Isothermal Compressibility Modulus Research Articles

Characterization of monolayers and liposomes that mimic lipid composition of HeLa cells

In this work, based on several studies, we develop an artificial lipid membrane to mimic the HeLa cell membrane using 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS) and cholesterol (CHOL). This is then a means to further study the fusion process of specific engineered liposomes. To characterize the mimicked HeLa cell membrane, we determined a series of surface pressure–area (π-A) isotherms and the isothermal compression modulus was calculated together with the dipole moment normal to the plane of the monolayer. The existence of laterally segregated domains was assessed using a fluorescence technique (Laurdan) and two microscopy techniques: Brewster angle microscopy (BAM) and atomic force microscopy (AFM) of Langmuir-Blodgett films (LBs) extracted at 30 mN m−1. To examine the nature and composition of the observed domains, force spectroscopy (FS) based on AFM was applied to the LBs. Finally, two engineered liposome formulations were tested in a fusion assay against mimicked HeLa cell membrane LBs, showing good results and thereby opening the door to further assays and uses.

Dynamic structure factor of a lipid bilayer in the presence of a high electric field.

The influence of a high average electric field (∼1 V/nm) in the hydrophobic interior of a bilayer lipid membrane on short-wavelength in-plane phononic motions of lipid chains is considered. The average electric field is assumed to be nearly constant on a picosecond time scale and a nanometer length scale. This field may be induced, for instance, by externally applied subnanosecond electric pulses or the membrane dipole potential. Using a generalized hydrodynamic approach, we derive a corresponding electrohydrodynamic model generalized to high wave numbers. In the considered approximation, all electric field effects are reduced only to a constant contribution to the generalized isothermal compressibility modulus. The corresponding dynamic structure factor for a lipid bilayer is derived. We show that due to polarization effects, the high field can critically impact the dynamics of longitudinal acousticlike modes at wave numbers near the major peak of the static structure factor. We estimate quantitatively that for typical lipid bilayers, transverse high electric fields can cause strong phonon energy softening, enhancement of phonon population, and formation of a gap in the dispersion of excitation frequency. The results obtained agree with simulations of the initiation of lipid bilayer electropores, suggesting that the proposed model reproduces the essential features of the field's impact on atomic density fluctuations. The proposed mechanism may have significant implications for the understanding of electroporation, passive molecular transport, and spontaneous pore formation in lipid bilayers.

Baric and dimensional changes of niobium properties

We have calculated the baric dependencies of thermophysical properties and melting temperature as well as the thermal equation of state for niobium based on the pair interatomic Mi-Lenard-Jones potential and the crystal Einstein model for niobium (Nb). Baric dependencies computations made along two isotherms 300 K and 3000 K are in good agreement with the experimental data for niobium. We have obtained the charts of pressure dependencies for the following properties: Debye temperature, the first, second and third Gruneisen parameters, isothermal compression modulus, isochoric and isobaric heat capacity, volumetric coefficient of thermal expansion and the melting temperature. The article investigates size dependencies of both specified properties and the melting temperature of niobium using an RP-model of nanocrystal.

Change in Thermophysical Properties and Melting Temperature of Niobium with Increasing Pressure

Based on the Mie–Lennard-Jones interatomic pair potential and the Einstein crystal model, the thermal equation of state and the pressure dependences of the thermophysical properties of niobium are obtained. The pressure dependences of the Debye temperature, the first, second, and third Gruneisen parameters, isothermal compression modulus, isochoric and isobaric heat capacity, thermal expansion coefficient, and the pressure derivatives of these parameters along the 300 and 3000 K isotherms are studied. The calculations showed good agreement with experimental data. Based on the results obtained, the pressure dependence of the melting point of niobium and its pressure derivative are calculated.

Effect of cholesterol on monolayer structure of different acyl chained phospholipids.

In this work we have investigated the effect of cholesterol (CHOL) in phospholipid monolayers on a series of phosphatidylcholines differing in acyl chain composition. We have used the CHOL proportion that abolishes the gel (Lβ)-to-liquid-crystalline (Lβ) transition in bilayers in order to investigate the mixing properties and laterally-segregated domains formed by specific phospholipid-CHOL ratios at the air-water interface. The binary monolayers where formed by mixing CHOL with 1,2-palmitoyl-sn-glycero-3-phos-phatidylcholine (DPPC);1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC); 1-pal-mitoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (PSPC); 1-palmitoyl-2-oleoyl-sn-gly-cero-3-phosphatidylcholine (POPC) and 1-palmitoyl-2-linoleyl-sn-glycero-3-phosphatidyl-choline (PLPC), respectively. From surface pressure-area (π-A) isotherms the isothermal compression modulus were calculated, and the mixing properties of the monolayers obtained by performing a basic surface thermodynamic analysis. From the excess Gibbs energy, the interaction parameter and the activity coefficients were also calculated. The study of the monolayers was complemented by determining the molecular dipole moment normal to the plane of the monolayer. The existence of laterally segregated domains was assessed by atomic force microscopy (AFM) of Langmuir-Blodgett films (LBs) extracted at 30 mNm-1. To get insight into the nature and composition of the observed domains force spectroscopy (FS) based on AFM was applied to the LBs.

