Increase In Bilayer Thickness Research Articles

Bismuth thickness-dependent structural and electronic properties of Bi/As2Se3 bilayer thin films

The structural, microstructural and electronic band gap properties of bilayer Bi/As2Se3 thin films prepared on glass substrate have been discussed in detail. The thin films prepared by thermal evaporation technique under high vacuum were characterized by X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM) and UV–visible–NIR spectrophotometer. The bilayer films with small Bi thickness showed amorphous nature, while large Bi thickness film led to the development of sharp XRD peaks corresponding to Bi and Bi2Se3 phase. The band gap of the studied film decreased with increasing Bi layer thickness which is explained on the basis of high concentration of localized defect states in the band gap region. The FESEM images showed the smooth and homogeneous surface for all the films. The Raman spectra showed the different structural units like AsSe3 pyramidal unit and Se8 rings that change with the Bi layer thickness. The present study reveals the interface diffusion of Bi into As2Se3 layer and interaction of Bi with Se due to exposure of the films to UV radiation.

Influence of Bilayer Size and Number in Multi-Bilayer DOPC Simulations at Full and Low Hydration.

Biophysical studies of model cell membranes at full and low hydration are usually carried out using scattering measurements on multi-bilayer systems. Molecular simulations of lipid bilayers aimed at reproducing those experimental conditions are usually conducted using single bilayers with different amounts of water. These simulation conditions may lead to artifacts arising from size effects and self-interactions because of periodic boundary conditions. We have tested the influence of the size and number of bilayers on membrane properties using the Lipid14 force field for lipids in molecular dynamics simulations of 1,2-dioleoyl- sn-glycero-3-phosphocholine bilayers at full hydration (44 water molecules per lipid), low hydration (18 water molecules per lipid), and dehydration (9 water molecules per lipid). A number of additional simulations were conducted with the Slipids force field for comparison. We have found that the average area per lipid (APL), thickness, mass density profiles, and acyl tail order parameters are insensitive to the size and the number of bilayers for all hydration states. The Lipid14 force field can also successfully reproduce the experimentally observed decrease in APL and corresponding increase in bilayer thickness upon dehydration, reflecting the increase in ordering as the system becomes more gel-like. Additionally, decreasing hydration levels were associated with a trend away from normal lateral diffusion and toward more subdiffusive regimes across both force fields. In summary, at least for the Lipid14 force field, the use of a single bilayer with 128 phospholipid molecules provides an adequate representation of multi-bilayer systems at varying levels of hydration.

Spatially resolved small-angle X-ray scattering for characterizing mechanoresponsive liposomes using microfluidics.

Atherosclerosis gives rise to blood vessel occlusion associated with blood flow alteration and substantial increase of average wall shear stress. This modification was proved acting as a purely physical trigger for targeted vasodilator release from a particular type of liposomes composed of 1,3-diaminophospholipids (Pad-PC-Pad). The flow-induced structural changes of these faceted liposomes, however, are completely unknown. Therefore, spatially resolved small-angle X-ray scattering was combined with microfluidics to uniquely study the purely physical mechanisms, which give rise to the highly efficient drug release from mechanoresponsive liposomes of nanometer size. The microfluidic device, designed to mimic a stenotic blood vessel, consisted of a 1-mm-wide channel with a constriction, 125 ​μm in diameter. Here, the changes of the average bilayer thickness and the mean size of the mechanoresponsive liposomes have been locally detected under flow conditions. Overall shape and bilayer thickness do change already near the constriction inlet, but the alteration is dominant near the outlet. At a flow rate of 0.2 ​μL/s, the liposome's bilayer thickness increased by 30 compared to the situation well before the constriction and under static condition. The detected bilayer thickness increase of the faceted liposomes is in line with the mechanically induced loss of interdigitation between the phospholipid amide chains. These results imply that rather the gradient force than the wall shear stress provokes structural changes of Pad-PC-Pad liposomes and the related drug release at stenoses. The approach, i.e. the combination of microfluidics and spatially resolved small-angle X-ray scattering, paves the way to design highly efficient and specific systems for the targeted drug delivery at constrictions with predefined morphology.

