Pressure-distance Curves Research Articles

Conformation of Single and Interacting Lipopolysaccharide Surfaces Bearing O-Side Chains

The outer surfaces of Gram-negative bacteria are composed of lipopolysaccharide (LPS) molecules exposing oligo- and polysaccharides to the aqueous environment. This unique, structurally complex biological interface is of great scientific interest as it mediates the interaction of bacteria with antimicrobial agents as well as with neighboring bacteria in colonies and biofilms. Structural studies on LPS surfaces, however, have so far dealt almost exclusively with rough mutant LPS of reduced molecular complexity and limited biological relevance. Here, by using neutron reflectometry, we structurally characterize planar monolayers of wild-type LPS from Escherichia coli O55:B5 featuring strain-specific O-side chains in the presence and absence of divalent cations and under controlled interaction conditions. The model used for the reflectivity analysis is self-consistent and based on the volume fraction profiles of all chemical components. The saccharide profiles are found to be bimodal, with dense inner oligosaccharides and more dilute, extended O-side chains. For interacting LPS monolayers, we establish the pressure-distance curve and determine the distance-dependent saccharide conformation.

Neutron reflectometry yields distance-dependent structures of nanometric polymer brushes interacting across water.

The interaction between surfaces displaying end-grafted hydrophilic polymer brushes plays important roles in biology and in many wet-technological applications. In this context, the conformation of the brushes upon their mutual approach is crucial, because it affects interaction forces and the brushes' shear-tribological properties. While this aspect has been addressed by theory, experimental data on polymer conformations under confinement are difficult to obtain. Here, we study interacting planar brushes of hydrophilic polymers with defined length and grafting density. Via ellipsometry and neutron reflectometry we obtain pressure-distance curves and determine distance-dependent polymer conformations in terms of brush compression and reciprocative interpenetration. While the pressure-distance curves are satisfactorily described by the Alexander-de-Gennes model, the pronounced brush interpenetration as seen by neutron reflectometry motivates detailed simulation-based studies capable of treating brush interpenetration on a quantitative level.

Numerical analysis of water flow characteristics after inrushing from the tunnel floor in process of karst tunnel excavation

In order to investigate water flow characteristics after inrushing in process of karst tunnel excavation, numerical simulations for five case studies of water inrush from the tunnel floor are carried out by using the FLUENT software on the background of Qiyueshan high risk karst tunnel. Firstly, the velocity-distance curves and pressure-distance curves are drawn by selecting a series of probing lines in a plane. Then, the variation characteristics of velocity and pressure are analyzed and the respective optimized escape routes are made. Finally, water flow characteristics after inrushing from the tunnel floor are discussed and summarized by comparing case studies under the conditions of different water-inrush positions and excavation situations. The results show that: (1) Tunnel constructors should first move to the tunnel side wall and then escape quickly when water inrush happens. (2) Tunnel constructors must not stay at the intersection area of the cross passage and tunnels when escaping. (3) When water inrush from floor happens in the left tunnel, if tunnel constructors meet the cross passage during escaping, they should pass through it rapidly, turn to the right tunnel and run to the entrance. (4) When water inrush from floor happens in the left tunnel, if there is not enough time to escape, tunnel constructors can run to the trolley and other equipment in the vicinity of the right tunnel working face. In addition, some rescuing equipment can be set up at the high location of the cross passage. (5) When water inrush from floor happens in the cross passage, tunnel constructors should move to the tunnel side wall quickly, turn to the tunnel without water inrush and run to the entrance. (6) When water inrush from floor happens in the cross passage, if there is not enough time to escape, tunnel constructors can run to the trolley and other equipment near by the left or the right tunnel working face. The results are of important practical significance and engineering value to ensure the safety of tunnel construction.

