Gel-liquid Phase Transition Research Articles

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.

Lipid lateral self-diffusion drop at liquid-gel phase transition.

A drop of lipid lateral self-diffusion coefficient at the liquid-gel phase transition in lipid membranes is calculated. So far this drop was missing theoretical description. Our microscopic model captures so-called subdiffusion regime, which takes place on 1 ps-100 ns timescale and reveals a jump of self-diffusion coefficient. Calculation of the jump is based on our recent study of liquid-gel phase transition. Subdiffusive regime is described within the free volume theory. Calculated values of self-diffusion coefficient are in agreement with quasielastic neutron scattering measurements. Self-diffusion coefficient is found to be composed of two factors: one is related to an area per lipid change at the phase transition, and the other one is due to an order of magnitude change in the stiffness of entropic repulsive potential.

Microscopic Description of the Thermodynamics of a Lipid Membrane at a Liquid–Gel Phase Transition

A microscopic model of the lipid membrane is constructed that provides analytically tractable description of the physical mechanism of the first order liquid-gel phase transition. We demonstrate that liquid-gel phase transition is cooperative effect of the three major interactions: inter-lipid van der Waals attraction, steric repulsion and hydrophobic tension. The model explicitly shows that temperature-dependent inter-lipid steric repulsion switches the system from liquid to gel phase when the temperature decreases. The switching manifests itself in the increase of lateral compressibility of the lipids as the temperature decreases, making phase with smaller area more preferable below the transition temperature. The model gives qualitatively correct picture of abrupt change at transition temperature of the area per lipid, membrane thickness and volume per hydrocarbon group in the lipid chains. The calculated dependence of phase transition temperature on lipid chain length is in quantitative agreement with experimental data. Steric repulsion between the lipid molecules is shown to be the only driver of the phase transition, as van der Waals attraction and hydrophobic tension are weakly temperature dependent.

Analytical Calculation of Diffusion Coefficient Drop at the Liquid-Gel Phase Transition in Lipid Membrane

Diffusion coefficient drop by an order of magnitude at the liquid-gel phase transition in the lipid membranes so far was missing theoretical description. Subdiffusion regime, which takes place on 1ps-10ns timescale, is captured by our microscopic model and shows a jump of the self-diffusion coefficient. We developed the analytical theory of the first order liquid-gel phase transition of a lipid bilayer using the microscopic model of semi-flexible strings. We have shown that the van der Waals attraction between the lipids tails is the essential component of the free energy. In the framework of the free volume theory, we calculated a diffusion coefficient in the subdiffusive regime drop by an order of magnitude at the main phase transition. We found that the main contribution to the diffusion drop is due to compression factor as opposed to activation-like factor. The calculated temperature dependencies of the major thermodynamic characteristics of the lipid membranes including diffusion coefficient, membrane thickness, area and volume per lipid molecule are in a good quantitative agreement with experimental data. The work was supported by the Russian Science Foundation (project #17-79-20440).

Gel phase in hydrated calcium dipicolinate

The mineralization of dipicolinic acid (DPA) molecules in bacterial spore cores with Ca2+ ions to form Ca-DPA is critical to the wet-heat resistance of spores. This resistance to “wet-heat” also depends on the physical properties of water and DPA in the hydrated Ca-DPA-rich protoplasm. Using reactive molecular dynamics simulations, we have determined the phase diagram of hydrated Ca-DPA as a function of temperature and water concentration, which shows the existence of a gel phase along with distinct solid-gel and gel-liquid phase transitions. Simulations reveal monotonically decreasing solid-gel-liquid transition temperatures with increasing hydration, which explains the experimental trend of wet-heat resistance of bacterial spores. Our observation of different phases of water also reconciles previous conflicting experimental findings on the state of water in bacterial spores. Further comparison with an unmineralized hydrated DPA system allows us to quantify the importance of Ca mineralization in decreasing diffusivity and increasing the heat resistance of the spore.

