Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
BBA - Biomembranes (v.1713, #1)
Unusual protein behavior illustrated with silk fibroin by Yoshiyuki Toshima (pp. 1-4).
We investigated the interaction between phospholipid membranes and silk fibroin recovered from the posterior silk gland of the silkworm. Observations of the planar lipid bilayer membrane and electron microscopic observations of liposomes showed that newly constructed silk fibroin, existing in the form of filaments, quickly penetrates phospholipid membranes without bursting them.
Keywords: Silk fibroin; Bilayer membrane; Transmembrane fibril; Silk gland
Structure and thermotropic phase behaviour of detergent-resistant membrane raft fractions isolated from human and ruminant erythrocytes by Peter J. Quinn; Cedric Tessier; Dominique Rainteau; Kamen S. Koumanov; Claude Wolf (pp. 5-14).
Detergent-resistant membrane raft fractions have been prepared from human, goat, and sheep erythrocyte ghosts using Triton X-100. The structure and thermotropic phase behaviour of the fractions have been examined by freeze-fracture electron microscopy and synchrotron X-ray diffraction methods. The raft fractions are found to consist of vesicles and multilamellar structures indicating considerable rearrangement of the original ghost membrane. Few membrane-associated particles typical of freeze-fracture replicas of intact erythrocyte membranes are observed in the fracture planes. Synchrotron X-ray diffraction studies during heating and cooling scans showed that multilamellar structures formed by stacks of raft membranes from all three species have d-spacings of about 6.5 nm. These structures can be distinguished from peaks corresponding to d-spacings of about 5.5 nm, which were assigned to scattering from single bilayer vesicles on the basis of the temperature dependence of their d-spacings compared with the multilamellar arrangements. The spacings obtained from multilamellar stacks and vesicular suspensions of raft membranes were, on average, more than 0.5 nm greater than corresponding arrangements of erythrocyte ghost membranes from which they were derived. The trypsinization of human erythrocyte ghosts results in a small decrease in lamellar d-spacing, but rafts prepared from trypsinized ghosts exhibit an additional lamellar repeat 0.4 nm less than a lamellar repeat coinciding with rafts prepared from untreated ghosts. The trypsinization of sheep erythrocyte ghosts results in the phase separation of two lamellar repeat structures ( d=6.00; 5.77 nm), but rafts from trypsinized ghosts produce a diffraction band almost identical to rafts from untreated ghosts. An examination of the structure and thermotropic phase behaviour of the dispersions of total polar lipid extracts of sheep detergent-resistant membrane preparations showed that a reversible phase separation of an inverted hexagonal structure from coexisting lamellar phase takes place upon heating above about 30 °C. Non-lamellar phases are not observed in erythrocytes or detergent-resistant membrane preparations heated up to 55 °C, suggesting that the lamellar arrangement is imposed on these membrane lipids by interaction with non-lipid components of rafts and/or that the topology of lipids in the erythrocyte membrane survives detergent treatment.
Keywords: Erythrocyte raft; Raft structure; Structure of membrane raft; Detergent-resistant membrane
The structure of DNA–DOPC aggregates formed in presence of calcium and magnesium ions: A small-angle synchrotron X-ray diffraction study by Daniela UhrÃková; Mária Hanulová; Sérgio S. Funari; Raylja S. Khusainova; FrantiÅ¡ek Å erÅ¡eň; Pavol Balgavý (pp. 15-28).
The structure of aggregates formed due to DNA interaction with dioleoylphosphatidylcholine (DOPC) vesicles in presence of Ca2+ and Mg2+ cations was investigated using synchrotron small-angle X-ray diffraction. For DOPC/DNA=1:1 mol/base and in the range of concentration of the cation2+ 0–76.5 mM, the diffractograms show the coexistence of two lamellar phases: Lx phase with repeat distance dLx∼8.26–7.39 nm identified as a phase where the DNA strands are intercalated in water layers between adjacent lipid bilayers, and LDOPC phase with repeat distance dDOPC∼6.45–5.65 nm identified as a phase of partially dehydrated DOPC bilayers without any divalent cations and DNA strands. The coexistence of these phases was investigated as a function of DOPC/DNA molar ratio, length of DNA fragments and temperature. If the amount of lipid increases, the fraction of partially dehydrated LDOPC phase is limited, depends on the portion of DNA in the sample and also on the length of DNA fragments. Thermal behaviour of DOPC+DNA+Ca2+ aggregates was investigated in the range 20–80 °C. The transversal thermal expansivities of both phases were evaluated.