The effect of H3O+ on the membrane morphology and hydrogen bonding of a phospholipid bilayer

At the 2017 meeting of the Australian Society for Biophysics, we presented the combined results from two recent studies showing how hydronium ions (H3O+) modulate the structure and ion permeability of phospholipid bilayers. In the first study, the impact of H3O+ on lipid packing had been identified using tethered bilayer lipid membranes in conjunction with electrical impedance spectroscopy and neutron reflectometry. The increased presence of H3O+ (i.e. lower pH) led to a significant reduction in membrane conductivity and increased membrane thickness. A first-order explanation for the effect was assigned to alterations in the steric packing of the membrane lipids. Changes in packing were described by a critical packing parameter (CPP) related to the interfacial area and volume and shape of the membrane lipids. We proposed that increasing the concentraton ofH3O+ resulted in stronger hydrogen bonding between the phosphate oxygens at the water-lipid interface leading to a reduced area per lipid and slightly increased membrane thickness. At the meeting, a molecular model for these pH effects based on the result of our second study was presented. Multiple μs-long, unrestrained molecular dynamic (MD) simulations of a phosphatidylcholine lipid bilayer were carried out and showed a concentrationdependent reduction in the area per lipid and an increase in bilayer thickness, in agreement with experimental data. Further, H3O+ preferentially accumulated at the water-lipid interface, suggesting the localised pH at the membrane surface is much lower than the bulk bathing solution. Another significant finding was that the hydrogen bonds formed by H3O+ ions with lipid headgroup oxygens are, on average, shorter in length and longer-lived than the ones formed in bulk water. In addition, the H3O+ ions resided for longer periods in association with the carbonyl oxygens than with either phosphate oxygen in lipids. In summary, the MD simulations support a model where the hydrogen bonding capacity of H3O+ for carbonyl and phosphate oxygens is the origin of the pH-induced changes in lipid packing in phospholipid membranes. These molecular-level studies are an important step towards a better understanding of the effect of pH on biological membranes.

A Membrane Thickness Sensor in TREK-1 Channels Transduces Mechanical Force

The transduction of force to a biological signal is critical to all living organisms and cells. Mechanosensitive ion channels participate, but the biophysical mechanism that transduces force remains unclear. Here we show that the sensing of membrane thinning through a helix tilt in TREK-1 channels transduces force. Reconstituted TREK-1 in PC lipids with a hydrophobic thickness of ~29 A (18:1 PC) robustly activated the channel; however, decreasing the thickness to ~26 A (16:1 PC) or increasing to ~36 A (22:1 PC) inhibited the channel. Increasing bilayer thickness by adding cholesterol (10-40%), or thinning the membrane with stretch, polyunsaturated fatty acids, or heat shifted the thickness dependence. The thickness sensitivity was located near a lipid-binding motif on a known gating helix at the end of the fourth transmembrane domain. We propose a stretch-activation model for TREK-1 where tension causes membrane thinning and TREK-1 senses membrane thinning to transduce the signal.

Self-assembly of bimetallic AuxPd1-x alloy nanoparticles via dewetting of bilayers through the systematic control of temperature, thickness, composition and stacking sequence

Bimetallic alloy nanoparticles (NPs) are attractive materials for various applications with their morphology and elemental composition dependent optical, electronic, magnetic and catalytic properties. This work demonstrates the evolution of AuxPd1-x alloy nanostructures by the solid-state dewetting of sequentially deposited bilayers of Au and Pd on sapphire (0001). Various shape, size and configuration of AuxPd1−x alloy NPs are fabricated by the systematic control of annealing temperature, deposition thickness, composition as well as stacking sequence. The evolution of alloy nanostructures is attributed to the surface diffusion, interface diffusion between bilayers, surface and interface energy minimization, Volmer-Weber growth model and equilibrium configuration. Depending upon the temperature, the surface morphologies evolve with the formation of pits, grains and voids and gradually develop into isolated semi-spherical alloy NPs by the expansion of voids and agglomeration of Au and Pd adatoms. On the other hand, small isolated to enlarged elongated and over-grown layer-like alloy nanostructures are fabricated due to the coalescence, partial diffusion and inter-diffusion with the increased bilayer thickness. In addition, the composition and stacking sequence of bilayers remarkably affect the final geometry of AuxPd1−x nanostructures due to the variation in the dewetting process. The optical analysis based on the UV–vis-NIR reflectance spectra reveals the surface morphology dependent plasmonic resonance, scattering, reflection and absorption properties of AuxPd1−x alloy nanostructures.