Intramolecular structural parameters are key modulators of the gel-liquid transition in coarse grained simulations of DPPC and DOPC lipid bilayers

The capability of coarse-grained models based on the MARTINI mapping to reproduce the gel-liquid phase transition in saturated and unsaturated model lipids was investigated. We found that the model is able to reproduce a lower critical temperature for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) with respect to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Nonetheless, the appearance of a gel phase for DOPC is strictly dependent on the intramolecular parameters chosen to model its molecular structure. In particular, we show that the bending angle at the coarse-grained bead corresponding to the unsaturated carbon-carbon bond acts as an order parameter determining the temperature of the phase transition. Structural analysis of the molecular dynamics simulations runs evidences that in the gel phase, the packing of the lipophilic tails of DOPC assume a different conformation than in the liquid phase. In the latter phase, the DOPC geometry resembles that of the relaxed free molecule. DPPC:DOPC mixtures show a single phase transition temperature, indicating that the observation of a phase separation between the two lipids requires the simulation of systems with sizes much larger than the ones used here.

Erlang flow of hydrophilic pore formation and closure events in a lipid bilayer during phase transition resulting from diffusion in the radius space.

The Smoluchowski equation with an energy profile of a special type and an assumed hydrophobic ("half") pore source term is used to describe the process of hydrophilic pore formation in a lipid bilayer at the gel-liquid phase transition. The source term reflects the occurrence of molecule packing defects in a lipid bilayer at phase transition. The time sequences of the pore formation and closure events are treated as non-stationary, second-order Erlang flows whose characteristics depend on the equation solution. The computed distributions of the time intervals between hydrophilic pores, and pore lifetimes agree with the previously published experimental interpulse interval and pulse duration histograms for the current fluctuations through planar bilayer membranes of DPPC immersed in a LiCl aqueous solution containing polyethylene glycol. Thus, the statistical analysis of pore formation and closure times leads us to conclude that firstly, the increased permeability of a lipid bilayer during the gel-liquid phase transition is accounted for by the emergence of additional hydrophobic defects in the heterogeneous structure of the bilayer and secondly, that the non-exponential distributions of the lipid channel closed and open times observed in experiments are evidence that the process of hydrophilic pore formation is not a one-step process but involves at least two dependent events.

Toward Chemically Resolved Computer Simulations of Dynamics and Remodeling of Biological Membranes

Cellular membranes are fundamental constituents of living organisms. Apart from defining the boundaries of the cells, they are involved in a wide range of biological functions, associated with both their structural and the dynamical properties. Biomembranes can undergo large-scale transformations when subject to specific environmental changes, including gel-liquid phase transitions, change of aggregation structure, formation of microtubules, or rupture into vesicles. All of these processes are dependent on a delicate interplay between intermolecular forces, molecular crowding, and entropy, and their understanding requires approaches that are able to capture and rationalize the details of all of the involved interactions. Molecular dynamics-based computational models at atom-level resolution are, in principle, the best way to perform such investigations. Unfortunately, the relevant spatial and time dimensionalities involved in membrane remodeling phenomena would require computational costs that are today unaffordable on a routinely basis. Such hurdles can be removed by coarse-graining the representations of the individual molecular components of the systems. This procedure anyway reduces the possibility of describing the chemical variations in the lipid mixtures composing biological membranes. New hybrid particle field multiscale approaches offer today a promising alternative to the more traditional particle-based simulations methods. By combining chemically distinguishable molecular representations with mesoscale-based computationally affordable potentials, they appear as one of the most promising ways to keep an accurate description of the chemical complexity of biological membranes and, at the same time, cover the required scales to describe remodeling events.

Low-melting-point polymeric nanoshells for thermal-triggered drug release under hyperthermia condition

