A peculiar phase transition of plasmalogen bilayer membrane under high pressure

Unperturbed hydrocarbon chains and liquid phase bilayer lipid chains: a computer simulation study

In this work, the properties of saturated and unsaturated fatty acid acyl chains 16:0, 18:0, 18:1(n-9)cis, 18:2(n-6)cis, 18:3(n-3)cis, 18:4(n-3)cis, 18:5(n-3)cis, 20:4(n-6)cis, 20:5(n-3)cis and 22:6(n-3)cis in a bilayer liquid crystalline state and similar hydrocarbon chains (with CH[Formula: see text] terminal groups instead of C=O groups) in the unperturbed state characterised by a lack of long-range interaction were investigated. The unperturbed hydrocarbon chains were modelled by Monte Carlo simulations at temperature [Formula: see text] K; sixteen fully hydrated homogeneous liquid crystalline phosphatidylcholine bilayers containing these chains were studied by molecular dynamics simulations at the same temperature. To eliminate effects of the simulation parameters, the molecular dynamics and Monte Carlo simulations were carried out using the same structural data and force field coefficients. From these computer simulations, the average distances between terminal carbon atoms of the chains (end-to-end distances) were calculated and compared. The trends in the end-to-end distances obtained for the unperturbed chains were found to be qualitatively similar to those obtained for the same lipid chains in the bilayers. So, for understanding of a number of processes in biological membranes (e.g., changes in fatty acid composition caused by environmental changes such as temperature and pressure), it is possible to use, at least as a first approximation, the relationships between the structure and properties for unperturbed or isolated hydrocarbon chains.

Preferential binding of Inorganic Mercury to specific lipid classes and its competition with Cadmium

Upon uptake of Hg and Cd into living systems, possible targets for metal induced toxicity include the membranes surrounding nervous, cardiovascular and renal cells. To further our understanding of the interactions of Hg and Cd with different lipid structures under physiologically relevant chloride and pH conditions (100 mM NaCl pH 7.4), we used fluorescence spectroscopy and dynamic light scattering to monitor changes in membrane fluidity and phase transition and liposome size. The metal effects were studied on zwitterionic, cationic and anionic lipids to elucidate electrostatically driven metal-lipid interactions. The effect of Hg-catalyzed cleavage of the vinyl ether bond in plasmalogens on these aforementioned properties was studied in addition to a thermodynamic characterization of this interaction by Isothermal Titration Calorimetry. The negatively charged Hg-chloride complexes formed under our experimental conditions induce membrane rigidity in membranes containing cationic lipids and plasmalogens while this effect is heavily reduced and entirely absent with zwitterionic and anionic lipids respectively. The K_D for the interaction of Hg with plasmalogen containing liposomes was between 4-30 μM. Furthermore, the presence of Cd affected the interaction of Hg with plasmalogen when negatively charged PS was also present. In this case, even the order of the metal addition was important.

How Do Membranes Respond to Pressure?

Bilayers formed by phospholipids are fundamental structures of biological membranes. The mechanical perturbation brought about by pressure significantly affects the membrane states of phospholipid bilayers. In this chapter, we focus our attention on the pressure responsivity for bilayers of some major phospholipids contained in biological membranes. At first, the membrane states and phase transitions of phospholipid bilayers depending on water content, temperature and pressure are explained by using the bilayer phase diagrams of dipalmitoylphosphatidylcholine (DPPC), which is the most familiar phospholipid in model membrane studies. Subsequently, the thermotropic and barotropic bilayer phase behavior of various kinds of phospholipids with different molecular structures is discussed from the comparison of their temperature--pressure phase diagrams to that of the DPPC bilayer. It turns out that a slight change in the molecular structure of the phospholipids produces a significant difference in the bilayer phase behavior. The systematic pressure studies on the phase behavior of the phospholipid bilayers reveal not only the pressure responsivity for the bilayers but also the role and meaning of several important phospholipids existing in real biological membranes.

Miscibility and interactions of animal and plant sterols with choline plasmalogen in binary and multicomponent model systems

In this work miscibility and interactions of sterols with choline plasmalogen (PC-plasm) in Langmuir monolayers were studied. Moreover, the properties of cholesterol/phosphatidylcholine/plasmalogen mixtures of different PC-plasm concentration were investigated. The foregoing systems were treated as a model of cancer cell membranes, which are of higher plasmalogen level than normal cells. Finally, the influence of β-sitosterol and stigmasterol (phytosterols differing in anticancer potency) on these mixtures was verified. The properties of monolayers were analyzed based on the parameters derived from the surface pressure-area isotherms and images taken with Brewster Angle Microscope. It was found that at 30% of sterol in sterol/plasmalogen monolayer the lipids are immiscible and 3D crystallites are formed within the film. Cholesterol molecules mix favorably with PC-plasm at Xchol ≥ 0.5, while the investigated phytosterols only at their prevailing proportion in binary system. The increase of choline plasmalogen in cholesterol/phosphatidylcholine monolayer causes destabilization of the system. Moreover, the incorporation of phytosterols into cholesterol/phosphatidylcholine+PC-plasm mixtures disturbed membrane morphology and this effect was stronger for β-sitosterol as compared to stigmasterol. It was concluded that the presence of vinyl ether bond at sn-1 position in PC-plasm molecule strongly affects miscibility of choline plasmalogen with sterols. The comparison of the collected data with those reported in literature allowed one to conclude that miscibility and interactions of sterols with PC-plasm are less favorable than those with phosphatidylcholine. It was also suggested that overexpression of plasmalogens in cancer cell membranes may be a factor differentiating sensitivity of cells to anticancer effect of phytosterols.

Thermotropic and barotropic phase behavior of phosphatidylcholine bilayers

Bilayers formed by phospholipids are frequently used as model biological membranes in various life science studies. A characteristic feature of phospholipid bilayers is to undergo a structural change called a phase transition in response to environmental changes of their surroundings. In this review, we focus our attention on phase transitions of some major phospholipids contained in biological membranes, phosphatidylcholines (PCs), depending on temperature and pressure. Bilayers of dipalmitoylphosphatidylcholine (DPPC), which is the most representative lipid in model membrane studies, will first be explained. Then, the bilayer phase behavior of various kinds of PCs with different molecular structures is revealed from the temperature-pressure phase diagrams, and the difference in phase stability among these PC bilayers is discussed in connection with the molecular structure of the PC molecules. Furthermore, the solvent effect on the phase behavior is also described briefly.

Study on the subgel-phase formation using an asymmetric phospholipid bilayer membrane by high-pressure fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.