Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

The linear compressibility of two-dimensional fatty acid mesophases has determined by grazing incidence x-ray diffraction. Surface pressure vs molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10m/N) are observed in the tilted phases. They are apparently independent of the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in L_2 or L_2' phases, S phase), and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted CS phase and for 1 direction of the S and L_2'' phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for L_2' and L_2'' phases and can be traced to subtle reorganizations upon untilting.Comment: 24 pages, 17 figure

Different forms of the bovine PrP gene have five or six copies of a short, G-C-rich element within the protein-coding exon

Current models of the virus-like agents of scrapie and bovine spongiform encephalopathy (BSE) have to take into account that structural changes in a host-encoded protein (PrP protein) exhibit an effect on the time course of these diseases and the survival time of any man or animal exposed to these pathogens. We report here the sequence of different forms of the bovine PrP gene which contain either five or six copies of a short, G-C-rich element which encodes the octapeptide Pro-His-Gly-Gly-Gly-Trp-Gly-Gln or its longer variants Pro-Gln/His-Gly-Gly-Gly-Gly-Trp-Gly-Gln. Out of 12 cattle, we found eight animals homozygous for genes with six copies of the Gly-rich peptide (6:6), while four were heterozygous (6:5). Two confirmed cases of BSE occurred in (6: 6) homozygous animals. Bovine spongiform encephalopathy (BSE) is a transmissible disease (Fraser et al., 1988; Dawson et al., 1990; Barlow & Middleton, 1990) which produces neuropathological lesions in cattle similar to those seen in ovine scrapie (Wells et al., 1987) and the rare human dementias Creutzfeldt-Jakob disease (CJD) and Gerstmann-Str/iussler syndrome (GSS) (Beck & Daniel, 1987). A cellular membrane protein (PrP) has a key role in the transmission and development of these diseases. This protein accumulates in the brain and other tissues during the protracted time course of these diseases and, in a disease-specific, protease-resistant isoform (SAF-PrP), has been purified by subcellular fractionation of scrapie

Computer analyses suggest interactions of non-muscle filamin with lipid membranes

AbstractIt is concluded from structure predictions of the primary amino acid sequence by computer analyses that two segments of non-muscle filamin could facilitate lipid membrane attachment or anchoring. Residues 49–71 of the amino-terminal may attach to phospholipid membranes, and residues 131–155 may anchor in the hydrophobic region of lipid membranes

Lateral current density fronts in asymmetric double-barrier resonant-tunneling structures

We present a theoretical analysis and numerical simulations of lateral current density fronts in bistable resonant-tunneling diodes with Z-shaped current-voltage characteristics. The bistability is due to the charge accumulation in the quantum well of the double-barrier structure. We focus on asymmetric structures in the regime of sequential incoherent tunneling and study the dependence of the bistability range, the front velocity and the front width on the structure parameters. We propose a sectional design of a structure that is suitable for experimental observation of front propagation and discuss potential problems of such measurements in view of our theoretical findings. We point out the possibility to use sectional resonant-tunneling structures as controllable three-terminal switches.Comment: to appear in J.Appl.Phy

Hole mobility in organic single crystals measured by a "flip-crystal" field-effect technique

We report on single crystal high mobility organic field-effect transistors (OFETs) prepared on prefabricated substrates using a "flip-crystal" approach. This method minimizes crystal handling and avoids direct processing of the crystal that may degrade the FET electrical characteristics. A chemical treatment process for the substrate ensures a reproducible device quality. With limited purification of the starting materials, hole mobilities of 10.7, 1.3, and 1.4 cm^2/Vs have been measured on rubrene, tetracene, and pentacene single crystals, respectively. Four-terminal measurements allow for the extraction of the "intrinsic" transistor channel resistance and the parasitic series contact resistances. The technique employed in this study shows potential as a general method for studying charge transport in field-accumulated carrier channels near the surface of organic single crystals.Comment: 26 pages, 7 figure

Interactions of lipid monolayers with the natural biopolymer hyaluronic acid

AbstractThe interaction of the natural mucopolysaccharide hyaluronic acid with different lipids, present in the natural membranes, was studied at the lipid/water interface using thermodynamic methods and X-ray diffraction. The results show that this biopolymer modifies the properties and the structure of the lipid monolayer. The two-dimensional crystalline lattice and domain structure of the charged octadecylamine monolayer are strongly disturbed by the hyaluronic acid, the monolayer compressibility increases and the monolayer collapse pressure drops down. In addition, the presence of charged lipid interfaces influences the structural organisation of the hyaluronic acid at the membrane/water interfaces. The impacts of these results on the structural organisation at the membrane interface are discussed

Coarsening in surface growth models without slope selection

We study conserved models of crystal growth in one dimension [$\partial_t z(x,t) =-\partial_x j(x,t)$] which are linearly unstable and develop a mound structure whose typical size L increases in time ($L = t^n$). If the local slope ($m =\partial_x z$) increases indefinitely, $n$ depends on the exponent $\gamma$ characterizing the large $m$ behaviour of the surface current $j$ ($j = 1/|m|^\gamma$): $n=1/4$ for $1< \gamma <3$ and $n=(1+\gamma)/(1+5\gamma)$ for $\gamma>3$.Comment: 7 pages, 2 EPS figures. To be published in J. Phys. A (Letter to the Editor