimensional Frank constants of a liquid crystal. In particular, the kHp term that favors membrane twist in Eq. (5) corresponds to the term in the Frank free energy that favors a helical pitch in a cholesteric liquid crystal. Consistent with this analogy, the authors point out that the typical radius of lipid tubules and helical ribbons is similar to the typical pitch of cholesteric liquid crystals. In addition, they use the three-dimensional liquid crystal approach to derive the structure of helical ribbons in mathematical detail. Their results are consistent with the three conclusions from the Helfrich-Prost model outlined above. 

The assumption of membrane softness is supported by a theoretical argument of Nelson et al., who showed that a flexible membrane cannot have crystalline order in thermal equilibrium at nonzero temperature, because thermal fluctuations induce dislocations, which destroy this order on long length scales.188 189 The assumption is also supported by two types of experimental evidence for diacetylenic lipid tubules. First, Treanor and Pace found a distinct fluid character in NMR and electron spin resonance experiments on lipid tubules.190 Second, Brandow et al. found that tubule membranes can flow to seal up cuts from an atomic force microscope tip, suggesting that the membrane has no shear modulus on experimental time scales.191 However, conflicting evidence comes from X-ray and electron diffraction experiments on diacetylenic lipid tubules. These experiments found sharp diffraction peaks, which indicate crystalline order in tubule membranes, at least over the length scales probed by the diffraction techniques.123,192 193... 

Synthetic lipids and peptides have been found to self-assemble into tubules [51,52]. Several groups have used these tubules as templates [17,51,53-56]. Much of this work has been the electroless deposition of metals [51,54]. Electrolessly plated Ni tubules were found to be effective field emission cathode sources [55]. Other materials templated in or on self-assembled lipid tubules include conducting polymer [56] and inorganic oxides [53]. Nanotubules from cellular cytoskeletons have also been used for electroless deposition of metals [57]. 

Fig. 1. Illustration of microcapillary capabilities of PDMS stamp, where a composite suspension of lipid tubules was drop casted onto a glass substrate (a-c). Once the lipid tubules were assembled into the PDMS channels, their confinement was confirmed through transmission mode optical microscopy. Reprinted with permission from ref. 14. Copyright 2008 American Chemical Society.

Zhao Y, Fang J. (2008) Direct printing of self-assembled lipid tubules on substrates. Langmuir 24 5113-5117. 

J.M. Schnur, Lipid Tubules A Paradigm for Molecularly Engineered Structures , Science, 262,1669 (1993)... 

Fig. 4.30 Transmission electron micrographs of chemically modified acetylcholine amphiphilic chiral assemblies (diacetylenic aldonamides), containing monomers (left) and pol)miers (right). In some cases the helical crystalline amphiphilic assemblies wrap into "lipid tubules". Images reproduced firom [57].

It stabilizes the lyotropic liquid crystalline state of biological assemblies relative to the crystalline state, due to the so-called chiral bilayer effect, which will be discussed in more detail in Section 4.2. For example, 10-nonacosanol, extruded from the lipophilic wax layer of pine needles, forms fluid lipid tubules rather than crystals. Although it is difficult to establish the enantiopurity of the natural product, the fact that synthetic pure enantiomers produce tubules while the racemate gives platelets suggests that the biologically relevant morphology is attained because of the enantiopurity of the biomolecule. °... 

Figure 21, Images (a,c) and Fourier transforms (b,d) of helical crystals of streptavidin formed on lipid tubules containing DODA-EOa-biotin (50). (a,c) Stain striations extend along the tubules. Protein densities are particularly visible at tube edges, corresponding to streptavidin molecules viewed edge-on. Scale bar 40 nm. (b,d) Distribution of Fourier transform amplitudes from the tubes shown in (a,c) corresponding to about 1700 streptavidin molecules. The fine spacing between layer lines indicates a helical repeat of 47 nm. Visible diffraction peaks extend up to 1.7 nm (arrowhead in (b)]. Reproduced from ref. 242 (Ringler et al., Chem. Eur. J. 1997, 3, 620) with permission ofWiley-VCH.

The resonance and environmental fluctuation effects in STM currents through adsorbed molecules have been analyzed. AFM has also been used to modify stmcturaUy lipid bilayers in a controlled procedure. The images showed that after the lipid tubule was scratched, the molecules rearrange after some time (24 h). 

Others196 have used lipid tubules as templates during chemical oxidation of pyrrole to form nanofibers with diameters between 10 and 50 nm and lengths reaching up to several hundred microns. 

Wilson-Kubalek et al. also produced specifically and nonspecifically functionalized unilamellar lipid tubules by using mixtures of a tubule-forming galactosylceramide and various charged or derivatized lipids [166]. Thus, nickel-doped lipids allowed the helical crystallization of histidine-tagged proteins. The authors also reproduced the helical crystallization of streptavidin. They even obtained helical arrays of relatively small proteins, such as actin and annexin, as well as large macromolecules, such as RNA polymerase (Fig. 21). 

Fig. 3. Dynamin s liposome-stimulated GTPase activity varies with liposome composition. (A) Dynamin s GTPase activity is greater when assayed in the presence of liposomes composed exclusively of DOPS, compared to liposomes prepared from DOPC PI4,5P2 (90 10 mol%). Liposomes composed of a higher mol% PI4,5P2 (see Fig. 2) are significantly more effective templates. (B) Negative-stain electron micrographs of dynamin-generated lipid tubules formed on DOPS and DOPC PI4,5P2 liposomes are indistinguishable, despite the observed differences in rates of GTP hydrolysis. Scale bar = 50 nm.

The analysis of dynamin helices formed on lipid tubules provides a picture of what happens to a dynamin assembly upon GTP hydrolysis, as dynamin adopts different conformations in the presence of different nucleotides. While the GTP bound state of dynamin (observed in the presence of GTP7S) forms a tight helix with a spacing of 11 nm, in the GDP bound state the spacing has nearly doubled to 20 nm (Stowell et al, 1999). 

A peculiar type of templates is represented by phosphoUpids. Lipid tubules were introduced by Schnur and coworkers [165] a couple of decades ago and more recently have been investigated as promising templating materials for the selective growth of PPy nanostructures which smprisingly self-assemble at the edges (not at the surface) of the phospholipidic tubules [146]. 

NRL is well positioned to participate in the development of new materials based on supramolecular interactions. Current NRL investigations of ferroelectric Uquid crystals and lipid tubules provide excellent examples of the kinds of novel structures and properties to be reahzed by taking proper account of both covalent and noncovalent bonds in controlling the architectural features of materials. These investigations have been unusnaUy prodnctive and merit high priority. 

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