Bilayer Phase Transition Research Articles

Topological hidden phase transition in honeycomb bilayers with a high Chern number

Topological hidden phase transition in honeycomb bilayers with a high Chern number

Magnetic phase transition and valley splitting in Janus GdBrI bilayer

Magnetic phase transition and valley splitting in Janus GdBrI bilayer

Probing Slipids Force Field for Phase Transitions in SOPC Lipid Bilayers with Various Cholesterol Concentrations

We explore the phase behavior of lipid bilayers containing SOPC (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) with various molar concentrations (0 mol%, 10 mol% and 30 mol%) of cholesterol. To this end, we performed extensive atomistic molecular dynamics simulations in conjunction with the Slipids force field with optimized parameters for the headgroups of phospholipids. We computed thermodynamic and structural quantities describing the ordering of the tails, the mobility of the heads and the arrangement of the lipids in the bilayers. We analyzed the behavior of the named quantities over the temperature range between 271 K and 283 K, where the experimentally determined melting temperature, Tm=279 K, lies, as well as at 400 K, which is used as a reference temperature. The obtained results are compared to available experimental data along with the outcome from molecular dynamics simulations of similar phospholipids containing different amounts of cholesterol. In the temperature interval of interest, we found evidence of the occurrence of a thermal-driven phase transition (melting) in both the pure system and the one with the lower concentration of cholesterol, while in the remaining system, the higher amount of cholesterol in the bilayer smears out the transitional behavior. Thus, we demonstrate the ability of the Slipids force field to predict the phase behavior of bilayers of SOPC and SOPC mixed with cholesterol.

An analysis of the dipalmitoylphosphatidylcholine bilayer gel phases predicted with molecular dynamics simulations using force fields from the GROMOS family

Dipalmitoylphosphatidylcholine is a phospholipid of major importance for biological systems. Molecular dynamics simulation investigations of this lipid focused on their behavior at human body temperature (≈ 37 °C or 310 K). For some applications, however, it is necessary to study its properties at room temperature (≈ 25 °C or 298 K). A small difference in temperature at this range is responsible for a phase transition in the lipid bilayer. Therefore, molecular dynamics simulations are carried out applying six different force fields from the GROMOS family to compare the less ordered phase (liquid-crystalline phase) with a more ordered phase (gel phase) of the hydrated bilayer of dipalmitoylphosphatidylcholine formed at 323 K and 298 K, respectively. The analysis of the bilayer structural quantities is used to evaluate order and packing at the two temperatures. The area per lipid and deuterium order parameter results are in agreement with simulation results from the literature and are also compared with experimental data. These parameters, however, do not provide a clear picture of the low-temperature gel phase present in the simulations using the force fields from the GROMOS family. To address this, a strategy for computing the lipid phase nematic order parameter is proposed, and a reference gel phase is simulated using the CHARMM36 force field for comparison. The results are consistent with other structural quantities but reveal a level of order that is below the expected for an ordered gel phase for the GROMOS force fields. Visual inspection of the simulation trajectories suggests the presence of a ripple phase. Overall, the GROMOS 54A8 provides the best performance among the tested GROMOS force fields for both investigated temperatures, even lacking a proper representation of the ordered gel phase.

Heating of nanoparticles and their environment by laser radiation and applications

This review considers the fundamental dynamic processes involved in the laser heating of metal nanoparticles and their subsequent cooling. Of particular interest are the absorption of laser energy by nanoparticles, the heating of a single nanoparticle or an ensemble thereof, and the dissipation of the energy of nanoparticles due to heat exchange with the environment. The goal is to consider the dependences and values of the temperatures of the nanoparticles and the environment, their time scales, and other parameters that describe these processes. Experimental results and analytical studies on the heating of single metal nanoparticles by laser pulses are discussed, including the laser thresholds for initiating subsequent photothermal processes, how temperature influences the optical properties, and the heating of gold nanoparticles by laser pulses. Experimental studies of the heating of an ensemble of nanoparticles and the results of an analytical study of the heating of an ensemble of nanoparticles and the environment by laser radiation are considered. Nanothermometry methods for nanoparticles under laser heating are considered, including changes in the refractive indices of metals and spectral thermometry of optical scattering of nanoparticles, Raman spectroscopy, the thermal distortion of the refractive index of an environment heated by a nanoparticle, and thermochemical phase transitions in lipid bilayers surrounding a heated nanoparticle. Understanding the sequence of events after radiation absorption and their time scales underlies many applications of nanoparticles. The application fields for the laser heating of nanoparticles are reviewed, including thermochemical reactions and selective nanophotothermolysis initiated in the environment by laser-heated nanoparticles, thermal radiation emission by nanoparticles and laser-induced incandescence, electron and ion emission of heated nanoparticles, and optothermal chemical catalysis. Applications of the laser heating of nanoparticles in laser nanomedicine are of particular interest. Significant emphasis is given to the proposed analytical approaches to modeling and calculating the heating processes under the action of a laser pulse on metal nanoparticles, taking into account the temperature dependences of the parameters. The proposed models can be used to estimate the parameters of lasers and nanoparticles in the various application fields for the laser heating of nanoparticles.

