Membrane Phase Transition Research Articles

Thermally Driven Membrane Phase Transitions Enable Content Reshuffling in Primitive Cells.

Self-assembling single-chain amphiphiles available in the prebiotic environment likely played a fundamental role in the advent of primitive cell cycles. However, the instability of prebiotic fatty acid-based membranes to temperature and pH seems to suggest that primitive cells could only host prebiotically relevant processes in a narrow range of nonfluctuating environmental conditions. Here we propose that membrane phase transitions, driven by environmental fluctuations, enabled the generation of daughter protocells with reshuffled content. A reversible membrane-to-oil phase transition accounts for the dissolution of fatty acid-based vesicles at high temperatures and the concomitant release of protocellular content. At low temperatures, fatty acid bilayers reassemble and encapsulate reshuffled material in a new cohort of protocells. Notably, we find that our disassembly/reassembly cycle drives the emergence of functional RNA-containing primitive cells from parent nonfunctional compartments. Thus, by exploiting the intrinsic instability of prebiotic fatty acid vesicles, our results point at an environmentally driven tunable prebiotic process, which supports the release and reshuffling of oligonucleotides and membrane components, potentially leading to a new generation of protocells with superior traits. In the absence of protocellular transport machinery, the environmentally driven disassembly/assembly cycle proposed herein would have plausibly supported protocellular content reshuffling transmitted to primitive cell progeny, hinting at a potential mechanism important to initiate Darwinian evolution of early life forms.

The Device Using a Polydimethylsiloxane Membrane and the Phase Transition of Water

The accumulation of ice on surfaces can cause harm in many industries. Our work describes an experiment and a simulation of the deicing effect based on a simple device combining a polydimethylsiloxane (PDMS) membrane and water phase transition. The device resulted in a minimum ice adhesion strength of 0.327 kPa, and the ice adhesion strength was still less than 5 kPa after 15 cycles, which meets the requirements of automatic deicing. It also held up after flushing with water and sand currents. In addition, our finite element simulation illustrates that the ice adhesion strength decreases greatly due to the change in initial stress distribution and the separation mode of ice. The fracture between the ice and membrane initiates from one side, and propagates gradually along the contour of ice while at the same time spreading rapidly towards the center. Compared with other icephobic methods, such as expensive and vulnerable micro/nano-surfaces or functional composite coatings, this low-cost and environment friendly device appears promising for large-scale deicing applications in various engineering fields.

How Do Urea and Trimethylamine N-Oxide Influence the Dehydration-Induced Phase Transition of a Lipid Membrane?

Living organisms are often exposed to extreme dehydration, which is detrimental to the structure and function of the cell membrane. The lipid membrane undergoes fluid-to-gel phase transition due to dehydration and thus loses fluidity and functionality. To protect the fluid phase of the bilayer these organisms adopt several strategies. Enhanced production of small polar organic solutes (also called osmolytes) is one such strategy. Urea and trimethylamine N-oxide (TMAO) are two osmolytes found in different organisms combating osmotic stress. Previous experiments have found that both these osmolytes have strong effects on lipid membrane under different hydration conditions. Urea prevents the dehydration-induced phase transition of the lipid membrane by directly interacting with the lipids, while TMAO does not inhibit the phase transition. To provide atomistic insights, we have carried out all-atom molecular dynamics (MD) simulation of a lipid membrane under varying hydration levels and studied the effect of these osmolytes on different structural and dynamic properties of the membrane. This study suggests that urea significantly inhibits the dehydration-induced fluid-to-gel phase transition by strongly interacting with the lipid membrane via hydrogen bonds, which balances the reduced lipid hydration due to the decreasing water content. In contrast, TMAO is excluded from the membrane surface due to unfavorable interaction with the lipids. This induces further dehydration of the lipids which reinforces the fluid-to-gel phase transition. We have also studied the counteractive role of TMAO on the effect of urea on lipid membrane when both the osmolytes are present. TMAO draws some urea molecules out of the membrane and thereby reduces the effect of urea on the lipid membrane at lower hydration levels. This is similar to the counteraction of urea's deleterious effects on protein by TMAO. All these observations are consistent with the experimental results and thus provide deep molecular insights into the role of these osmolytes in protecting the fluid phase of the membrane, the key survival strategy against osmotic-stress-induced dehydration.

