Tubules diacetylenic lipids

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... 

Furthermore, Oda et al. pointed out that there are two topologically distinct types of chiral bilayers, as shown in Figure 5.46.165 Helical ribbons (helix A) have cylindrical curvature with an inner face and an outer face and are the precursors of tubules. These are, for example, the same structures that are observed in the diacetylenic lipid systems discussed in Section 4.1. By contrast, twisted ribbons (helix B) have Gaussian saddlelike curvature, with two equally curved faces and a C2 symmetry axis. They are similar to the aldonamide and peptide ribbons discussed in Sections 2 and 3, respectively. The twisted ribbons in the tartrate-gemini surfactant system were found to be stable in water for alkyl chains with 14-16 carbons. Only micelles form... 

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... 

As discussed in Sects. 2.3 and 2.4, mixing the two enantiomers generally results in important changes in fiber morphology. Racemates tend to form platelets that do not express any chirahty. hi a couple of cases [35,36] as, for example, for diacetylenic lipid 22 [33,34], a mixture of right-handed and left-handed helices is observed instead of platelets. Data are rarely available concerning the behavior of mixtures of enantiomers other than 1 1 racemic mixtures. In the case of diacetylenic lipid 22, the phase separation between the enantiomers should lead to tubules of opposite handedness, the propor-... 

Tubules and helical ribbons formed by diacetylenic lipids such as 22 (Scheme 4) have been extensively used as templates for metalhc coatings [173], and the deposition of gold nanoparticles [174] or polypyrrole threads [175]. Their coassembly with silica precursors gives rise to organic/inorganic hybrids having mesoscopic helical and tubular shapes and multUamellar walls [176]. 

Diacetylenes in phospholipid bilayers have been the subject of extensive studies in our laboratory, not only because of the highly conjugated polymers they form, but also because of their ability to transform bilayers into interesting microstructures. Consequent to our synthesis and characterization of several isomeric diacetylenic phospholipids, we have found that the polymerization in diacetylenic bilayers is not complete. In order to achieve participation of all diacetylenic lipid monomer in the polymerization process, diacetylenic phospholipid was mixed with a spacer lipid, which contained similar number of methylenes as were between the ester linkage and the diacetylene of the polymerizable lipid. Depending upon the composition of the mixtures different morphologies, ranging from tubules to liposomes, have been observed. Polymerization efficiency has been found to be dependent on the composition of the two lipids and in all cases the polymerization was more rapid and efficient than the pure diacetylenic system. We present the results on the polymerization properties of the diacetylenic phosphatidylcholines in the presence of a spacer lipid which is an acetylene-terminated phosphatidylcholine. 

There are two reasons to think this situation might occur. The first reason is experimental. As discussed in Sections 2-5, in most experiments on chiral materials, tubules and helical ribbons are observed with only one sense of handedness. However, there are a few exceptions in experiments on diacetylenic phospholipids,144 diacetylenic phosphonate lipids,145 146 and bile.162 In these exceptional cases, some helices are observed with the opposite sense of handedness from the majority. In the work on diacetylenic phospholipids, the minority handedness was observed only during the kinetic process of tubule formation at high lipid concentration,144 which is a condition that should promote metastable states. Hence, these experiments may indeed show a case of biased chiral symmetry-breaking in which the molecular chirality favors a state of one handedness and disfavors a mirror image state. 

Tubules have also been prepared by swelling thin films of polymerizable diacetylenic phosphatidylhydroxyethanol (choline functionally in 21 is replaced by hydroxyethanol) in aqueous metal ion solutions above the phase transition temperature of the lipid. Various cylindrical structures were observed upon swelling the lipid in the presence of mono- and divalent cations. In contrast, no definable microstructures were noted in the absence of cations [362],... 

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].

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