Bilayer superstructure

In Section II,A, we consider the types of spectra expected for rapid anisotropic motion within a bilayer superstructure (Fig. 6A). It has bwn known for some time that certain lipids will form a completely different type of structure in which the molecules project radially from the center of a cylinder (Fig. 6B Luzzati and Husson, 1962). As mentioned in Section II,B, rapid lateral diffusion within a bilayer phase only affects P-NMR line shapes if the lamellae enclose a particle of small radius. The cylinders in a hexagonal phase have a very small radius, and therefore lateral diffusion about the cylinder axis can cause further averaging of tensor components. The unique axis of the system now becomes the axis of the cylinder, and thus we label its chemical-shift component ap. However, noticing that along this axis the field would be roughly normal to the fatty acyl chains, we would expect the value of this chemical shift to be similar to. On the other hand. 

Lipofullerenes such as 35-37 self-assemble within lipid bilayers into rod-like structures of nanoscopic dimensions [61, 62]. These anisotropic superstructures may be important for future membrane technology. Significantly, lipofullerenes 35 and 37 have very low melting points, 22 and 67 °C (DSC, heating scan), respectively, with 35 being the first liquid fuUerene derivative at room temperature. 

Approaches to artificial ion channels have, for instance, made use of macrocyclic units [6.72,6.74] (see also below), of peptide [8.183-8.185] and cyclic peptide [8.186] components, of non-peptidic polymers [8.187] and of various amphiphilic molecules [6.11, 8.188, 8.189]. The properties of such molecules incorporated in bilayer membranes may be studied by techniques such as ion conductance [6.69], patch-clamp [8.190] or NMR [8.191, 8.192] measurements. However, the nature of the superstructure formed and the mechanism of ion passage (carrier, channel, pore, defect) are difficult to determine and often remain a matter of conjecture. 

It was generally found that double chain ammonium glutamic acid amphiphiles of the general formula 2Cn-Glu-CmN+ (15) typically form typical bilayer vesicles above their phase transition temperature.118 It is interesting to refer to the compound 2Ci2-Glu-CnN+ (15b, n = 12)111,119 in which the predominant superstructures were flexible filaments with... 

The last example we would like to discuss is a lattice of holes formed in stoichiometric hexagonal (h) BN double layers on Rh(lll), see Fig. 5(c) and [99]. The lattice is composed of holes in the BN-bilayer with a diameter of 24 2 A, and an average distance of 32 2 A. The holes in the upper layer are offset with respect to the smaller holes in the lower layer. We note that well-ordered superstructures with a large period have already been observed some time ago by means of LEED for borazine adsorption onto Re(0001) [102], while borazine adsorption onto other close-packed metal surfaces, such as Pt(lll), Pd(lll), and Ni(lll), leads to the self-limiting growth of commensurate ABN monolayers [103,104]. For BN/Rh(lll) it is not clear at present whether the Rh(lll) substrate is exposed at the bottom of the holes. If this was the case the surface would not only be periodic in morphology but also in chemistry, and therefore would constitute a very useful template for the growth of ordered superlattices of metals, semiconductors, and molecules. 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as imderpotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface structures can be classified (a) simple superstructures with small packing densities and (b) close-packed (bulklike) or even compressed structures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63. 64. 65. 66. 67. 68. 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au... 

Nakashima, N., Asakuma, S., Kunitake, T. (1985). Optical microscopic study of helical superstructures of chiral bilayer membranes, J. Am. Chem. Soc., 1107 509. 

The transmission of chirality in membranes and other types of aggregates formed by surfactants was widely studied. The superstructures of bilayers formed by gemini dicationic surfactants based on quaternary ammonium centers can be modulated through the employ of enantiomeric tartrate salts. The twisted ribbons (Fig. 4) are formed when an enantiomeric excess of one of the tartrates is present, with the period (helical pitch) shorten-... 

The various ribbons presented above consist of amphiphilic molecules arranged in bilayers. The long axis of the molecules is perpendicular to the ribbon plane or slightly tilted from this direction. But ribbons or tapes can also be formed from the assembly of molecular rods oriented with their long axis parallel to the width of the ribbon. This is the case for some peptides that form extended /3-sheet tapes which stack to form chiral superstructures (Fig. 3) [73]. It is also the case for numerous gelators consisting of a central aromatic core and chiral cholesteryl saccharidic moieties on the sides, such as the porphyrin derivative shown in Fig. 4 [74]. Chirality effects in these... 

Yamada, K. Diara, H. Ide, T. Fukumoto, T. Hirayama, C. Formation of heUcal superstructure from single-waDed bilayers by amphiphiles with oligo-L-glutamic acid-head group. Chem. Lett. 1984,10, 1713. 

FIGURE 9.5 Polypropylene crystallized in the y-form consisting of bilayers of two parallel helices (a) reveak a characteristic peak at 0 = 10° in its WAXS pattern (b) lamellae will aggregate and form arborescent lamellar superstructures as shown in a Scanning Force Microscopy phase image (c). 

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