Transient Permeabilization of Living Cells: Combining Shear Flow and Acoustofluidic Trapping for the Facilitated Uptake of Molecules

Here, we present a novel approach for the transient permeabilization of cells. We combined laminar shear flow in a microchannel with chaotic advection employing surface acoustic waves. First, as a fundamental result on the one hand, and as a kind of reference measurement for the more complex acoustofluidic approach on the other hand, we studied the permeabilization of cells in pure shear flow in a microchannel with Y-geometry. As a proof of principle, we used fluorescent dyes as model drugs and investigated their internalization into HeLa cells. We found that drug uptake scaled non-linearly with flow rate and thus shear stress. For calcein, we obtained a maximal enhancement factor of about 12 at an optimum flow rate of Q = 500 µL/h in the geometry used here compared to static incubation. This result is discussed in the light of structural phase transitions of lipid membranes accompanied by non-linear effects, as the plasma membrane is the main barrier to overcome. Second, we demonstrated the enhanced permeabilization of acoustically trapped cells in surface acoustic wave induced vortices in a microchannel, with an enhancement factor of about 18 compared to quasi-static incubation. Moreover, we optimized the trapping conditions regarding flow rate, the power level of the surface acoustic wave, and trapping time. Finally, we showed that our method is not limited to small molecules but can also be applied to compounds with higher molecular weight.

Temperature-Controlled Assembly and Characterization of a Droplet Interface Bilayer.

The droplet interface bilayer (DIB) method for assembling lipid bilayers (i.e., DIBs) between lipid-coated aqueous droplets in oil offers key benefits versus other methods: DIBs are stable and often long-lasting, bilayer area can be reversibly tuned, leaflet asymmetry is readily controlled via droplet compositions, and tissue-like networks of bilayers can be obtained by adjoining many droplets. Forming DIBs requires spontaneous assembly of lipids into high density lipid monolayers at the surfaces of the droplets. While this occurs readily at room temperature for common synthetic lipids, a sufficient monolayer or stable bilayer fails to form at similar conditions for lipids with melting points above room temperature, including some cellular lipid extracts. This behavior has likely limited the compositions-and perhaps the biological relevance-of DIBs in model membrane studies. To address this problem, an experimental protocol is presented to carefully heat the oil reservoir hosting DIB droplets and characterize the effects of temperature on the lipid membrane. Specifically, this protocol shows how to use a thermally conductive aluminum fixture and resistive heating elements controlled by a feedback loop to prescribe elevated temperatures, which improves monolayer assembly and bilayer formation for a wider set of lipid types. Structural characteristics of the membrane, as well as the thermotropic phase transitions of the lipids comprising the bilayer, are quantified by measuring the changes in electrical capacitance of the DIB. Together, this procedure can aid in evaluating biophysical phenomena in model membranes over various temperatures, including determining an effective melting temperature (TM) for multi-component lipid mixtures. This capability will thus allow for closer replication of natural phase transitions in model membranes and encourage the formation and use of model membranes from a wider swath of membrane constituents, including those that better capture the heterogeneity of their cellular counterparts.

Time Dependent Impact of Reactive Oxidants on Seminal Attributes of Murrah Bull during Cryopreservation and Storage

Background: Cryopreservation is an invaluable technique yet it is also known to be detrimental to sperm function and fertility due to cryo-injury and concomitant generation of reactive oxidants. During laboratory manipulation for the cryopreservation and freeze-thaw process, spermatozoa undergo osmotic stress, ionic imbalance, metabolic decoupling, membrane phase transition, destabilization of the cytoskeleton and antioxidant depletion which communally hampers the semen quality.Methods: With the aim of determining implications of cryopreservation and storage, semen samples were collected by artificial vagina technique from 12 Murrah bulls and subsequently examined at 0 hour (before cryopreservation) and at 24 hour, 1 month and 2 month of storage for various seminal attributes. Simultaneously seminal plasma was separated and preserved at -20oC till the analysis of biochemical indicators of semen quality viz., nitric oxide (NO), total antioxidant quantity (TAC) and lipid peroxidation status (TBARS). Result: A sharp reduction (p less than 0.01) in the semen quality was observed only at 24 h after cryopreservation except for viability. Significant reduction (p less than 0.05) in viable counts was observed up to 1 month interval. The capacitated sperm percentage was greater (p less than 0.01) in the cryopreserved semen as compared to fresh ejaculate. The mean ± SE levels of NO (μmol/L), TAC and TBARS (Units/ml) was 2.31±0.27, 0.73±0.04 and 1.11±0.16 respectively in seminal plasma of neat semen stored at -20oC, while the values in the extended seminal plasma after cryopreservation was 2.37±0.31, 0.44±0.03 and 0.65±0.03 respectively. So it can be inferred that most of the damage encountered by spermatozoa is during the initial period of freezing, but the damage associated by various stressors cannot be ignored.