Experimental investigation of high temperature oxidation during self-propagating reaction in Zr/Al reactive multilayer films

High temperature oxidation behavior during a fast self-propagating reaction is reported in binary 1Zr/1Al reactive multilayer films. Magnetron sputtered deposited freestanding Zr/Al films with different bilayer thicknesses were investigated experimentally to characterize the oxidation behavior during a self-propagating reaction, ignited by an electrical arc. X-ray diffraction (XRD) analysis of reacted film, ignited in air indicates the formation of oxides of both Zr and Al besides intermetallic compounds. In argon 4.6 atmosphere, oxidation has been reduced, but it is also evident that a very small amount of only ZrO2 forms. The temperature profiles measured by a high-speed two-color pyrometer exhibit the temperature plateau after reaching the maximum temperature. The static temperatures in plateau vary from 1475 °C to 1588 °C depending on bilayer thicknesses. This temperature plateau becomes more pronounced with increasing bilayer thickness. However, an extended time duration at high temperature plateau characterizes the significant oxidation in air during a self-propagating reaction, which can influence reaction properties and thus broaden the scope of applications.

Lipid-Dependent Activation and Desensitization Mechanism of MthK

MthK is a prokaryotic Ca2+-activated K+ channel with several high-resolution structures, and serves as a model for investigating gating mechanisms of K+ channels. Previous investigations using proteoliposome-based Tl+ flux assay indicated that MthK underwent desensitization in seconds after Ca2+ activation. In contrast, MthK studied with electrophysiology in a planar lipid bilayer system show no evidence of desensitization. We hypothesized that this is caused by the different bilayer properties between the two preparations. The lipid composition of the bilayers in both preparations is the same (DOPC:POPG, 3:1). However, the differences in curvatures (liposomes vs. planar bilayers) and thicknesses (decane, used for planar bilayer formation, leads to increased bilayer thickness) may affect channel gating. Since MthK does not appear to desensitize in the thicker, decane-containing planar bilayers, we expect that increasing the bilayer thickness in liposomes will cause slower or no desensitization in the flux assays. Conversely, decreasing the bilayer thickness in the planar bilayers will lead to desensitization in the electrophysiology assays. We performed planar bilayer and Tl+ flux experiments using MthK reconstituted in liposomes made with lipids with different lipid acyl chain lengths (PC, from C16:1 to C22:1) while maintaining the POPG mole fraction. Changing the acyl chain length did not affect the voltage-dependent gating behavior of MthK when tested in planar bilayers. In addition, MthK did not undergo desensitization in any lipid compositions tested. On the contrary, varying the acyl chain length affected both MthK activation and inactivation in flux assays. We found that MthK inactivates more slowly in liposomes with longer acyl chain lengths (C20:1 and C22:1), and faster in liposomes with shorter acyl chain lengths (C16:1). Additionally, MthK activation was markedly slower in liposomes with longer acyl-chain lipids. The mechanism for lipid-dependent activation and desensitization of MthK is under investigation.

Effect of phosphatidylcholine bilayer thickness and molecular order on the binding of the antimicrobial peptide maculatin 1.1