Specific Role of Glycolipids in the Regular Stacking of Membranes Reconstituted from Thylakoid Lipid Extracts

Chloroplasts contain stacks of flattened photosynthetic membranes (thylakoids) that host the photosynthetic protein complexes. Using neutron diffraction under controlled temperature and humidity, we observe that solid supported films prepared from protein-free thylakoid lipid extracts self-organize as regular bilayer stacks, the only functional supra-molecular organization for photosynthesis. We show that this lamellar order is close to the reversible phase transition from hexagonal (HII) to lamellar (Lα). This transition and the observed phase coexistence can be modulated by fine-tuning the lipid profile and the hydration of the lipid mixture. In a developmental context, like the etioplast-to-chloroplast transition, or in response to environmental variations (stress), this property might contribute to the highly dynamic flexibility of the structure of plastids. Furthermore, our analyses highlight the critical role of galactolipids - most importantly DGDG - as the contributing components to membrane stacking via hydrogen bonds between polar heads of adjacent bilayers. This role of galactolipids might be determinant in other glycolipid-rich membrane-ordered systems such as myelin sheath.Diffraction patterns obtained on the D16 instrument (ILL, Grenoble, France) will be presented together with a detailed analysis of phases and pressure-distance curves obtained for various lipid compositions ranging from purified individual thylakoid lipids to complex mixtures approaching the natural thylakoid membrane composition.

Compression of High Grafting Density Opposing Polymer Brushes Using Molecular Dynamics Simulations in Explicit Solvent

Opposing polymer brush layers at high grafting density were examined under confinement and characterized with respect to structure and interaction forces using molecular dynamics simulations in an explicit solvent. The brush system underwent a static compression, where the system is simulated at several discrete separation distances. These simulations are all at the same solvent chemical potential as a non-interacting reference state to produce a realistic compression. Normal pressure-distance profiles were generated and compared to density profiles at each separation distance to determine structure-property relationships. Significant interpenetration of brush layers occurred at high compression, to the extent that each brush reached the opposing surface. Higher interpenetration corresponded to a sharp increase in the pressure-distance curve, suggesting a correlation between interpenetration and interaction forces. We find clear differences from literature values using implicit solvent techniques.

Direct Measurement of Inter-filament Forces in Neurofilament Networks: Synchrotron X-ray Diffraction Study under Osmotic Pressure

Neurofilaments (NFs) are the major protein constituents in neuronal processes (axons and dendrites) that impart mechanical stability and act as structural scaffolds. The filaments assemble from 3 different subunit proteins (NF-L, NF-M, NF-H) to form a 10 nm diameter flexible polymer with radiating unstructured sidearms. Recent work, showed that at high protein concentration, the NFs form a nematic hydrogel network with a well-defined interfilament spacing as can be measured by synchrotron small angle x-ray scattering (SAXS) [1]. The x-ray/phase behavior study showed the role the different subunit protein compositions play in the interfilament interaction. In order to directly elucidate the interfilament forces responsible for the mechanical properties of NFs hydrogel, we conducted a SAXS-osmotic pressure study, which yielded pressure-distance curves at different subunit compositions and monovalent salts. We show that filaments composed with NF-L and NF-M strongly attract each other through their polyampholyte sidearms, in particularly at high monovalent salt. However, filaments comprised of NF-L and NF-H, at high NF-H grafting density, show a distinctly different pressure-distance dependency, with much larger interfilament spacing and weaker salt dependence. Supported by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347, and the Human Frontier Science Program organization.[1] J.B. Jones, C.R. Safinya, Biophys. J. 95, 823 (2008).

Equation of state of colloids coated by polyelectrolyte multilayers

We propose an adaptation of the osmotic stress technique for determining equations of state of colloids covered by polymer films. We describe here the case of silica particles covered by 6 or 7 layers of polyelectrolyte multilayers. The establishment of a pressure-distance curve allows the interpretation of colloidal interactions over more than four orders of magnitude in pressure. This large range allows a clear separation between three distinct regimes: layer compression, brush extension and at very low pressures a regime of weak repulsion. The physical origins of the regimes are discussed and described by models of colloidal interactions.