Effects of inclusions and the presence of an adsorbing surface on lateral and flip-flop phase transitions in fluid membranes

In this paper, we first investigate the effects of inclusions on the critical behavior of a fluid bilayer biomembrane made of two incompatible amphiphiles A and B. Upon variation of certain appropriate parameters (temperature, lateral pressure, etc.), these lipid molecules can undergo a lateral or transverse phase separation into macro-phase domains. Such transitions are of ordered liquid-disordered liquid type, and the free energy allowing the description of the physical system is a two-order parameters field theory. We show that the presence of a random distribution of inclusions (proteins, enzymes, etc.) may generate a high order coupling interaction term Ωφ2ψ2, which tends to favor flip-flop transition. We evaluate in particular the effect, close to the consolute point, of inclusions on the behavior of the composition fluctuations, φ and ψ, as well as that of the partial and the overall compressibilities in terms of temperature T and chemical potential difference Δμ. Moreover, we show that the presence of inclusions (such as translocases) leaves the values of the critical exponents unchanged, but that the amplitudes are shifted to their high values. We have also established clear links with some experimental work, which concerns a vesicle membrane, proving in particular the existence of a critical temperature Tc in the medium coupling regime. In the second part of this work, we investigate how the critical behavior of the system is affected near an adsorbing wall. We assume that one or both amphiphiles are strongly adsorbed on the surface. We assist therefore to a quenched composition on the wall. The segregation is assumed to occur at low temperature. The influence, on the critical properties of the fluid membrane, of the quenched surface fluctuations is discussed, in all cases, and the composition profile determined.

Effect of TMAO on the Structure and Phase Transition of Lipid Membranes: Potential Role of TMAO in Stabilizing Cell Membranes under Osmotic Stress.

Extremophiles adopt strategies to deal with different environmental stresses, some of which are severely damaging to their cell membrane. To combat high osmotic stress, deep-sea organisms synthesize osmolytes, small polar organic molecules, like trimethylamine-N-oxide (TMAO), and incorporate them in the cell. TMAO is known to protect cells from high osmotic or hydrostatic pressure. Several experimental and simulation studies have revealed the roles of such osmolytes on stabilizing proteins. In contrast, the effect of osmolytes on the lipid membrane is poorly understood and broadly debated. A recent experiment has found strong evidence of the possible role of TMAO in stabilizing lipid membranes. Using the molecular dynamics (MD) simulation technique, we have demonstrated the effect of TMAO on two saturated fully hydrated lipid membranes in their fluid and gel phases. We have captured the impact of TMAO's concentration on the membrane's structural properties along with the fluid/gel phase transition temperatures. On increasing the concentration of TMAO, we see a substantial increase in the packing density of the membrane (estimated by area, thickness, and volume) and enhancement in the orientational order of lipid molecules. Having repulsive interaction with the lipid head group, the TMAO molecules are expelled away from the membrane surface, which induces dehydration of the lipid head groups, increasing the packing density. The addition of TMAO also increases the fluid/gel phase transition temperature of the membrane. All of these results are in close agreement with the experimental observations. This study, therefore, provides a molecular-level understanding of how TMAO can influence the cell membrane of deep-sea organisms and help in combating the osmotic stress condition.

Dember photovoltaic effect as method for structural characterization of phospholipidic membranes

Dember photovoltaic effect (Dember effect) was used to study permeability of a thin layer of lyotropic liquid crystal (dipalmitoylphosphatidylcholine) at various temperatures. By photo dissociation of a photosensitive ionic dye dissolved in a lyotropic liquid crystal, an inhomogeneous distribution of charge carriers was induced in a thin liquid crystal layer. The diffusion relaxation of these charge distributions depends on the structure of lyotropic liquid crystal and show up itself in the temperature dependence of pulsed photoelectric response. The data obtained are in a good agreement with the results of dielectric measurements. In perspective, above method can be applied to examine permeability of natural and synthetic lipid vesicles (cells, exosomes, liposomes and other vesicles) relevant in medicine and to characterize the structural phase transitions in lipid membranes when modified by molecules of interest (e.g. cholesterol in liposomes to increase rigidity and stability).

Study on Phase Transitions of Phospholipid Bilayer Membranes under High Pressure

Study on Phase Transitions of Phospholipid Bilayer Membranes under High Pressure

Metal-Organic Frameworks Corset with a Thermosetting Polymer for Improved Molecular-Sieving Property of Mixed-Matrix Membranes.