Antimicrobial peptides (AMPs) interact directly with bacterial membrane lipids. Thus, changes in the lipid composition of bacterial membranes can have profound effects on the activity of AMPs. In order to understand the effect of bilayer thickness and molecular order on the activity of AMPs, the interaction of maculatin 1.1 (Mac1.1) with phosphatidylcholine (PC) model membranes composed of different monounsaturated acyl chain lengths between 14 and 22 carbons was characterised by dual polarisation interferometry (DPI) and 31P and 1H solid-state NMR techniques. The thickness and bilayer order of each PC bilayer showed a linear dependence on the acyl chain length. The binding of Mac1.1 exhibited a biphasic dependency between the amount of bound Mac1.1 and bilayer thickness, whereby the mass of bound peptide increased from C14 to C16 and then decreased from C16 to C22. Significant perturbation of 31P chemical shift anisotropy (CSA) values was only observed for DOPC (C18) and DEPC (C22), respectively. In the case of DEPC, the greater range in CSA indicated different headgroup conformations or environments in the presence of Mac1.1. Overall, the results indicated that there is a significant change in the bilayer order upon binding of Mac1.1 and this change occurred in a co-operative manner at higher concentrations of Mac1.1 with increasing bilayer thickness and order. Overall, an optimum bilayer thickness and lipid order may be required for effective membrane perturbation by Mac1.1 and increasing the bilayer thickness and order may counteract the activity of Mac1.1 and play a role in antimicrobial resistance to AMPs.

Self-Assembly in Aqueous Oppositely Charged Gemini Surfactants: A Correlation between Morphology and Solubilization Efficacy.

Micellization behavior of oppositely charged gemini surfactants (anionic surfactant; phosphoric acid, P,P'-1,4-butanedieyl, P,P'-didodecylester, disodium salt (12-4-12A), and cationic surfactant; butanediyl-1,4, bis (N,N-hexadecyl ammonium) dibromide (16-4-16) or (D-isosorbate-1,4-diyl bis(N,N-dimethyl-N-hexadecylammonium acetoxy) dichloride (16-Isb-16)) has been studied (individually or of a gemini mixture) by conductivity and surface tension measurements. Critical micelle concentration (CMC) data show both synergistic (for 12-4-12A + 16-Isb-16) and antagonistic (for 12-4-12A + 16-4-16) interactions between the two components. Small angle neutron scattering (SANS) measurement shows formation of various aggregates, spherical (or ellipsoidal), rod-shaped, and vesicular, by changing the mole fraction (at fixed total surfactant concentration, 10 mM). Viscosity, zeta (ζ)-potential, and transmission electron microscopy (TEM) data are found in conformity of SANS results. Surprisingly, two morphologies (vesicles and rod-shaped micelles) show stability in a wide temperature range (303-343 K). The behavior has been explained on the basis of temperature induced dehydration and depletion of micellar charge. Aqueous gemini mixtures, of different morphologies, have been used for the determination of solubilization efficacy (using UV-visible spectrophotometer) toward polycyclic aromatic hydrocarbons (PAHs: anthracene; pyrene or fluorene). Molar solubilization ratio (MSR) data suggest that vesicles enhance the solubilization efficacy. SANS analysis shows that vesicle bilayer thickness increases upon PAH solubilization. The order of bilayer thickness increase is found to be anthracene > pyrene > fluorene, which is in the same order as the aqueous solubility of PAHs. This is the first report which correlates morphology to the solubilization efficacy.

Engineering Escherichia coli membrane phospholipid head distribution improves tolerance and production of biorenewables

Economically competitive microbial production of biorenewable fuels and chemicals is often impeded by toxicity of the product to the microbe. Membrane damage is often identified as a major mechanism of this toxicity. Prior efforts to strengthen the microbial membrane by changing the phospholipid distribution have largely focused on the fatty acid tails. Herein, a novel strategy of phospholipid head engineering is demonstrated in Escherichia coli. Specifically, increasing the expression of phosphatidylserine synthase (+pssA) was found to significantly increase both the tolerance and production of octanoic acid, a representative membrane-damaging solvent. Tolerance of other industrially-relevant inhibitors, such as furfural, acetate, toluene, ethanol and low pH was also increased. In addition to the increase in the relative abundance of the phosphoethanolamine (PE) head group in the +pssA strain, there were also changes in the fatty acid tail composition, resulting in an increase in average length, percent unsaturation and decreased abundance of cyclic rings. This +pssA strain had significant changes in: membrane integrity, surface potential, electrochemical potential and hydrophobicity; sensitivity to intracellular acidification; and distribution of the phospholipid tails, including an increase in average length and percent unsaturation and decreased abundance of cyclic rings. Molecular dynamics simulations demonstrated that the +PE membrane had increased resistance to penetration of ethanol into the hydrophobic core and also the membrane thickness. Further hybrid models in which only the head group distribution or fatty acid tail distribution was altered showed that the increase in PE content is responsible for the increase in bilayer thickness, but the increased hydrophobic core thickness is due to altered distribution of both the head groups and fatty acid tails. This work demonstrates the importance of consideration of the membrane head groups, as well as a modeling approach, in membrane engineering efforts.