Short-range pressures between liquid bilayer membranes

For many years a large, short-range repulsive interaction has been observed between a variety of hydrated surfaces. The physical origin of this ubiquitous interaction has been controversial. In the case of lipid bilayers, proposed mechanisms include a hydration pressure, due to water polarization and/or hydrogen-bond reorientation by the bilayer surface, and several types of entropic (steric) pressures, due to motion of individual lipid molecules or undulations of the entire bilayer. This review focuses on a number of recent osmotic stress/X-ray diffraction experiments performed with phosphatidylcholine bilayers designed to determine the distance range where each of these pressures dominates. At very short interbilayer separations (less than about 4 Å), the pressure-distance curve depends on the volume fraction of head groups at the interface, indicating the presence of a large steric barrier arising from direct interactions between head groups from opposing bilayers. The range of this steric pressure can be increased by the addition of lipids with larger head groups, such as glycolipids or lipids with covalently attached polymers (polyethylene glycol lipids). For intermediate interbilayer separations (about 2–10 Å), the pressure-distance curves are similar for liquid-crystalline and crystaline phosphatidylcholine bilayers, the pressure-fluid spacing realationship is nearly independent of temperature, and the magnitude of the pressure depends on the dipole potential. In this range of fluid spacings we argue that the pressure can be best accounted for by a hydration pressure. For interbilayer spacing greater than about 10 Å, the magnitude and range of the observed pressure depend on temperature and on the bending modulus of the bilayer. These observations provide compelling evidence for the presence of a longer-range undulation pressure, that markedly influences the hydration properties of phospholipid bilayers.

Processing of materials—monolithic to composites

A multimillion rupee 500-ton hydraulic extrusion/forging facility established at NPL, New Delhi, has been used to undertake extensive studies in forming, the process of plastically deforming, which is the most important way of shaping materials. Wrought materials are used extensively for making useful products employing extrusion and forging, the two important secondary processing techniques used to convert materials into useful shapes and sizes and also to improve the mechanical and metallurgical properties. The success of these processes depends on the proper control and eventual optimization of different process variables.Different ferrous and non-ferrous materials including 316 and 321 stainless steels (using glass lubrication), aluminium brass, admiralty brass and 70/30 and 90/10 cupro nickels have been extruded at a lab-scale production level to determine the influence of different extrusion variables such as temperature, strain rate, extrusion ratio, different die design and several different compositions of lubricants on pressure requirement and surface quality of the products.In the case of ferrous extrusions, the choice of billet-container lubricant has a marked effect on the shape of pressure-distance curves by influencing the effective coefficient of friction and heat transfer coefficient. Temperature, extrusion ratio and strain rate mainly influence the level of pressure-distance curves. Significance of specially designed dish shaped dies, avoiding the dead metal zone and the importance of extrusion limit diagrams for different materials on the available press capacity has also been highlighted.Deformation behaviour of the hi-tech materials; discontinuously reinforced metal matrix composites (MMCs) and aluminium-lithium alloys using hot extrusion and closed die forging, have been carried out. Parameters have been optimized to extrude rods and thin walled circular MMC tubes of aluminium alloy (2124/6061)-SiCp MMCs having varying volume fractions of SiCp. This developmental work has been carried out using powder metallurgy, liquid metallurgy and spray atomization and deposition routes, in joint collaboration with several R & D organizations.Hot extrusion being an important production technique, results of these experiments can be directly transferred to industries. Similar deformation studies either by closed die or by isothermal forging have been made to develop components.The pilot plant level facility is available for any industry for carrying out developmental R & D work. NPL welcomes such collaboration.

Long- and short-range interactions between phospholipid/ganglioside GM1 bilayers.

The structure and interactive properties of liquid-crystalline egg phosphatidylcholine (EPC) bilayers containing the ganglioside GM1 and its uncharged analogue, asialoGM1 (AGM1), have been obtained by X-ray diffraction analysis of osmotically stressed liposomes. Both electron density profiles and reciprocal space modeling indicate that (1) the incorporation of up to 30 mol % GM1 into EPC bilayers has little effect on bilayer organization and (2) the oligosaccharide portion of the GM1 molecule extends at least 12 A beyond the EPC head group into the fluid space, implying that the GM1 head group is nearly fully extended from the bilayer surface. Pressure-distance relations for EPC:GM1 bilayers in 100 mM ionic strength buffer show that, for large bilayer separations, the interbilayer repulsive pressure decays exponentially with a decay length and magnitude expected for electrostatic repulsion arising from the charged GM1. However, at interbilayer separations of < or = 30 A for 7:3 and 8:2 EPC:GM1 and < or = 22 A for 9:1 EPC:GM1, the pressure-distance curves have distinct upward breaks, with the sharpness of this break depending strongly on the amount of GM1 in the bilayer. For 7:3 EPC:GM1 bilayers, the break is quite sharp so that the distance between bilayers does not decrease below 28 A with further increases in applied pressure. For EPC:GM1 8:2 and 9:1 bilayers, the upward break becomes softer with decreasing GM1 concentration. For uncharged EPC:AGM1 bilayers, the repulsive pressure extends only to an equilibrium fluid separation of about 36 A, but has a similar behavior to the pressure-distance data for EPC:GM1 for separations below 20 A. We argue that the nonelectrostatic repulsive pressures arise primarily from the steric interactions between the hydrated oligosaccharide head groups that protrude from the bilayer surface.

