Structure, habitat and seed of Mariopteris ZEILLER

The genus Mariopteris ZEILLER is distributed from the Namurian A to the early Stephanian in Europe. In the Dobrudzha Basin, numerous specimens are found mainly in flood plain claystones and siltstones. The habitat and reconstruction of the plants suggests a creeping manner of growth. Climatic changes are reflected in the architecture of species. One specimen has an attached seed proving the classification of the genus as a pteridosperm.</p

Vein reticulation in seed ferns of the Dobrudzha coalfield Bulgaria

Alethopteris, Lonchopteris, Paripteris, Linopteris and Reticulopteris are abundant pteridosperm fronds in the Pennsylvanian-age Dobrudzha Coalfield. The transition from fan-shape to reticulate veining in Lonchopteris, Linopteris and Reticulopteris and in Laveineopteris to Reticulopteris is documented. The most probable cause for this is the decrease in humidity; in addition to temperature controls on the geographic distribution. The form and positioning of anastomoses are discussed.</p

Low amounts of PEG-lipid induce cubic phase in phosphatidylethanolamine dispersions

AbstractBy using time-resolved X-ray diffraction we demonstrate that low amounts (5–10 mol%) of a phospholipid with two saturated hydrocarbon acyl chains 14 carbon atoms long and PEG550 chain covalently attached to its phosphoethanolamine polar head group, DMPE(PEG550), induce spontaneous formation of a cubic phase with lattice constant 20.5 nm (cubic aspect #8, space group Im3m) in aqueous dispersions of dielaidoylphosphatidylethanolamine (DEPE). This phase displays a highly resolved X-ray diffraction pattern with 17 low-angle reflections. The cubic phase was found to intrude in the temperature range between the lamellar liquid crystalline (Lα) phase and the inverted hexagonal phase (HII) known to form in pure DEPE/water dispersions. A higher DMPE(PEG550) amount of 20 mol% was found to eliminate the non-lamellar phases in the temperature scale up to 100°C. DMPE grafted with PEG5000 only shifts the Lα-HII transition of DEPE to higher temperatures but does not promote formation of cubic phase. These findings indicate that, consistent with their bulky head groups, the PEG-lipids decrease the tendency for negative interfacial mean curvature of the DEPE bilayers

New phases induced by sucrose in saturated phosphatidylethanolamines: an expanded lamellar gel phase and a cubic phase

AbstractA new lamellar gel phase (Lβ) with expanded lamellar period was found at low temperatures in dihexadecylphosphatidylethanolamine (DHPE) and dipalmitoylphosphatidylethanolamine (DPPE) dispersions in concentrated sucrose solutions (1–2.4 M). It forms via a cooperative, relatively broad transition upon cooling of the Lβ gel phase of these lipids. According to the X-ray data, the transformation between Lβ and Lβ is reversible, with a temperature hysteresis of 6–10°C and a transition width of about 10°C. No specific volume changes and a very small heat absorption of about 0.05 kcal/mol accompany this transition. The Lβ−Lβ transition temperature strongly depends on the disaccharide concentration. From a value of about 10°C below the melting transition of DHPE, it drops by 25°C with decrease of sucrose concentration from 2.4 M to 1 M. The low-temperature gel phase Lβ has a repeat spacing by 8–10 Å larger than that of the Lβ gel phase and a single symmetric 4.2 Å wide-angle peak. It has been observed in 1. 1.25, 1.5 and 2.4 M solutions of sucrose, but not in 0.5 M of sucrose. The data clearly indicate that the expanded lamellar period of the Lβ phase results from a cooperative, reversible with the temperature, increase of the interlamellar space of the Lβ gel phase. Other sugars (trehalose, maltose, fructose, glucose) induce similar expanded low-temperature gel phases in DHPE and DPPE. The Lβ phase is osmotically insensitive. Its lamellar period does not depend on the sucrose concentration, while the lattice spacings of the Lα, Lβ and HH phases decrease linearly with increase of sucrose concentration. Another notable sugar effect is the induction of a cubic phase in these lipids. It forms during the reverse HH−Lα phase transition and coexists with the Lα phase in the whole temperature range between the HH and Lβ phases. The cubic phase has only been observed at sucrose concentrations of 1 M and above. In accordance with previous data, sucrose suppresses the Lα phase in both lipids and brings about a direct Lβ−HH phase transition in DHPE. A rapid, reversible gel-subgel transformation takes place at about 17°C in both DPPE and DHPE. Its properties do not depend on the sucrose concentration. The observed new effects of disaccharides on the properties of lipid dispersions might be relevant to their action as natural protectants