Chiral nematic and fluctuation-induced first-order phase transitions in AB-stacked kagome bilayers

Chiral nematic and fluctuation-induced first-order phase transitions in AB-stacked kagome bilayers

Studies of some bio-inspired liquid crystals

ABSTRACTThe main phase transition of lipid bilayers corresponds to a defined change from an ordered (gel) to a disordered (fluid liquid crystal) state of hydrocarbon chains, occurring in general at a defined temperature Tm. However, aqueous dispersions of pure anionic phospholipids, and particularly DMPG, may exhibit a ‘melting regime’, from Tmon until Tmoff, over ~10°C. The melting process depends on pH, ionic strength and several details of sample preparation. This paper makes a review on seven papers of the author with different collaborations, with the original proposal of pores in the DMPG melting transition. The focus is on structural results obtained from X-ray scattering, varying concentration and temperature, but integrating with results from other experimental techniques. Initially the effect of salt addition was separated from the effect of DMPG concentration. At DMPG concentrations higher than 70 mM a lamellar phase starts and a detailed study of the temperature variation of 150 mM DMPG allowed arrival to detailed pore model, focused in the surface fractions of pores in the bilayers. The model admits large and small pores in the melting regime and agrees with the new Lamellar Phase with pores (Lp) starting at T3. The large biological relevance is stressed.

Breaking sensitivity limits of QCM-D: An integrated nano-honeycomb sensor for characterization molecular interactions with lipid bilayers.

In the last few years Quartz Crystal Microbalance with Dissipation (QCM-D) emerged as an indispensable label-free method for monitoring formation of lipid bilayers on surfaces and assessing quality of the bilayers formed. QCM-D provides time-resolved data on mass and viscoelastic properties of molecular layers at the sensor surface and therefore is suitable for detecting vesicle adhesion, rupture, and spreading to form planar surface-supported bilayers. However, the small surface areas of QCM sensors impose severe limitation on mass sensitivity of the method. This is, perhaps, the main reason why QCM-D is rarely employed for studying lipid bilayer phase transitions and membrane binding events. To overcome these limitations, we have integrated the QCM sensor with an array of rigid honeycomb-packed anodic aluminum oxide (AAO) nanopores of ca. 25-80 nm in diameter and 2-10 μm in length by anodizing aluminum film deposited on the electrode surface. The resulting nanostructured QCM sensor increased surface area and improved sensitivity of the method by >100-fold. AAO-QCM electrode also provided a nanoscale template for self-assembly of lipid bilayers into nanotubular structures without any chemical attachment. We then employed such nanopore-confined bilayers to observe the main and more difficult to characterize pre-transition of membranes prepared from either pure 1,2-dimyristoyl-sn-glycerol-3-phosphocholine (DMPC) or 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) lipids. Observed changes in QCM frequency reflected variations in lipid density upon phase transitions and occurred at temperatures in agreement with literature data. The signal-to-noise ratio of the phase transition peaks increased by 300-fold when compared to conventional QCM-D sensors. To further explore potential applications of AAO-QCM-D, ethanol-induced biphasic effects of phospholipid bilayers were studied. High sensitivity of AAO-QCM electrodes makes the method highly attractive for developing biosensing applications based on detecting changes in lipid packing at the nanoscale.

Preparation and Characterization of Trastuzumab Fab-Conjugated Liposomes (Immunoliposomes).