Purpose: The aim of this paper was to synthesise core-shell nanostructures comprised of mesoporous silica core and a low melting-point polyethylene glycol (PEG) nanoshell with a sharp gel–liquid phase transition for rapid drug release at hyperthermia temperature range. Materials and methods: The phase transition behaviours of PEGs with molecular weights of 1000, 1500, and 2000 Da were analysed using differential scanning calorimetry (DSC) to determine the optimal formulation with phase transition in the hyperthermia range. The ‘graft-to’ method was employed to synthesise core–shell nanostructures using the selected PEG formulation. The drug loading and release behaviours of these nanocarriers were examined by ultra-violet visible spectroscopy (UV-Vis) using doxorubicin as a model drug. Magnetic resonance-guided focused ultrasound (MRgFUS) was also applied as a typical thermal modality to evaluate the rate of drug release from the core-shell nanostructures. Results: The PEG molecular weight of 1500 Da presented the optimal phase transition temperature for thermal-triggered release under hyperthermia conditions. Drug release measurements at different temperatures using UV-Vis methods showed a 20.2 ± 4.3% leakage in aqueous solution at 37 °C after 30 min, while this value was significantly increased to 68.2 ± 3.7% at 50 °C. A 45.5 ± 3.1% drug release was also obtained after sonication of the drug-loaded nanoparticles for 5 × 20 s using MRgFUS. Conclusion: Although the ratio of drug leakage at physiological temperatures was relatively high, the sharp transition temperature, high loading efficiency, and fast drug release at hyperthermia temperature range could make these core-shell nanoparticles prominent for enhancing the efficacy of various hyperthermia modalities in the treatment of cancer tumours.

Phospholipid Architecture of the Bovine Milk Fat Globule Membrane Using Giant Unilamellar Vesicles as a Model

Giant unilamellar vesicles (GUVs) were constructed using an electroformation technique to mimic the morphology of the native milk fat globule membrane (MFGM) for the purpose of structural investigation. Bovine milk derived phospholipids were selected to manufacture GUVs which were characterized by confocal laser scanning microscopy after fluorescent staining. Circular nonfluorescent dark regions were observed in a 3/7 (mol/mol) surface mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3 phosphoethanolamine. Linear shaped dark lipid domains were found in GUVs containing sphingomyelin (SM) in the absence of cholesterol. The dark regions were interpreted as a gel phase formed by a high gel-liquid phase transition temperature (Tm) of DPPC and SM. This study provides a strategy for investigating the lipid structural organization within the native MFGM using a model lipid bilayer system and reveals that a SM and cholesterol association network is not the only requirement for nonfluorescent lipid domain formation and that PE is preferably located in the inner leaflet of the phospholipid bilayer.

Phase behavior of DODAB aqueous solution

Phase behavior of DODAB aqueous solution, prepared without sonication, was studied by adiabatic scanning calorimetry. Measurements revealed four phase transitions with the temperatures 35.2, 39.6, 44.6, and 52.4°C at heating and one transition at the temperature 40.4°C at cooling. The first three transitions at heating occur in unilamellar vesicles. The first and third transitions correspond to the subgel-gel and gelliquid phase transitions, corresponding enthalpy jumps are equal to 33 and 49 kJ/mol. The second transition appears after some aging and is similar to gel-ripple phase transition in a DPPC solution, with the enthalpy jump under the transition exceeding 7.4 kJ/mol. The transition occurs in unilamellar vesicles. The transition at the temperature 52.4°C occurs in another subsystem of the solution, which we believe to be multilamellar vesicles. The enthalpy jump at this transition is equal to 97 kJ/mol, and data analysis suggests that this is a subgel-liquid transition. The phase transition at cooling is the liquid-gel transition in unilamellar vesicles. During the measurements, a slow evolution of the solution occurs, consisting in a change of concentrations of unilamellar and multilamellar vesicles. This transformation mainly occurs at low temperatures.

Insertion properties of cholesterylcyclodextrins in phospholipid membranes: a molecular study

Amphiphilic cyclodextrins (CDs) are good candidates to functionalize natural membranes, as well as synthetic vesicles. In this paper, we fully describe and compare the insertion properties of the permethylated mono-cholesteryl α-CD (TASC) and its mono- and di-cholesteryl β-CD analogues (TBSC and TBdSC) in dipalmitoyl-L-α-phosphatidylcholine (DPPC) mono- and bi-layers as membrane models from the macroscopic to the molecular scale. By calculating the inverse compressibility moduli and free excess Gibbs energies from the Langmuir isotherms, the influence of the CD type, CD ratio and number of cholesteryl anchors on the membrane properties have been established. TBdSC, with its two cholesteryl residues, seems to be anchored best to the membrane compared to CD derivatives with only one anchor. Furthermore, TASC appears to be more firmly inserted into the membrane than TBSC. The in-plane structure is characterized by Brewster angle microscopy (BAM) at the air–water interface and atomic force microscopy (AFM) of the mono- and bi-layers deposited on mica. Depending on the compression, full miscibility of the cholesteryl CDs and the phospholipids is observed at low surface pressures and a clear demixing tendency is apparent during compression. CD-modified bilayers are stable and are subject to a gel–liquid phase transition upon heating. Due to their bulky CD moiety, the amphiphilic CDs exhibit a distinct fluidizing effect, shifting the DPPC's gel–liquid transition. The structure of the mixed TASC/DPPC mono- and bi-layers perpendicular to the surface is investigated with Angstrom resolution by neutron reflectivity. In this way a molecular model of the insertion has been established, which suggests that the CD cavities partly protrude into the subphase, which should leave them accessible for complex formation.

