The temperature-dependent self-assembly of the single-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(trimethylammonio)ethyl phosphate] (PC-C32-PC) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray scattering, rheological measurements, and dynamic light scattering (DLS). At room temperature this compound, in which two phosphocholine headgroups are connected by a C(32) alkyl chain, proved to be capable of gelling water very efficiently by forming a dense network of nanofibers (Kohler et al. Angew. Chem., Int. Ed. 2004, 43, 245). A specific feature of this self-assembly process is that it is not driven by hydrogen bonds but solely by hydrophobic interactions of the long alkyl chains. The nanofibers have a thickness of roughly the molecular length and show a helical superstructure. A model for the molecular structure of the fibrils which considers the extreme constitution of the bolaamphiphile is proposed. Upon heating the suspensions three different phase transitions can be detected. Above 49 degrees C, the temperature of the main transition where the alkyl chains become "fluid", a clear low-viscosity solution is obtained due to a breakdown of the fibrils into smaller aggregates. Through mechanical stress the gel structure can be destroyed as well, indicating a low stability of these fibers. The gel formation is reversible, but as a drastic rearrangement of the molecules takes place, metastable states occur.

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Bipolar lipids with only one long alkyl chain and two polar headgroups at the ends are model compounds for the chemically more complicated bola lipids from Archea bacteria, for instance, the bis(diphytanyl) diglycerol tetraethers. [1] In Irlbachia alata and Anthocleista djalonensis, the bola lipid docosan-1,22-diyl-bis-[2-(trimethylammonio)ethylphosphate] (irlbacholine) has a C 22 alkyl chain that connects two polar phosphocholine groups and has been used in plant medicine. It was found to be effective against the fungus Trichophyton rubrum, which affects the skin, and other fungal infections. [2] In the course of studies on the possibility of using similar bola lipids with one long alkyl chain as membrane stabilizers, a bola phospholipid with a C 32 alkyl chain was synthesized, namely the lipid dotriacontan-1,32-diyl-bis-[2-(trimethylammonio)ethylphosphate] (PC-C32-PC, 1; Figure 1).[3] During the initial characterization of 1 we observed unusual aggregation behavior, which necessitated further physico-chemical investigations. To obtain sufficient quantities of 1 a more efficient synthetic route to long-chain bola compounds was necessary.We prepared lipid 1 in gram quantities by a newly developed simple and effective procedure. The construction of the chain with 32 carbon atoms is possible in a one-step synthesis by a copper-catalyzed coupling of undec-10-en-1-ylmagnesium bromide with 1,10-dibromodecane in a molar ratio of 2:1 (see Experimental Section). Dotriacontan-1,31-diene is then converted into dotriacontan-1,32-diol by hydroboration with disiamyl borane and subsequent oxidative hydrolysis. The importance of the new reaction is underlined by the possibility of functionalizing the 1-and 32-positions. The phosphocholine moieties were introduced by the reaction of dotriacontan-1,32-diol with 2-bromoethyl phosphoric acid dichloride in chloroform. This classic phosphorylation reagent was more effective than more modern variants. The trimethylammonium groups in 1 were introduced by established procedures. [4] Because of the long alkyl chain and the large polar headgroups at both chain ends of 1, it forms novel types of aggregate structures not observed previously. We investigated the self-assembly of the bipolar compound in aqueous solution using differential scanning calorimetry, electron microscopy, X-ray scattering, viscosity measurements, and FT-IR spectroscopy.In highly dilute aqueous solution (c < 0.5 wt %), a very viscous, almost transparent gel is observed. The 0.1-wt % sample shown in Figure 2 has a water/1 molar ratio of ca. 45 000:1; we see that an enormous amount of water can be "immobilized" by the gel structure. At temperatures above 50 8C or through mechanical stress the gel character disappears and a fluid solution is obtained; the viscosity drops by about five orders of magnitude. Preliminary viscosity measurements at room temperature show an indication of a yield stress (t 0 *) of roughly 7.2 Pa for a 0.8-wt % solution (Figure 3). Temperature-dependent viscosity measurements indicate a viscosity...

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The peptide sequence B18, derived from the membrane-associated sea urchin sperm protein bindin, triggers fusion between lipid vesicles. It exhibits many similarities to viral fusion peptides and may have a corresponding function in fertilization. The lipid-peptide and peptide-peptide interactions of B18 are investigated here at the ultrastructural level by electron microscopy and x-ray diffraction. The histidine-rich peptide is shown to self-associate into two distinctly different supramolecular structures, depending on the presence of Zn(2+), which controls its fusogenic activity. In aqueous buffer the peptide per se assembles into beta-sheet amyloid fibrils, whereas in the presence of Zn(2+) it forms smooth globular clusters. When B18 per se is added to uncharged large unilamellar vesicles, they become visibly disrupted by the fibrils, but no genuine fusion is observed. Only in the presence of Zn(2+) does the peptide induce extensive fusion of vesicles, which is evident from their dramatic increase in size. Besides these morphological changes, we observed distinct fibrillar and particulate structures in the bilayer, which are attributed to B18 in either of its two self-assembled forms. We conclude that membrane fusion involves an alpha-helical peptide conformation, which can oligomerize further in the membrane. The role of Zn(2+) is to promote this local helical structure in B18 and to prevent its inactivation as beta-sheet fibrils.

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In the present work, we describe the synthesis and temperature-dependent aggregation behavior of two examples of a new class of highly asymmetrical glycerol diether bolaphospholipids. The bolalipids contain a long alkyl chain (C32) bound to glycerol in the sn-3 position, carrying a hydroxyl group at the ω position. The C16 alkyl chain in the sn-2 position either possesses a racemic methyl branch at the 10 position of the short alkyl chain (lipid II) or does not (lipid I). The sn-1 position of the glycerol is linked to a zwitterionic phosphocholine moiety. The temperature-dependent aggregation behavior of both bolalipids was studied using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and X-ray scattering. Aggregate structures were visualized by transmission electron microscopy (TEM). We show that both bolalipids self-assemble into large lamellar sheetlike aggregates. Closed lipid vesicles or other aggregate structures such as tubes or nanofibers, as usually found for diglycerol tetraether lipids, were not observed. Within the lamellae the bolalipid molecules are arranged in an antiparallel (interdigitated) orientation. Lipid I, without an additional methyl moiety in the short alkyl chain, shows a lamellar phase with high crystallinity up to a temperature of 34 °C, which was not observed before for other phospholipids.

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