Fusion Peptides Promote Formation of Bilayer Cubic Phases in Lipid Dispersions. An X-Ray Diffraction Study

AbstractSmall angle x-ray diffraction revealed a strong influence of the N-terminal influenza hemagglutinin fusion peptide on the formation of nonlamellar lipid phases. Comparative measurements were made on a series of three peptides, a 20-residue wild-type X-31 influenza virus fusion peptide, GLFGAIAGFIENGWEGMIDG, and its two point-mutant, fusion-incompetent peptides G1E and G13L, in mixtures with hydrated phospholipids, either dipalmitoleoylphosphatidylethanolamine (DPoPE), or monomethylated dioleoyl phosphatidylethanolamine (DOPE-Me), at lipid/peptide molar ratios of 200:1 and 50:1. All three peptides suppressed the HII phase and shifted the Lα–HII transition to higher temperatures, simultaneously promoting formation of inverted bicontinuous cubic phases, QII, which becomes inserted between the Lα and HII phases on the temperature scale. Peptide-induced QII had strongly reduced lattice constants in comparison to the QII phases that form in pure lipids. QII formation was favored at the expense of both Lα and HII phases. The wild-type fusion peptide, WT-20, was distinguished from G1E and G13L by the markedly greater magnitude of its effect. WT-20 disordered the Lα phase and completely abolished the HII phase in DOPE-Me/WT-20 50:1 dispersions, converted the QII phase type from Im3m to Pn3m and reduced the unit cell size from ∼38 nm for the Im3m phase of DOPE-Me dispersions to ∼15 nm for the Pn3m phase in DOPE-Me/WT-20 peptide mixtures. The strong reduction of the cubic phase lattice parameter suggests that the fusion-promoting WT-20 peptide may function by favoring bilayer states of more negative Gaussian curvature and promoting fusion along pathways involving Pn3m phase-like fusion pore intermediates rather than pathways involving HII phase-like intermediates

Salt Tolerance of Archaeal Extremely Halophilic Lipid Membranes

The membranes of extremely halophilic Archaea are characterized by the abundance of a diacidic phospholipid, archaetidylglycerol methylphosphate (PGP-Me), which accounts for 50-80 mol% of the polar lipids, and by the absence of phospholipids with choline, ethanolamine, inositol, and serine head groups. These membranes are stable in concentrated 3-5 m NaCl solutions, whereas membranes of non-halophilic Archaea, which do not contain PGP-Me, are unstable and leaky under such conditions. By x-ray diffraction and vesicle permeability measurements, we demonstrate that PGP-Me contributes in an essential way to membrane stability in hypersaline environments. Large unilamellar vesicles (LUV) prepared from the polar lipids of extreme halophiles, Halobacterium halobium and Halobacterium salinarum, retain entrapped carboxyfluorescein and resist aggregation in the whole range 0-4 m NaCl, similarly to LUV prepared from purified PGP-Me. By contrast, LUV made of polar lipid extracts from moderately halophilic and non-halophilic Archaea (Methanococcus jannaschii, Methanosarcina mazei, Methanobrevibacter smithii) are leaky and aggregate at high salt concentrations. However, adding PGP-Me to M. mazei lipids results in gradual enhancement of LUV stability, correlating with the PGP-Me content. The LUV data are substantiated by the x-ray results, which show that H. halobium and M. mazei lipids have dissimilar phase behavior and form different structures at high NaCl concentrations. H. halobium lipids maintain an expanded lamellar structure with spacing of 8.5-9 nm, which is stable up to at least 100 degrees C in 2 m NaCl and up to approximately 60 degrees C in 4 m NaCl. However, M. mazei lipids form non-lamellar structures, represented by the Pn3m cubic phase and the inverted hexagonal H(II) phase. From these data, the forces preventing membrane aggregation in halophilic Archaea appear to be steric repulsion, because of the large head group of PGP-Me, or possibly out-of-plane bilayer undulations, rather than electrostatic repulsion attributed to the doubly charged PGP-Me head group