Influence of the bilayer thickness of nanostructured multilayer MoN/CrN coating on its microstructure, hardness, and elemental composition

Multilayer nanostructured coatings consisting of alternating MoN and CrN layers were obtained by vacuum cathode evaporation under various conditions of deposition. The transition from micron sizes of bilayers to the nanometer scale in the coatings under investigation leads to an increase in hardness from 15 to 35.5 GPa (with a layer thickness of about 35 nm). At the same time, when the number of bilayers in the coating decreases, the average Vickers hardness increases from 1267 HV0.05 to 3307 HV0.05. An increase in the value of the potential supplied to the substrate from–20 to–150 V leads to the formation of growth textures in coating layers with the [100] axis, and to an increase in the intensity of reflections with increasing bilayer thickness. Elemental analysis carried out with the help of Rutherford backscattering, secondary ion mass spectrometry and energy dispersion spectra showed a good separation of the MoN and CrN layers near the surface of the coatings.

Roughness replication in neutron supermirrors

Neutron supermirrors (SMs), the major components of neutron optical devices, are depth-graded d-spacing multilayers of several hundreds to several thousands of bilayers. The interface roughness is a major factor in the reflectivity of multilayers. This influence is especially significant if the number of bilayers is large. In this work, the interface roughness and its correlations were studied in DC-sputtered Ni–Ti neutron supermirrors. Detector scans were carried out to observe off-specular neutron scattering in selected regions of the q space from (increasing bilayer thickness) normal- and (decreasing bilayer thickness) reverse-layer-sequence SMs. In-plane and out-of-plane roughness correlations are manifested in diffuse scatter plateaus and peaks which are interpreted in terms of resonant diffuse scattering. Distorted wave Born approximation simulations quantitatively reproduce the characteristic features of the measured detector scans with reasonable roughness correlation parameters, i.e. in-plane and out-of-plane correlation lengths, common interface roughness, and Hurst parameters. The different character of resonant diffuse scattering from normal- and reverse-layer-sequence SMs is qualitatively explained and systematized using quasi-kinematical considerations in terms of material and SM parameters. The total off-specular intensity of the supermirrors was found to be non-monotonic with respect to the specular reflectivity at the corresponding angle of incidence.

Investigation of Al/CuO multilayered thermite ignition

The ignition of the Al/CuO multilayered material is studied experimentally to explore the effects of the heating surface area, layering, and film thickness on the ignition characteristics and reaction performances. After the description of the micro-initiator devices and ignition conditions, we show that the heating surface area must be properly calibrated to optimize the nanothermite ignition performances. We demonstrated experimentally that a heating surface area of 0.25 mm2 is sufficient to ignite a multilayered thermite film of 1.6 mm wide by a few cm long, with a success rate of 100%. A new analytical and phenomenological ignition model based on atomic diffusion across layers and thermal exchange is also proposed. This model considers that CuO first decomposes into Cu2O, and then the oxygen diffuses across the Cu2O and Al2O3 layers before reaching the Al layer, where it reacts to form Al2O3. The theoretical results in terms of ignition response times confirm the experimental observation. The increase of the heating surface area leads to an increase of the ignition response time and ignition power threshold (go/no go condition). We also provide evidence that, for any heating surface area, the ignition time rapidly decreases when the electrical power density increases until an asymptotic value. This time point is referred to as the minimum response ignition time, which is a characteristic of the multilayered thermite itself. At the stoichiometric ratio (Al thickness is half of the CuO thickness), the minimum ignition response time can be easily tuned from 59 μs to 418 ms by tuning the heating surface area. The minimum ignition response time increases when the bilayer thickness increases. This work not only provides a set of micro-initiator design rules to obtain the best ignition conditions and reaction performances but also details a reliable and robust MicroElectroMechanical Systems process to fabricate igniters and brings new understanding of phenomena governing the ignition process of Al/CuO multilayers.

Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties

Atomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110–300 °C), film thickness (20–300 nm), bilayer thickness (0.1–100 nm), and TiO2 content (0%–100%). Al2O3 was grown from Me3Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.

Refractive index and bandgap variation in Al2O3-ZnO ultrathin multilayers prepared by atomic layer deposition

This research focuses on the study of the refractive index and bandgap behavior in ultrathin multilayer films of Al2O3-ZnO bilayers grown via atomic layer deposition (ALD) technique on Si(100) substrates. The multilayer configuration stack consists in alternate layers of constant thickness Al2O3 (2 nm) and varying thickness ZnO films in order to obtain a total thickness of ∼100 nm. A set of 10 samples based on bilayers with various 2:X thickness ratios were prepared, where X refers to the ZnO layer thickness. X is proportional to the number of cycles (N) of the ZnO precursor, varying from 1 to 100. The sample morphology was studied via Atomic Force Microscopy and the results show that the surface roughness of the multilayers varies from 0.2 to 1.2 nm, as the ZnO layer thickness increases. In all cases, the roughness values remain below 2% of the total thickness of the multilayer. The refractive index n(λ) and optical bandgap, Eg, of each multilayer sample were studied via spectroscopic ellipsometry (SE). A General Oscillator optical model was utilized to fit the experimental data in order to obtain the total thickness, refractive index and absorption coefficient. Cross-sectional mode scanning electron microscope images verified the multilayer total thickness and corroborated the accuracy of the optical model. The refractive index varies significantly from values close to the Al2O3 refractive index when the bilayer thickness is small, up to values corresponding closely to ZnO for thicker bilayers. The refractive index, as a function of bilayer thickness, varies between 1.63 and 2.3, for λ = 370 nm (UV region), showing high sensitivity. In addition, the optical bandgap energy, Eg, determined using the Tauc model, decreases when the bilayer thickness increases, with a maximum variation of ΔEg ∼1.6 eV. These results reveal that the refractive index and optical bandgap of Al2O3-ZnO material can be modulated systematically as a function of the bilayer thickness. Such behavior is of great importance for optoelectronics applications, in particular for the development of devices with response in the UV spectral range.

The Influence of Lipid Bilayer Physicochemical Properties on Gramicidin A Conformer Preferences

The conformational preferences adopted by gramicidin A (GA) dimers inserted into phospholipid bilayers are reported as a function of the bilayer cholesterol content, temperature, and incubation time. Through use of vesicle capture-freeze drying methodology, GA dimers were captured in lipid bilayers and the conformational preferences of the complex were analyzed using ion mobility-mass spectrometry. Perturbations that affect the physicochemical interactions in the lipid bilayer such as cholesterol incorporation, temperature, and incubation time directly alter the conformer preferences of the complex. Regardless of bilayer cholesterol concentration, the antiparallel double helix (ADH) conformation was observed to be most abundant for GA dimers in bilayers composed of lipids with 12 to 22 carbon acyl chains. Incorporation of cholesterol into lipid bilayers yields increased bilayer thickness and rigidity, and an increased abundance of parallel double helix (PDH) and single-stranded head-to-head (SSHH) dimers were observed. Bilayers prepared using 1,2-dilauroyl-sn-glycero-3-phosphocholine, a lipid with 12 carbon acyl chains, yielded a nascent conformer that decreased in abundance as a function of bilayer cholesterol content. High resolution ion mobility-mass spectrometry data revealed two peaks in the ADH region suggesting that ADH populations are composed of two distinct conformers. The conformer preferences of GA dimers from 1,2-distearoyl-sn-glycero-3-phosphocholine bilayers were significantly different for samples incubated at 4°C vs. 60°C; increased cholesterol content yielded more PDH and SSHH at 60°C. The addition of cholesterol as well as incubating samples of 1,2-distearoyl-sn-glycero-3-phosphocholine at 60°C for 24–72 h yielded an increase in PDH and SSHH abundance.

Effects of Low Concentrations of Docosahexaenoic Acid on the Structure and Phase Behavior of Model Lipid Membranes.