Keywords: DNA; Dioleoylphosphatidylcholine; Ca; 2+; Mg; 2+; Cationic vesicles; Small-angle X-ray diffraction
Effects of farnesol on the physical properties of DMPC membranes by Amy C. Rowat; Danielle Keller; John H. Ipsen (pp. 29-39).
Farnesol interacts with membranes in a wide variety of biological contexts, yet our understanding of how it affects lipid bilayers is not yet complete. This study investigates how the 15-carbon isoprenoid, farnesol, influences the phase behaviour, lateral organization, and mechanical stability of dimyristol phosphatidylcholine (DMPC) model membranes. Differential scanning calorimetry (DSC) of multilamellar DMPC–farnesol mixtures (up to 26 mol% farnesol) demonstrates how this isoprenoid lowers and broadens the gel–fluid phase transition. A gel–fluid coexistence region becomes progressively more dominant with increasing farnesol concentration and at concentrations of and greater than 10.8 mol%, an upper transition emerges at about 35 °C. Atomic force microscopy images of supported farnesol–DMPC bilayers containing 10 and 20 mol% farnesol provide structural evidence of gel–fluid coexistence around the main transition. Above this coexistence region, membranes exhibit homogeneous lateral organization but at temperatures below the main gel–fluid coexistence region, another form of phase coexistence is observed. The solid nature of the gel phase is confirmed using micropipette aspiration. The combined thermodynamic, structural, and mechanical data allow us to construct a phase diagram. Our results show that farnesol preferentially partitions into the fluid phase and induces phase coexistence in membranes below the main transition of the pure lipid.
Keywords: DSC; AFM; Phase behaviour; Mechanics; Lateral organization; Lipid bilayer; Isoprenoid
Structure and thermal history dependant phase behavior of hydrated synthetic sphingomyelin analogue: 1,2-dimyristamido-1,2-deoxyphosphatidylcholine by Hiroshi Takahashi; Yukihisa Okumura; Junzo Sunamoto (pp. 40-50).
The physical properties of hydrated multilamellar sample of 1,2-dimyristamido-1,2-deoxyphosphatidylcholine (DDPC) were investigated by means of differential scanning calorimetry (DSC), static X-ray diffraction, and simultaneous DSC and X-ray diffraction. The DDPC is a synthetic sphingomyelin analogue and has two amide bonds in its hydrophobic parts. This paper reports on metastable phase behavior of the hydrated DDPC sample. By cooling from a chain-melted state at the rates of greater than 4 °C min−1, hydrated DDPC bilayers form a metastable gel phase. In the gel phase, the hydrophobic chains are tilted with respect to the bilayer normal, as like the gel phase of glycero-phosphatidylcholines. By heating, the metastable gel phase is transformed in to a stable phase associated with an exothermic heat event at 18.3 °C (Δ H=14.6 kJ mol−1) and then the stable phase is transformed into a liquid–crystalline phase at 25.6 °C (Δ H=42 kJ mol−1). The incubation at 17 °C for more than 1 h also induces the formation of the stable phase. In the stable phase, the hydrophobic chains are packed into highly ordered crystal-like structure. However, the X-ray diffraction pattern of the stable phase suggested that the entire DDPC molecules do not form a two-dimensional molecular ordered lattice, differing from normal subgel phase of glycero-phosphatidylcholines. The structure and phase behavior of DDPC revealed by the present study are discussed from the viewpoint of hydrogen bonds.
Keywords: Acylamino-phospholipid; Phase behavior; Metastability; Hydrogen bond; Differential scanning calorimetry; X-ray diffraction
Further aspects on the hemolytic activity of the antibiotic lipopeptide iturin A by Francisco J. Aranda; José A. Teruel; Antonio Ortiz (pp. 51-56).