Immunoliposomes are made by conjugating antibodies or antibody fragments on liposome surfaces. Antibody fragments Fab', single-chain Fv fragments (scFv), or new constructs such as nanobodies are commonly used instead of whole IgGs for reduced risk immunogenicity. Here we described the preparation and characterization of immunoliposome-containing trastuzumab Fabs on the surface. The targeting ligand Fab-PEG-DSPE was synthesized by site-specific coupling between the C-terminal cysteine of the Fab and the maleimide group at the distal end of a DSPE-PEG. It was incorporated into preformed liposomes at 60°C above the lipid bilayer phase transition temperature. The binding avidity of the immunoliposomes containing different Fab valencies was characterized using biolayer interferometry.

Fluid-gel coexistence in lipid membranes under differential stress

A widely conserved property of many biological lipid bilayers is their asymmetry. In addition to having distinct compositions on its two sides, a membrane can also exhibit different tensions in its two leaflets, a state known as differential stress. Here, we examine how this stress can influence the phase behavior of the constituent lipid monolayers of a single-component membrane. For temperatures sufficiently close to, but still above, the main transition, molecular dynamics simulations show the emergence of finite gel domains within the compressed leaflet. We describe the thermodynamics of this phenomenon by adding two empirical single-leaflet free energies for the fluid-gel transition, each evaluated at its respective asymmetry-dependent lipid density. Finite size effects arising in simulation are included in the theory through a geometry-dependent interfacial term. Our model reproduces the phase coexistence observed in simulation. It could therefore be used to connect the “hidden variable” of differential stress to experimentally observable properties of the main phase transition. These ideas could be generalized to any first-order bilayer phase transition in the presence of asymmetry, including liquid-ordered/liquid-disordered phase separation.

Improvement of Thermal Stability of Amphipathic Peptide-Phospholipid Nanodiscs via Lateral Association of α-Helices by Disulfide Cross-Linking.

Amphipathic α-helical peptides have been reported to form discoidal particles or nanodiscs with phospholipids, in which a lipid bilayer patch is encircled by peptides. Peptide-based nanodiscs have broad applicability because of their ease of preparation, size flexibility, and structural plasticity. We previously revealed that the nanodiscs formed by apolipoprotein-A-I-derived peptide 18A showed temperature-dependent structural destabilization above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayer. It has been suggested that this destabilization is due to the migration of peptides bound to the edge of the discs to the bilayer surface. In this study, we designed a peptide that could stabilize nanodisc structures against the phase transition of lipid bilayers by disulfide cross-linking of peptides. An 18A-dimer cross-linked by a proline residue, 37pA (Ac-18A-P-18A-CONH2), also showed thermal destabilization of nanodiscs like 18A. However, cross-linking the sides of the two α-helices of the cysteine-substituted analogue 37pA-C2 with disulfide bonds led to the formation of nanodiscs that were more stable to temperature changes. This stabilizing effect was mainly due to the formation of a cyclic 37pA-C2 monomer by intramolecular disulfide cross-linking. These results suggest that the lateral association of two α-helices, which is the basis of the double-belt structure, is an important factor for the implementation of stable nanodiscs. The results of this study will help in development of more stable nanoparticles with membrane proteins in the future.

Structural changes of interfacial water upon fluid-ripple-gel phase transitions of bilayers

All atom molecular dynamics simulations of fully hydrated dimyristoylphosphatidylcholine lipid bilayers are performed for total 9.45 μs showing a fluid to a ripple to a gel phase transition. Changes in the radial distribution functions and angular distribution functions of the interface water (IW) signify a shift from a distorted tetrahedral to a tetrahedral arrangement upon decrease in temperature at the gel phase. The potential of mean forces of the IW follow two cross-overs near both phase transitions unlike the BW. Our analyses reveal a strong correlation between bilayer phase transitions and the structural properties of the IW.