Instability of Myelin Tubes under Dehydration: deswelling of layered cylindrical structures

We report experimental observations of an undulational instability of myelin figures. Motivated by this, we examine theoretically the deformation and possible instability of concentric, cylindrical, multi-lamellar membrane structures. Under conditions of osmotic stress (swelling or dehydration), we find a stable, deformed state in which the layer deformation is given by \delta R ~ r^{\sqrt{B_A/(hB)}}, where B_A is the area compression modulus, B is the inter-layer compression modulus, and h is the repeat distance of layers. Also, above a finite threshold of dehydration (or osmotic stress), we find that the system becomes unstable to undulations, first with a characteristic wavelength of order \sqrt{xi d_0}, where xi is the standard smectic penetration depth and d_0 is the thickness of dehydrated region.Comment: 5 pages + 3 figures [revtex 4

Including debris cover effects in a distributed model of glacier ablation

Distributed glacier melt models generally assume that the glacier surface consists of bare exposed ice and snow. In reality, many glaciers are wholly or partially covered in layers of debris that tend to suppress ablation rates. In this paper, an existing physically based point model for the ablation of debris-covered ice is incorporated in a distributed melt model and applied to Haut Glacier d’Arolla, Switzerland, which has three large patches of debris cover on its surface. The model is based on a 10 m resolution digital elevation model (DEM) of the area; each glacier pixel in the DEM is defined as either bare or debris-covered ice, and may be covered in snow that must be melted off before ice ablation is assumed to occur. Each debris-covered pixel is assigned a debris thickness value using probability distributions based on over 1000 manual thickness measurements. Locally observed meteorological data are used to run energy balance calculations in every pixel, using an approach suitable for snow, bare ice or debris-covered ice as appropriate. The use of the debris model significantly reduces the total ablation in the debris-covered areas, however the precise reduction is sensitive to the temperature extrapolation used in the model distribution because air near the debris surface tends to be slightly warmer than over bare ice. Overall results suggest that the debris patches, which cover 10% of the glacierized area, reduce total runoff from the glacierized part of the basin by up to 7%

Structure of symmetric and asymmetric "ripple" phases in lipid bilayers

We reproduce the symmetric and asymmetric ``rippled'' $P_{\beta'}$ states of lipid membranes by Monte Carlo simulations of a coarse-grained molecular model for lipid-solvent mixtures. The structure and properties compare favorably with experiments. The asymmetric ripple state is characterized by a periodic array of fully interdigitated ``defect'' lines. The symmetric ripple state maintains a bilayer structure, but is otherwise structurally similar. The formation of both ripple states is driven by the propensity of lipid molecules with large head groups to exhibit splay.Comment: 4 pages, 4 figure

Thermotropic and structural effects of poly(malic acid) on fully hydrated multilamellar DPPC–water systems

The thermotropic and structural effects of low molecular weight poly(malic acid) (PMLA) on fully hydrated multilamellar dipalmitoylphosphatidylcholine (DPPC)-water systems were investigated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and freeze-fracture transmission electron microscopy (FFTEM). Systems of 20 wt% DPPC concentration and 1 and 5 wt% PMLA to lipid ratios were studied. The PMLA derivatives changed the thermal behavior of DPPC significantly and caused a drastic loss in correlation between lamellae in the three characteristic thermotropic states (i.e., in the gel, rippled gel and liquid crystalline phases). In the presence of PBS or NaCl, the perturbation was more moderate. The structural behavior on the atomic level was revealed by FTIR spectroscopy. The molecular interactions between DPPC and PMLA were simulated via modeling its measured infrared spectra, and their peculiar spectral features were interpreted. Through this interpretation, the poly(malic acid) is inferred to attach to the headgroups of the phospholipids through hydrogen bonds between the free hydroxil groups of PMLA and the phosphodiester groups of DPPC