Lipid-Protein Interactions in Nanodiscs: How to Enhance Stability

Lipid-protein interactions can function as “co-factors” that affect the properties / function of transmembrane proteins. Herein, the interaction between anionic dimyristoylphosphatidylglycerol (DMPG) and zwitterionic dimyristoylphosphatidylcholine (DMPC) with the amphiphatic membrane scaffold protein (MSP), were studied. Two 25 kDa MSP wrap around the circumference of discoidal bilayer in a belt-like manner to form a nanodisc [1,2]. The membrane-like structure of nanodiscs has been used for reconstitution of membrane proteins in a native-like environment. Differential scanning calorimetry was employed to characterize lipid-protein interactions in these particles by evaluating changes in MSP denaturation temperature and lipid gel-liquid phase transition as a function of nanodisc lipid composition and ionic strength. Small-angle X-ray scattering and size-exclusion chromatography were used to determine the overall structure of the nanodisc. We suggest the nanodisc lipid is divided into a lipid rim that interacts with the internal face of the MSP helical segments, while the centrally located nanodisc lipids maintain a more bulk-like lipid behavior. This finding is important for reconstitution of membrane proteins since the presence of a ‘lipid rim’ serves to prevent contact between the membrane protein and the MSP. Furthermore, the presence of two distinct lipid environments reduces the available area for reconstituted membrane proteins in the nanodisc. We also show that the negatively charged DMPG has a higher preference for the rim due to its negatively charged headgroup. Finally, we conclude that DMPG stabilizes the nanodisc in a twofold manner: i) DMPG ’freezes’ the MSP conformation preventing flexibility / dissociation that may lead to aggregation. ii) DMPG also contributes to prevention of aggregation due to electrostatic repulsion between the negatively charged lipids on neighboring nanodiscs.[1] T.H. Bayburt et al.: Nano Letters 2 (2002) 853.[2] N. Skar-Gislinge and J.B. Simonsen et al.: JACS 132 (2010) 13713.

Ion-channel-like behavior in lipid bilayer membranes at the melting transition

It is well known that at the gel-liquid phase-transition temperature a lipid bilayer membrane exhibits an increased ion permeability. We analyze the quantized currents in which the increased permeability presents itself. The open time histogram shows a "-3/2" power law which implies an open-closed transition rate that decreases like k(t)∝t(-1) as time evolves. We propose a "pore freezing" model to explain the observations. We discuss how this model also leads to the 1/fα noise that is commonly observed in currents across biological and artificial membranes.

Temperature Dependence of the Interaction of Antimicrobial Peptides With Mixed Lipid Bilayers

The interactions of two α-helical antimicrobial peptides, aurein 1.2 (13 residues) and maculatin 1.1 (21 residues), with model membranes have been examined using solid-state NMR and surface plasmon resonance techniques. P-31 NMR of multilamellar (MLV) dimyristoylphosphatidycholine (DMPC) vesicles with aurein 1.2 revealed minor disruptions in the bilayer above the gel-liquid phase transition. However, below the phase transition temperature an isotropic signal was observed, indicating that the peptide disrupted the bilayer and formed small, rapidly tumbling aggregates ∼ 22 nm in diameter as determined by light scattering measurements. However, the isotropic signal was not seen with the longer peptide. Additional experiments conducted using different lipid compositions revealed that both fluidity and temperature influence the peptide interaction. Gel phase lipid bilayers were more strongly affected by the peptide although similar effects were observed at lower temperatures in unsaturated chain lipid bilayers in the liquid crystalline state.A preliminary study on membranes mimicking the lipid composition of S. aureus has demonstrated a disruptive effect on the bilayer organization by addition of maculatin 1.1, a potent antibacterial peptide. As revealed in P-31 static NMR spectra of MLV composed of dimyristoylphosphatidylglycerol (DMPG) and tetramyristoylcardiolipin (TMCL), the peptide promoted formation of a dominant isotropic phase at 15°C, well below the liquid-crystalline transition temperature; while the lamellar organization was mainly restored above 50°C and an intermediate state was observed at 30°C. Interestingly, relaxation experiments on MLV without peptide indicated coexistence of two populations in the temperature range 30-50°C, most likely composed of fluid DMPG and rigid TMCL. The antimicrobial peptide may insert preferentially at domain boundaries, using defects in membrane packing to lower energy costs. Further experiments are ongoing to determine the nature of the isotropic phase and its relevance to antimicrobial activity.