The bacterial lipopeptide iturin A is able to cause hemolysis of human erythrocytes in a dose-dependent manner. Hemolysis takes place at iturin concentrations below its critical micellar concentration. Relative kinetics determinations clearly show that K+ leakage occurs prior to hemoglobin release. Furthermore, hemolysis can be prevented by addition to the outer solution of osmotic protectants of appropriate size. Altogether these results indicate that iturin A-induced hemolysis follows a colloid-osmotic mechanism, with the formation of a membrane pore of average diameter 32 Å. Iturin A is capable of inducing leakage of an aqueous fluorescent probe trapped in human erythrocyte ghosts, but not in large unilamellar liposomes made of various lipid compositions. The different permeabilizing effects of iturin A on model and biological membranes are discussed on the light of the presented results.
Keywords: Iturin; Lipopeptides; Biosurfactants; Liposomes; Hemolysis
Phase behavior of freeze-dried phospholipid–cholesterol mixtures stabilized with trehalose by Satoshi Ohtake; Carolina Schebor; Sean P. Palecek; Juan J. de Pablo (pp. 57-64).
A study is presented of the role of cholesterol content on the gel-to-liquid crystalline phase transition of freeze-dried liposomes stabilized with trehalose, a well known lyoprotectant. The phospholipids considered in this work, DPPC and DPPE, belong to the two predominant phospholipid species found in numerous biological membranes. Cholesterol is found in abundance in mammalian plasma membranes. DSC measurements reveal that cholesterol-containing liposomes exhibit multiple phase transitions upon dehydration. Addition of trehalose to these systems lowers the phase transition temperature and limits the phase separation of the lipidic components upon freeze-drying. This work provides strong evidence for the effectiveness of trehalose in stabilizing cholesterol-containing membranes upon lyophilization.
Keywords: Cholesterol; DPPC; DPPE; Phase transition; Freeze-dried
Diffusion measurements of water, ubiquinone and lipid bilayer inside a cylindrical nanoporous support: A stimulated echo pulsed-field gradient MAS-NMR investigation by Olivier Wattraint; Catherine Sarazin (pp. 65-72).
Stimulated echo pulsed-field gradient1H magic angle spinning NMR has been used to investigate the mobility of water, ubiquinone and tethered phospholipids, components of a biomimetic model membrane. The diffusion constant of water corresponds to an isotropic motion in a cylinder. When the lipid bilayer is obtained after the fusion of small unilamellar vesicles, the extracted value of lipid diffusion indicates unrestricted motion. The cylindrical arrangement of the lipids permits a simplification of data analysis since the normal bilayer is perpendicular to the gradient axis. This feature leads to a linear relation between the logarithm of the attenuation of the signal intensity and a factor depending on the gradient strength, for lipids covering the inner wall of aluminium oxide nanopores as well as for lipids adsorbed on a polymer sheet rolled into a cylinder. The effect of the bilayer formation on water diffusion has also been observed. The lateral diffusion coefficient of ubiquinone is in the same order of magnitude as the lipid lateral diffusion coefficient, in agreement with its localization within the bilayer.
Keywords: Abbreviations; AAO; anodic aluminum oxide; DOPE; 1,2-dioleoyl-sn glycero-3-phosphatidylethanolamine; EggPC; egg yolk L-α-phosphatidylcholine; HR-MAS; high resolution magic angle spinning; MLVs; multilamellar vesicles; NMR; nuclear magnetic resonance; PEG; polyethylene glycol; NHS; N-hydrosuccinimide; PET; polyethylene terephtalate; PFG-STE; pulsed-field gradient stimulated echo; POPC; 1-palmitoyl-2-oleoyl-; sn; -glycero-3-phosphatidylcholine; SUVs; small unilamellar vesiclesMAS-NMR PFG-STE; Tethered phospholipid bilayers; Nanoporous anodic aluminum oxide support; Oriented model membrane; Ubiquinone; Diffusion constants