In this paper we report an X-ray diffraction study on the phase behavior of binary lipid mixtures of 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (DHA-PE) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at low concentrations below 5.0 mol% DHA-PE. Our results show that DHA-PE induces phase separation into a DHA rich liquid crystalline (Lα) phase and a DHA poor gel (Lβ′) phase at overall DHA-PE concentrations as low as 0.1 mol%. In addition, we find that the structure of the Lβ′ phase, from which the DHA-PE molecules are largely excluded, is modified in the phase-separated state at low DHA-PE concentrations, with a decrease in bilayer thickness of 1.34 nm for 0.1 mol% at room temperature, compared to pure DPPC bilayers. This result is contrary to that seen in similar studies on mono-unsaturated lipids where an increase in bilayer thickness is observed. The surprising effect of such low DHA-PE concentrations on membrane structure may be important in understanding the role of highly polyunsaturated lipids in biological membrane-based structures and similar artificial surfactant systems.

Rhodopsin/Lipid Hydrophobic Matching—Rhodopsin Oligomerization and Function

Lipid composition of the membrane and rhodopsin packing density strongly modulate the early steps of the visual response of photoreceptor membranes. In this study, lipid-order and bovine rhodopsin function in proteoliposomes composed of the sn-1 chain perdeuterated lipids 14:0d27-14:1-PC, 16:0d31-16:1-PC, 18:0d35-18:1-PC, or 20:0d39-20:1-PC at rhodopsin/lipid molar ratios from 1:70 to 1:1000 (mol/mol) were investigated. Clear evidence for matching of hydrophobic regions on rhodopsin transmembrane helices and hydrophobic thickness of lipid bilayers was observed from 2H nuclear magnetic resonance order parameter measurements at low rhodopsin concentrations. Thin bilayers stretched to match the length of transmembrane helices observed as increase of sn-1 chain order, while thicker bilayers were compressed near the protein. A quantitative analysis of lipid-order parameter changes suggested that the protein adjusts its conformation to bilayer hydrophobic thickness as well, which confirmed our earlier circular-dichroism measurements. Changes in lipid order parameters upon rhodopsin incorporation vanished for bilayers with a hydrophobic thickness of 27 ± 1 Å, suggesting that this is the bilayer thickness at which rhodopsin packs in bilayers at the lowest membrane perturbation. The lipid-order parameter studies also indicated that a hydrophobic mismatch between rhodopsin and lipids triggers rhodopsin oligomerization with increasing rhodopsin concentrations. Both hydrophobic mismatch and rhodopsin oligomerization result in substantial shifts of the equilibrium between the photointermediates metarhodopsin I and metarhodopsin II; increasing bilayer thickness favors formation of metarhodopsin II while oligomerization favors metarhodopsin I. The results highlight the importance of hydrophobic matching for rhodopsin structure, oligomerization, and function.

Interactions of borneol with DPPC phospholipid membranes: a molecular dynamics simulation study.

Borneol, known as a “guide” drug in traditional Chinese medicine, is widely used as a natural penetration enhancer in modern clinical applications. Despite a large number of experimental studies on borneol’s penetration enhancing effect, the molecular basis of its action on bio-membranes is still unclear. We carried out a series of coarse-grained molecular dynamics simulations with the borneol concentration ranging from 3.31% to 54.59% (v/v, lipid-free basis) to study the interactions of borneol with aDPPC(1,2-dipalmitoylsn-glycero-3-phosphatidylcholine) bilayer membrane, and the temperature effects were also considered. At concentrations below 21.89%, borneol’s presence only caused DPPC bilayer thinning and an increase in fluidity; A rise in temperature could promote the diffusing progress of borneol. When the concentration was 21.89% or above, inverted micelle-like structures were formed within the bilayer interior, which led to increased bilayer thickness, and an optimum temperature was found for the interaction of borneol with the DPPC bilayer membrane. These findings revealed that the choice of optimal concentration and temperature is critical for a given application in which borneol is used as a penetration enhancer. Our results not only clarify some molecular basis for borneol’s penetration enhancing effects, but also provide some guidance for the development and applications of new preparations containing borneol.