Pressure Effects on Artificial and Cellular Membranes.

Lipids, which provide valuable model systems for membranes, display a variety of polymorphic phases, depending on their molecular structure and environmental conditions. By use of x-ray and neutron diffraction, infrared spectroscopy and calorimetry, the temperature and pressure dependent structure and phase behaviour of several lipid systems, differing in chain configuration and headgroup structure, have been studied. Besides lamellar phases also non-lamellar phases, such as the inverted hexagonal phase and bicontinuous cubic phases, have been investigated. Hydrostatic pressure has been used as a physical parameter for studying the stability and energetics of lyotropic mesophases, but also because high pressure is an important feature of certain natural membrane environments (e.g., marine biotopes) and because the high-pressure phase behaviour of biomolecules is of biotechnological interest. Neutron scattering in combination with the H/D contrast variation technique has been used to the study of lateral organization of phase-separated binary lipid mixtures with distinct mixing properties. Within their two-phase coexistence regions large-scale concentration fluctuations appear, and the morphology of these fluctuations can be characterized as a complex heterogeneous system of coexisting clusters having fractal-like properties. By using the pressure-jump relaxation technique in combination with time-resolved synchrotron x-ray diffraction, the kinetics of different lipid phase transformations were also investigated. The time constants for completion of the transitions are dependent on the direction of the transition, the symmetry and topology of the structures involved, and also on the pressure-jump amplitude. In several cases also intermediate structures can be detected under non-equilibrium conditions.

The effects of high pressure on the phase state and molecular structure of pure lipid bilayers are discussed. The relations of delta H, delta S and delta V in phase transitions are straightforward and are discernible in heterogeneous bilayers in natural membranes. The effects of pressure on the dynamic properties of bilayer constituents are less clearly understood, but order parameters obtained at pressure by different techniques show agreement. The extent and significance of hydration is poorly understood. Four physiological functions are discussed: passive permeability, active transport, membrane excitability and synaptic transmission. It is shown that a full interpretation of the kinetic effects of pressure on these processes requires much more detailed molecular information than is available at present.

Phase behavior of lipid bilayers at high pressure is critical to biological processes. Using coarse grained molecular dynamic simulations, we report critical characteristics of dipalmitoylphosphatidylcholine bilayers with applied high pressure, and also show their phase transition by cooling bilayer patches. Our results indicate that the phase transition temperature of dipalmitoylphosphatidylcholine bilayers obviously shifts with pressure increasing in the rate of 37 °C kbar(-1), which are in agreement with experimental data. Moreover, the main phase transition is revealed to be strongly dependent on lipid area. A critical lipid area of ~0.57 nm(2) is found on the main phase transition boundary. Similar structures of acyl chains lead to the same sensitivity of phase transition temperature of different lipids to the pressure. Based on the lateral density and pressure profiles, we also discuss the different effects on bilayer structure induced by high temperature and high pressure, e.g., increasing temperature induces higher degree of interdigitation of lipid tails and thinner bilayers, and increasing pressure maintains the degree of interdigitation and bilayer thickness.

Effect of Mechanical Stresses on Lipid Membrane Phase Properties

Effects of physiologically relevant pressures of helium on the structure of cholesterol-containing lipid bilayers. A neutron diffraction study

Pressure effects on the structure and phase behavior of phospholipid-gramicidin bilayer membranes

The effect of hydrogen pressure and temperature on kinetics of hydrogen-induced direct phase transformation in hard magnetic alloy

Pressure Effects on the Structure and Phase Behaviour of Phospholipid-Gramicidin Bilayers

The cytoplasmic membrane of Methanospirillum hungatei was isolated from osmotic lysates of spheroplasts, with yields of 7-8% of the cell dry weight. Cytoplasmic contamination was negligible, as judged by the removal of soluble enzymes. The cytoplasmic membrane consists of lipid (35-37%), primarily as a biphytanyldiglycerol tetraether glycolipid; protein (45-50%); and carbohydrate (10-12%). Ultra-thin sections showed that the trilaminar membrane formed vesicles with a maximum diameter of 0.4 microns. Protrusions of membrane projecting from the vesicles were seen often in negatively stained preparations. Fractionation of M. hungatei cells grown in the presence of [14C]mevalonic acid revealed that 90% of the phytanyl lipids were present in the cytoplasmic membrane band, with two minor bands accounting for the remainder of the label. Approximately 50% of the galactose, glucose, and mannose present in the cytoplasmic membrane was found in lipid extracts, while the remainder of these sugars and 98% of the rhamnose were present as nonlipid sugars. The cell sheath, isolated with a yield of 13% of the cell dry weight, contained the same sugars as the cytoplasmic membrane, but in very different proportions. Amino acid analysis of the membrane proteins showed that hydrophobic amino acid residues made up 37% of the total, neutral amino acids, 39%, basic, 8%, acidic, 16%, and that half-cysteine was present. Sodium dodecyl sulfate-polyacrylamide gel patterns of solubilized cytoplasmic membrane proteins revealed major bands at 195, 74.5, 44, 32, and 30 KDa. Significant amounts of nickel co-isolated with the cytoplasmic membrane, accounting for 0.16% of the membrane dry weight.

The response of lipid membranes to changes in external pressure is important for many biological processes, and it can also be exploited for technological applications. In this work, we employ all-atom molecular dynamics simulations to characterize the changes in the physical properties of phospholipid bilayers brought about by high pressure (1000 bar). In particular, we study how the response differs, in relation to different chain unsaturation levels, by comparing monounsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and biunsaturated dioleoyl-phosphatidylcholine (DOPC) bilayers. Various structural, mechanical, and dynamical features are found to be altered by the pressure increase in both bilayers. Notably, for most properties, including bilayer area and thickness, lipid order parameters, lateral pressure profile, and curvature frustration energy, we observe significantly more pronounced effects for monounsaturated POPC than biunsaturated DOPC. Possible biological implications of the results obtained are discussed, especially in relation to how different lipids can control the structure and function of membrane proteins.

Predicting the mechanism of MBP binding to cholesterol is meaningful in understanding how MBP participate in lateral membrane organization. The interaction of MBP with cholesterol monolayer was investigated at three surface pressures on 10 mM Tris-HCl buffer with the different concentrations of MBP. The results show that π-A isotherms shift to larger molecular area at all pressures. By means of analyzing π-T curves, a surface pressure increase was obtained. Results indicated that the greater the protein concentration in the subphase, the larger the increase of surface pressure. In addition, changes in monolayer surface morphology and domain formation were performed by AFM. These results provide more direct and convincing evidence for the MBP interaction with cholesterol. The MBP-cholesterol interaction suggests a significant concentrations and surface pressure dependence and is probably governed by hydrogen bonds. The date presented could help to understand at least one of the molecular mechanisms through which MBP affects lateral organization of the cholesterol membrane.

Sequential selective supercritical fluid extraction (S3FE) of triglycerides and flavonolignans from milk thistle (Silybum marianum L, Gaertn)

Effects of hydrostatic pressure on bilayer phase behavior and dynamics of dilauroylphosphatidylcholine

The impact of deuteration on natural and synthetic lipids: A neutron diffraction study

Environmental factors influencing the inactivation of Cronobacter sakazakii by high hydrostatic pressure