Temperature- and Pressure-Induced Bilayer Phase Transitions of an Amide-Linked Phosphatidylcholine: A Contrasting Effect of Chain-Linkage Type

Abstract An unnatural phospholipid with two amide linkages, dipalmitoylamidodeoxyphosphatidylcholine (DPADPC), was synthesized and its bilayer phase transitions were observed by differential scanning calorimetry under atmospheric pressure and light-transmittance measurements under high pressure. The DPADPC bilayer membrane showed different phase transitions depending on the thermal history of the lipid sample under atmospheric pressure. The thermal-history dependent transitions were also observed under high pressure. The temperature–pressure phase diagram and thermodynamic quantities of the phase transitions of the DPADPC bilayer membrane were compared with those of bilayer membranes of hydrophobic chain-linkage isomers, ester-linked dipalmitoylphosphatidylcholine (DPPC) and ether-linked dihexadecylphosphatidylcholine (DHPC). The thermodynamic quantities of the main transition for these PC bilayer membranes were similar to one another, whereas the suppression of the interdigitation and great stabilization of the hydrated crystal phase were observed for the DPADPC bilayer membrane. Furthermore, the results of nuclear magnetic resonance measurements indicated that the motility of PC molecules in both gel and liquid crystal phases of the DPADPC bilayer membranes were much lower than those of the DPPC and DHPC bilayer membranes. From the above results, we concluded that the molecular interaction in the PC bilayer membranes increases in the order of ether-, ester- and amide-linkage isomers.

Antibacterial Mechanism of Multifunctional MXene Nanosheets: Domain Formation and Phase Transition in Lipid Bilayer.

MXenes, two-dimensional metal carbides or nitrides with multifunctional surfaces, are one of the most promising antibacterial nanoscale materials. However, their putative bactericidal mechanism is elusive. To study their bactericidal mechanism, we investigated the interaction between a MXene nanosheet and a model bacterial membrane by molecular dynamics simulations and found that an adsorbed MXene on a membrane surface induced a local phase transition in a domain where the fluidity of the phospholipid in this domain at room temperature was comparable with that of the gel phase. The domain also showed a denser and thinner phospholipid membrane structure than the peripheral phospholipids. By comparing it with our previous experiments of the bactericidal activity of MXenes, we proposed the leakage of intercellular molecules at the phase boundary defects as a possible bactericidal mechanism of MXenes that leads to cell lysis. This study provides a useful model for tailoring new bactericidal nanomaterials.

The molecular design of drug-ionic liquids for transdermal drug delivery: Mechanistic study of counterions structure on complex formation and skin permeation

Though ionic liquids (ILs) as novel enhancers had garnered wide attention, detailed studies elucidating molecular design of drug-ILs were missing and mechanisms of their formation and skin permeation were still lacking. Herein, we systematically investigated effects of counterions structures on formation and skin permeation of drug-ILs. Firstly, effects of counterions on formation of drug-ILs were dependent on polarizability, molecular weight (M.W.) and polar surface area of counterions. It was caused by strong charge assisted hydrogen bond and van der Waals interactions revealed through FT-IR, X-ray photoelectron spectroscopy and molecular docking, which undermined ionic interactions and reduced total interaction strength, thereby produced lower lattice energy. Then, skin permeability of drug-ILs had a good parabola relationship with M.W., polarizability and log P of counterions. The underlying mechanism was the increased drug miscibility with stratum corneum, which caused conformational disorder and phase transition of lipid bilayers characterized by ATR-FTIR, DSC and confocal laser scanning microscopy. Finally, the drug-ILs proved to be non-irritating using in vivo skin erythema analysis. In conclusion, the quantitative structure-activity relationship models based on counterions structure to predict formation and skin permeation of drug-ILs were developed, which provided basic theory for design of drug-ILs with high permeation-enhancing efficiency.

Tunable Magnetism and Insulator–Metal Transition in Bilayer Perovskites

Two-dimensional (2D) transition-metal oxide perovskites greatly expand the field of available 2D multifunctional material systems. Here, based on density functional theory calculations, we predicted the presence of ferromagnetism orders accompanying with an insulator-metal phase transition in bilayer $KNbO_{3}$ and $KTaO_{3}$ by applying strain engineering and/or external electric field. Our results will contribute to the applications of few-layer transition metal oxide perovskites in the emerging spintronics and straintronics.