Metal-organic frameworks (MOFs) are promising materials for gas separation membranes. However, the framework flexibility affects their molecular-sieving properties. Herein, we restrict the flexibility of zeolitic imidazolate framework-7 (ZIF-7) by controlling its phase transition in mixed-matrix membranes (MMMs), relying on the so-called "space-confinement effect" of a novel thermosetting polymer, poly 2,2'-(p-oxydiphenyl)-5,5'-bibenzimidazole (OPBI) polymer. Compared with the pure OPBI membrane, the optimized membranes containing 30 wt % ZIF-7 with a narrow-pore (np) phase (ZIF-7-II) exhibited a significant improvement in H2/CO2 separation, e.g., the H2/CO2 ideal selectivity increased ∼2.8 times, surpassing the state-of-the-art upper bound of polymeric membranes and exhibited excellent stability at increased pressure and temperature (8 bar, 180 °C).

Phase Transition in a Memristive Suspended MoS2 Monolayer Probed by Opto- and Electro-Mechanics.

Semiconducting monolayers of a 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T' phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the 2H-1T' phase transition, which imposes a 3% directional elongation of the topological 1T' phase with respect to the semiconducting 2H. We note that only a few percent 2H-1T' phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with first-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2 is important for the implementation of future devices.

Correlation between adhesion strength and phase behaviour in solid-supported lipid membranes

Fundamental understanding of vesicle adhesion in the size range ≤ 200 nm is of major importance when addressing biologically relevant processes involving the presence of small vesicles like exosomes or endosomes. Using quartz crystal microbalance with dissipation monitoring, we investigate the correlation between vesicle deformation and eventual membrane rupture on surfaces with different adhesion levels, as well as their respective thermotropic phase transitions. In particular, phase transitions of solid-supported membranes on Au resemble the cooperative behaviour of lipid membrane transitions in bulk. In contrast, solid-supported membranes on SiO2 exhibit broadened ‘double-peak’ transitions, rendering a ‘decoupling’ effect during melting due to stronger interactions with SiO2. This paper provides a comprehensive view of the correlation between size, geometry and phase transitions observed in the layer of adsorbed lipid vesicles/membranes. It paves the way to explore structural changes on more complex biointerfaces by acoustic-based sensors.

Flavonoids modulate liposomal membrane structure, regulate mitochondrial membrane permeability and prevent erythrocyte oxidative damage

In the present work, we investigated the interaction of flavonoids (quercetin, naringenin and catechin) with cellular and artificial membranes. The flavonoids considerably inhibited membrane lipid peroxidation in rat erythrocytes treated with tert-butyl hydroperoxide (700 μM), and the IC50 values for prevention of this process were equal to 9.7 ± 0.8 μM, 8.8 ± 0.7 μM, and 37.8 ± 4.4 μM in the case of quercetin, catechin and naringenin, respectively, and slightly decreased glutathione oxidation. In isolated rat liver mitochondria, quercetin, catechin and naringenin (10–50 μM) dose-dependently increased the sensitivity to Ca2+ ions - induced mitochondrial permeability transition. Using the probes TMA-DPH and DPH we showed that quercetin rather than catechin and naringenin strongly decreased the microfluidity of the 1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomal membrane bilayer at different depths. On the contrary, using the probe Laurdan we observed that naringenin transfer the bilayer to a more ordered state, whereas quercetin dose-dependently decreased the order of lipid molecule packing and increased hydration in the region of polar head groups. The incorporation of the flavonoids, quercetin and naringenin and not catechin, into the liposomes induced an increase in the zeta potential of the membrane and enlarged the area of the bilayer as well as lowered the temperature and the enthalpy of the membrane phase transition. The effects of the flavonoids were connected with modification of membrane fluidity, packing, stability, electrokinetic properties, size and permeability, prevention of oxidative stress, which depended on the nature of the flavonoid molecule and the nature of the membrane.

Magnetic and electronic phase transitions probed by nanomechanical resonators

The reduced dimensionality of two-dimensional (2D) materials results in characteristic types of magnetically and electronically ordered phases. However, only few methods are available to study this order, in particular in ultrathin insulating antiferromagnets that couple weakly to magnetic and electronic probes. Here, we demonstrate that phase transitions in thin membranes of 2D antiferromagnetic FePS3, MnPS3 and NiPS3 can be probed mechanically via the temperature-dependent resonance frequency and quality factor. The observed relation between mechanical motion and antiferromagnetic order is shown to be mediated by the specific heat and reveals a strong dependence of the Néel temperature of FePS3 on electrostatically induced strain. The methodology is not restricted to magnetic order, as we demonstrate by probing an electronic charge-density-wave phase in 2H-TaS2. It thus offers the potential to characterize phase transitions in a wide variety of materials, including those that are antiferromagnetic, insulating or so thin that conventional bulk characterization methods become unsuitable.