Protein–membrane interaction: effect of myelin basic protein on the dynamics of oriented lipids

We have studied the effect of physiological amounts of myelin basic protein (MBP) on pure dimyristoyl l-α-phosphatidic acid (DMPA) oriented membranes. The investigation has been carried out using several complementary experimental methods to provide a detailed characterization of the proteo-lipid complexes. In particular, taking advantage of the power of the quasi-elastic neutron scattering (QENS) technique as optimal probe in biology, a significant effect is suggested to be induced by MBP on the anisotropy of lipid dynamics across the liquid–gel phase transition. Thus, the enhancement of the spatially restricted, vertical translation motion of DMPA is suggested to be the main responsible for the increased contribution of the out of plane lipid dynamics observed at 340 K.

205 液体-ゲル相転移を持つ液膜の形状変動相転移に関する研究

205 液体-ゲル相転移を持つ液膜の形状変動相転移に関する研究

1128 脂質 2 重膜の液体-ゲル相転移に対する流動性と形状変動の影響に関する研究

The liquid-gel phase transition of bilayer lipid membranes (BLM) is known to be of the first order and closely related with internal degrees of freedom of the amphiphilic molecules. It is also well known that the Potts model undergoes a first order phase transition. Therefore we study the liquid-gel phase transition of the BLM by 10-states Potts model on triangulated surfaces by Monte Carlo simulations. We find that there are non-trivial effects of both fluidity and shape fluctuations on the liquid-gel phase transition and moreover that this first order phase transition is altered to a second order one.

The role of chromophore in the lipid–protein interactions in bacteriorhodopsin–phosphatidylcholine vesicles

By fluorescence and phase properties of a 1-acyl-2-[8-(2-anthroyl)-octanoyl]- sn-glycero-3-phosphocholine probe, the influence of the chromophore on the phase transition of bacteriorhodopsin–lipid vesicles was investigated. It was observed that removal of the chromophore led to the down-shifting of the phase transition temperatures. The temperatures corresponding to the beginning and ending of the gel–liquid phase transition were also influenced. This demonstrated that the liquid phase is reached more easily when the chromophore is bleached. The results indicate that removal of the chromophore alters the protein–lipid interactions. It is suggested that this alteration might be related to the change in the lipid molecular packing.

Effect of temperature and salinity stress on growth and lipid composition of Shewanella gelidimarina.

The maximum growth temperature, the optimal growth temperature, and the estimated normal physiological range for growth of Shewanella gelidimarina are functions of water activity (a(w)), which can be manipulated by changing the concentration of sodium chloride. The growth temperatures at the boundaries of the normal physiological range for growth were characterized by increased variability in fatty acid composition. Under hyper- and hypoosmotic stress conditions at an a(w) of 0.993 (1.0% [wt/vol] NaCl) and at an a(w) of 0.977 (4.0% [wt/vol] NaCl) the proportion of certain fatty acids (monounsaturated and branched-chain fatty acids) was highly regulated and was inversely related to the growth rate over the entire temperature range. The physical states of lipids extracted from samples grown at stressful a(w) values at the boundaries of the normal physiological range exhibited no abrupt gel-liquid phase transitions when the lipids were analyzed as liposomes. Lipid packing and adaptational fatty acid composition responses are clearly influenced by differences in the temperature-salinity regime, which are reflected in overall cell function characteristics, such as the growth rate and the normal physiological range for growth.