Lipids diacetylenic

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... 

Figure 5.52 (a) Space-filling model of three-dimensional structure of diacetylenic lipid DCg PC... 

According to this argument, diacetylenic lipids do not have an unusually high value of the chiral field h compared with other lipids. Rather, diacetylenic lipids have an unusual negative value of the coefficient t, which gives a free... 

This scenario for molecular packing leading to biased chiral symmetrybreaking is quite speculative. However, it makes an important point for molecular modeling of lipid membranes The unusual feature of diacetylenic lipids does not have to be associated with the stereocenter of the molecules but rather may be a broken symmetry in the packing of the kinks in the acyl chains. This speculation needs to be investigated by detailed molecular modeling calculations. 

Figure 9. VIS Multiplot of absorbance of a monolayer of diacetylene lipid (5) vs. polymerization time. Constant surface pressure 10 mN m l 20°C N2 atmosphere.

In both of these cases, the ligand (sialic acid) for the analyte of interest (influenza vims) was covalently linked to the PDA backbone generated upon photopolymerization. Functional sensors based on ligands that are noncovalently incorporated into liposomes have also been reported (Charych et al. 1996 Pan and Charych 1997). Mixed liposomes as well as mixed thin films on glass containing a combination of the ganglioside GMl and diacetylene lipids detect the presence of cholera toxin, a protein that binds to GMl. 

Peek BM, Callahan JH, Namboodiri K, Singh A, Gaber BP. Effect of vesicle size on the pol5fmerization of a diacetylene lipid. Macromolecules 1994 27 292-297. 

Domain formation in binary mixtures of a polymerizable lipid and non-polymerizable lipid is well established for diacetylenic lipids. The rigid diacetylenic unit facilitates the formation of enriched domains in the condensed phase of monolayers or the solid-analogous phase of bilayers. Since diacetylenes polymerize most readily in solid-like states, most studies have focused on conditions that favor domain formation. Only in the case of a mixture of a charged diacetylenic lipid and a zwitterionic PC was phase separation not observed. Ringsdorf and coworkers first reported the polymerization of a phase-separated two-dimensional assembly in 1981 [33], Monolayer films were prepared from mixtures consisting of a diacetylenicPC (6) (Fig. 5) and a nonpolymerizable distearoyl PE (DSPE). 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

Aqueous dispersions of polymerizable lipids and surfactants can be polymerized by UV irradiation (Fig. 18). In the case of diacetylenic lipids the transition from monomeric to polymeric bilayers can be observed visually and spectroscopically. This was first discussed by Hub, 9) and Chapman 20). As in monomolecular layers (3.2.2) short irradiation brings about the blue conformation of the poly(diacetylene) chain. In contrast, upon prolonged irradiation or upon heating blue vesicles above the phase transition temperature of the monomeric hydrated lipid the red form of the polymer is formed 23,120). The visible spectra of the red form in monolayers and liposomes are qualitatively identical (Fig. 19). 

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

What reasons are there for mixing polymerizable lipids with natural ones Polymerized membrane systems, especially those based on diacetylenic lipids, have proven to be excessively rigid and to show no phase transition. Addition of natural lipids could help to retain a certain membrane mobility even in the polymerized state, with almost unaffected stability. Furthermore, natural lipids can provide a suitable environment for the incorporation of membrane proteins into polymerizable membranes (see 4.2.3). Besides this, enzymatic hydrolysis of the natural membrane component can be used for selectively opening up a vesicle in order to release entrapped substances in a defined manner (see 4.2.2). Therefore, it is interesting to learn about the miscibility of polymerizable and natural lipids and also about the polymerization behavior of these mixtures. Investigations on this subject have thus far focused on mixtures of natural lipids with polymerizable lipids carrying diacetylene moieties. 

Due to the topochemical restrictions of diacetylene polymerization, diacetylenic lipids are solely polymerizable in the solid—analogous phase. During the polyreaction an area contraction occurs leading to a denser packing of the alkyl chains. In addition to surface pressure/area isotherms the polymerization behavior of diacetylenic lipids containing mixed films give information about the miscibility of the components forming the monolayer ... 

Provided the components are completely miscible and hexagonally packed in a mixed film below a molar ratio of 0.25 of diacetylenic lipid, each of the 6 nearest neighbors of a polymerizable lipid molecule is a nonpolymerizable natural lipid. Due to the low lateral diffusion rate in the condensed phase diacetylene polymerization should either become impossible or at least proceed at a considerably lower rate. 

After vesicle polymerization the phase transition of the diacetylenic lipid has almost completely disappeared, while the phase transition of DSPC is unaffected by polymerization (Fig. 33b). The same holds true for mixtures of (23) with DOPC 62. ... 

Preliminary experiments with 6-CF loaded mixed vesicles of dipalmitoylphosphat-idylcholine (DPPC) and diacetylenic lipid (22) exhibit only a very slow release of 6-CF after the addition of phopholipase A2. Reasons for this may be headgroup interactions which decrease the enzymatic activity or the fact that multilamellar vesicles were used for this study. Further experiments will be carried out with giant liposomes (visible under the light microscope) which are unilamellar and provide a higher enclosure percentage and low surface curvature (see 4.3.2). 

In contrast to diacetylenes, lipids with alkene-functionalized chains (e.g., acryloyl, dienoyl) can be polymerized in the La phase to a high degree of conversion [74], O Brien and coworkers systematically studied polymerization of acryloyl and dienoyl lipids in bilayer vesicles (see [25,26] for reviews). Subsequently, the Saavedra and O Brien groups prepared and characterized solid supported bilayers composed of dienoyl lipids (Fig. 6) [11, 75-77], Several parameters relating... 

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