Biomimetic light-activatable graphene-based nanoarchitecture for synergistic chemophotothermal therapy

• Lipid coated biocompatible rGO-mesoporous-silica nanocomposites with was prepared with enhanced hydrophilicity and stability. • The high loading capacity of the nanodrug carriers was proved. • The in vivo antitumor evaluations confirmed the efficacy of nanodrug carriers. The development of biomimetic technology offers excellent opportunities for the construction of therapeutic platforms with enhanced biocompatibility and antitumor performance. In this study, lipid-bilayer-coated reduced graphene oxide (rGO) modified with mesoporous-silica nanosheets was constructed via facile lipid self-assembly for effective synergistic chemophotothermal therapy. These nanodrug carriers were suitable for a high loading rate (53.3%) of doxorubicin (DOX). In vitro , after cellular internalization of these nanosheets, upon near infrared light (NIR) irradiation, the rGO in the nanoarchitecture generated heat not only for hyperthermia treatment but also for triggering lipid bilayer phase transition and on-demand controlled DOX release for chemotherapy. As expected, systematic in vivo antitumor evaluations confirmed the efficient solid tumor removal without recurrence. Our results revealed the excellent integration of the mesoporous-silica-coated rGO and lipid bilayer, which enabled the construction of a platform with the merits of both materials within a single unit. Thus, our results also indicate the great potential for the applications of graphene-based nanomaterials in biomedicine.

Effect of the Hydrocarbon Chain Disorder in Phosphatidylcholine Bilayers on Gigahertz Sound Velocity.

Suspensions of multilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and hydrated and dehydrated aligned multilamellar samples of DMPC were studied by Brillouin spectroscopy in the temperature range from 90 to 333 K. The sound velocity of the longitudinal acoustic wave was evaluated from the Brillouin spectra. It was found that phase transition, hydration state, and planar or vesicular form of bilayers affect the gigahertz sound velocity. Usually, the temperature dependence of the sound velocity is weak in solid substances. Amazingly, the sound velocity of hydrated DMPC samples showed significant temperature-induced changes of up to 1.8 times, even within the solid-like gel phase. We explained this effect by temperature-induced excitations of the disordered conformational states of the hydrocarbon chains as well as anharmonic effects. In addition, the relevance of the gigahertz sound velocity to the description of subterahertz Raman features was demonstrated.

Strain-induced phase transition in CrI3 bilayers

A monolayer of CrI3 is a two-dimensional crystal in its equilibrium configuration is a ferromagnetic semiconductor. In contrast, two coupled layers can be ferromagnetic, or antiferromagnetic depending on the stacking. We study the magnetic phase diagram upon the strain of the antiferromagnetically coupled bilayer with C2/m symmetry. We found that strain can be an efficient tool to tune the magnetic phase of the structure. A tensile strain stabilizes the antiferromagnetic phase, while a compressive strain turns the system ferromagnetic. We associate that behavior to the relative displacement between layers induced by the strain. We also study the evolution of the magnetic anisotropy, the magnetic exchange coupling, and how the Curie temperature is affected by the strain.

The effect of neuroleptic drugs on DPPC/sphingomyelin/cholesterol membranes

The hydrophobic nature of neuroleptic drugs renders that these molecules interact not only with protein receptors, but also with the lipids constituting the membrane bilayer. We present a systematic study of the effect of seven neuroleptic drugs on a biomembrane model composed of DPPC, sphingomyelin, and cholesterol. Differential scanning calorimetry (DSC) measurements were used to monitor the gel-fluid phase transition of the lipid bilayer at three pH values and also as a function of drug concentration. The implementation of a new methodology to mix lipids homogeneously allowed us to assemble bilayers completely free of organic solvents. The seven neuroleptics were: trifluoperazine, haloperidol decanoate, clozapine, quetiapine, olanzapine, aripiprazole, and amisulpride. The DSC results show that the insertion of the drug into the bilayer produces a fluidization and a disordering of the bilayer. The bilayer perturbation is qualitatively the same for all the studied drugs, but quantitatively different. The driving force for the neuroleptic drug to place itself in the lipid bilayer is entropic in nature, signaling to the importance of the size and geometry of the drugs. The drug protonated species produce stronger effects than their non-protonated forms. At high concentrations two of the neuroleptics revert the fluidization effect and another completely abolishes the gel-fluid transition. The DSC data and the associated discussion contribute to the understanding of the interactions between neuroleptic drugs and lipid membranes.