Contributions of hydration and steric (entropic) pressures to the interactions between phosphatidylcholine bilayers: experiments with the subgel phase.

The total repulsive interaction between electrically neutral, fluid bilayer membranes is thought to have a number of components, including a hydration pressure, due to the reorientation of water by the bilayer, and steric (entropic) pressures due to bilayer undulations, head group motion, and molecular protrusions. For fully hydrated, crystalline bilayers these three steric pressures should be relatively small, and the major repulsive pressure present should be the hydration pressure. Therefore, to isolate the contribution of hydration pressure to the total interbilayer interaction, we have measured pressure-distance data by X-ray diffraction analysis of osmotically stressed dipalmitoylphosphatidylcholine (DPPC) multilayers in the subgel phase, where wide-angle and low-angle X-ray data show the bilayers are crystalline. As applied pressure was increased from 0 to 1 x 10(6) dyn/cm2, the interbilayer fluid space (df) decreased less than 1 A from its value at full hydration of 8.4 A. As the pressure was increased from 1 x 10(6) to 3 x 10(7) dyn/cm2, df decreased from about 8 to 4 A. For this range of df, the repulsive pressure decayed exponentially with df with a decay length of 1.4 A. Further increases in applied pressure did not appreciably decrease df, so that there was a sharp upward break in the pressure-distance curve at an interbilayer spacing of about 3 A. In contrast, pressure-distance relations for gel (L beta') phase and liquid-crystalline (L alpha) phase bilayers had much softer upward breaks at df < 5 A and extended to larger df at zero applied pressure. However, the pressure-distance curves for all three phases decayed exponentially with approximately the same decay length for 4 < df < 8 A. We interpret these data to mean the following: (1) the repulsion observed for df < 5 A is primarily a steric pressure whose range depends on head group motion; (2) the steric undulation pressure plays an important role in determining the hydration properties and the range of the total repulsive pressure for fluid membranes; (3) the same underlying mechanisms govern the repulsive pressure for all phases for 4 < df < 8 A; (4) these mechanisms include a pressure due to reorientation of water molecules; and (5) the hydration pressure component extents a maximum of about two water molecules from the bilayer surface for the subgel phase.

Glass-lubricated hot extrusion of stainless steel

AbstractExperimental extrusion has been carried out on type 321 and type 316 stainless steels to study the influence of temperature, ram speed, extrusion ratio, die design, and several different compositions of glass lubricants on extrusion pressure–distance curves and on the surface quality of the extruded products. The choice of billet/container lubricant has a marked effect on the shape of the pressure–distance curves by influencing both the effective coefficient of friction and the heat-transfer coefficient. Temperature, extrusion ratio, and die lubricant mainly influence the level of the pressure–distance curve. The surface quality of the extruded product, assessed in terms of scoring and roughness, varies widely with extrusion conditions but does not appear to correlate directly with the efficiency of lubrication, as judged from the pressure curves. The best surface quality is obtained with/ast extrusion speed and temperatures low in the normal reheating range, as defined by the extrusion limit diagram.

Shock Propagation in Spherically Symmetric Exponential Atmospheres

Computer calculations have been made of the propagation of the shock from an explosion at the origin of a spherically symmetric, exponential, ideal-gas atmosphere. A generalization of Sachs scaling has been devised which allows a very convenient systematization of the pressure-distance curves, and which shows that, for a suitable normalization, these curves are straight lines throughout a significant range of yield, distance, and degree of atmospheric inhomogeneity. This technique is also used to investigate the validity of modified Sachs scaling, an approximate method of generating pressure-distance curves for an inhomogeneous atmosphere from the known results for a homogeneous atmosphere. It is found that modified Sachs scaling yields results which deviate from the hydrodynamic code calculations by at most 20% in the weak shock regime. This result is essentially independent of yield. Some comments are made on the applicability of these methods to the problem of shocks from explosions in horizontally stratified, real-gas atmospheres.