On the Origin of the Anomalous Behavior of Lipid Membrane Properties in the Vicinity of the Chain-Melting Phase Transition

Biomembranes are key objects of numerous studies in biology and biophysics of great importance to medicine. A few nanometers thin quasi two-dimensional liquid crystalline membranes with bending rigidity of a few kT exhibit unusual properties and they are the focus of theoretical and experimental physics. The first order chain-melting phase transition of lipid membranes is observed to be accompanied by a pseudocritical behavior of membrane physical-chemical properties. However, the investigation of the nature of the anomalous swelling of a stack of lipid membranes in the vicinity of the transition by different groups led to conflicting conclusions about the level of critical density fluctuations and their impact on the membrane softening. Correspondingly, conclusions about the contribution of Helfrich’s undulations to the effect of swelling were different. In our work we present a comprehensive complementary neutron small-angle and spin-echo study directly showing the presence of significant critical fluctuations in the vicinity of the transition which induce membrane softening. However, contrary to the existing paradigm, we demonstrate that the increased undulation forces cannot explain the anomalous swelling. We suggest that the observed effect is instead determined by the dominating increase of short-range entropic repulsion.

Biophysical Characterization of Membrane Phase Transition Profiles for the Discrimination of Outer Membrane Vesicles (OMVs) From Escherichia coli Grown at Different Temperatures

Dynamic Light Scattering (DLS), Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM) are physical techniques widely employed to characterize the morphology and the structure of vesicles such as liposomes or human extracellular vesicles (exosomes). Bacterial extracellular vesicles are similar in size to human exosomes, although their function and membrane properties have not been elucidated in such detail as in the case of exosomes. Here, we applied the above cited techniques, in synergy with the thermotropic characterization of the vesicles lipid membrane using a turbidimetric technique to the study of vesicles produced by Gram-negative bacteria (Outer Membrane Vesicles, OMVs) grown at different temperatures. This study demonstrated that our combined approach is useful to discriminate vesicles of different origin or coming from bacteria cultured under different experimental conditions. We envisage that in a near future the techniques employed in our work will be further implemented to discriminate complex mixtures of bacterial vesicles, thus showing great promises for biomedical or diagnostic applications.

Annexin-V stabilizes membrane defects by inducing lipid phase transition

Annexins are abundant cytoplasmic proteins, which bind to membranes that expose negatively charged phospholipids in a Ca2+-dependent manner. During cell injuries, the entry of extracellular Ca2+ activates the annexin membrane-binding ability, subsequently initiating membrane repair processes. However, the mechanistic action of annexins in membrane repair remains largely unknown. Here, we use high-speed atomic force microscopy (HS-AFM), fluorescence recovery after photobleaching (FRAP), confocal laser scanning microscopy (CLSM) and molecular dynamics simulations (MDSs) to analyze how annexin-V (A5) binds to phosphatidylserine (PS)-rich membranes leading to high Ca2+-concentrations at membrane, and then to changes in the dynamics and organization of lipids, eventually to a membrane phase transition. A5 self-assembly into lattices further stabilizes and likely structures the membrane into a gel phase. Our findings are compatible with the patch resealing through vesicle fusion mechanism in membrane repair and indicate that A5 retains negatively charged lipids in the inner leaflet in an injured cell.

Thermodynamic Study on Phase Transitions of Phospholipid Bilayer Membranes

Thermodynamic Study on Phase Transitions of Phospholipid Bilayer Membranes

Origin of lipid tilt in flat monolayers and bilayers.

This paper continues the series of our works devoted to the liquid-gel phase transition in lipid membranes. Previously we described a variation of area per lipid, membrane thickness, and diffusion coefficient at the temperature-driven liquid-gel phase transition in bilayers. Here we expand the application of our analytic model approach to include a description of the lipid tilt and also extend the investigation to include Langmuir and self-assembled monolayers. The theory describes tilt formation at the temperature-driven liquid-gel phase transition in bilayers and the pressure-driven phase transition in Langmuir monolayers. Neither uniform tilt nor liquid-gel phase transition is found in self-assembled monolayers chemically bonded to the substrate.