Propofol modulates the lipid phase transition and localizes near the headgroup of membranes

The compound 2,6-diisopropylphenol (Propofol, PRF) is widely used for inducing general anesthesia, but the mechanism of PRF action remains relatively poorly understood at the molecular level. This work examines the possibility that a potential mode of action of PRF is to modulate the lipid order in target membranes. The effect on monolayers and bilayers of dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) was probed using Langmuir monolayer isotherms, differential scanning calorimetry (DSC), isothermal titration calorimetry (ITC) and molecular dynamics (MD) simulations. Increasing amounts of PRF in a DPPC monolayer causes a decrease in isothermal compressibility modulus at the phase transition. A partition constant for PRF in DPPC liposomes on the order of K≈1500M−1 was found, and the partitioning was found to be enthalpy-driven above the melting temperature (Tm). A decrease in Tm with PRF content was found whereas the bilayer melting enthalpy ΔHm remains almost constant. The last finding indicates that PRF incorporates into the membrane at a depth near the phosphatidylcholine headgroup, in agreement with our MD-simulations. The simulations also reveal that PRF partitions into the membrane on a timescale of 0.5μs and has a cholesterol-like ordering effect on DPPC in the fluid phase. The vertical location of the PRF binding site in a bacterial ligand-gated ion channel coincides with the location found in our MD-simulations. Our results suggest that multiple physicochemical mechanisms may determine anesthetic potency of PRF, including effects on proteins that are mediated through the bilayer.

Microscopic dynamics of nanoparticle monolayers at air–water interface

We present results of surface mechanical and particle tracking measurements of nanoparticles trapped at the air–water interface as a function of their areal density. We monitor both the surface pressure (Π) and isothermal compression modulus (ϵ) as well as the dynamics of nanoparticle clusters, using fluorescence confocal microscopy while they are compressed to very high density near the two dimensional close packing density Φ∼0.82. We observe non-monotonic variation in both ϵ and the dynamic heterogeneity, characterized by the dynamical susceptibility χ4 with Φ, in such high density monolayers. We provide insight into the underlying nature of such transitions in close packed high density nanoparticle monolayers in terms of the morphology and flexibility of these soft colloidal particles. We discuss the significance our results in the context of related studies on two dimensional granular or colloidal systems.

Morphological transitions in polymer monolayers under compression

We present a systematic investigation of morphological transitions in poly vinylacetate Langmuir monolayers. On compression, the polymer monolayer is converted to a continuous membrane with a thickness of approximately 2-3 nm. Above a certain surface concentration the monolayer, on water, undergoes a morphological transition-buckling, leading to formation of striped patterns of period of lambda(b) approximately 160 nm, as determined from in situ grazing incidence small angle x-ray scattering measurements. The obtained value is much smaller than what has been typically observed for Langmuir monolayers on water or thin films on soft substrates. Using existing theories for buckling of fluidlike films on fluid substrates, we obtain very low values of bending rigidity and Young's modulus of the polymer monolayer compared to that observed earlier for lipid or polymeric monolayers. Since buckling in these monolayers occurs only above a certain surface concentration, we have looked at the possibility that the buckling in these films occurs due to changes in their mechanical properties under compression. Using the model of Huang and Suo of buckling of solidlike films on viscoelastic substrates, we find values of the mechanical properties, which are much closer to the bulk values but still significantly lower. Although the reduction could be along the lines of what has been observed earlier for ultrathin polymer film or surface layers of polymers, the possibility of micromechanical effects also determining the buckling in such polymer monolayers cannot be ruled out. We have provided possible explanation of the buckling of the poly vinylacetate monolayers in terms of the change in isothermal compression modulus with surface concentration.

Effect of Unsaturated Lipid Chains on Dimensions, Molecular Order and Hydration of Membranes

We conducted infrared linear dichroism and X-ray measurements to characterize membranes of sn-1 chain perdeuterated, polyunsaturated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0−22:6ω3 PC, SDPC-d35) and of monounsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (16:0−18:1 PC, POPC-d31) as a function of hydration and temperature. Our novel approach is to use IR order parameters of C−H and C−D vibrations of both sn-1 and sn-2 chains for a separate estimation of the mean length and effective cross sectional area of the saturated and unsaturated chains in the bilayers. In the gel phase, the thickness of the hydrophobic core of SDPC bilayers exceeded the thickness of POPC bilayers, because the polyunsaturated chains adopted an extended conformation. However, in the fluid phase, the hydrophobic thickness of SDPC membranes, with the longer 18:0 and 22:6 acyl chains, was almost equal to the thickness of the 16:0−18:1 chains in POPC membranes. The isothermal compressibility modulus of the docosahexaenoyl chains was smaller than the modulus of the stearoyl chains of SDPC, whereas the palmitoyl and oleoyl chains of POPC are similarly compressible. Analysis in terms of a simple polymer brush model showed that unsaturation increases chain flexibility and chain mean area in lipid membranes. Temperature dependent measurements indicated that polyunsaturation decreased the sensitivity of the bilayer to temperature changes. The study of lipid hydration-sensitive spectral parameters suggested that this tendency may be at least partially explained by a weakening of water binding to polar moieties of polyunsaturated lipids with increasing temperature. Although the influence of hydrocarbon chain polyunsaturation on hydration of phosphate groups and the entire lipid was minor, we observed a modification of lipid carbonyl group hydration. Hydration properties and phase behavior of SDPC suggest an influence from polyunsaturation on the lateral pressure profile across the bilayer.

Mechano-chemistry of closed, vesicular membrane systems

The definition of a surface pressure in the encapsulating membrane surface of a large vesicle or biological cell cannot be stated in the same manner as for an insoluble monolayer, nor can a surface pressure be measured directly, as in a monolayer system. Our approach is to determine a suitable definition for surface pressure in closed membrane systems of large vesicles and biological cells; this surface pressure will represent an internal equation of state for the membrane that can be compared with the observed monolayer surface pressure versus area behavior. Using mechanical experiments to produce isotropic membrane tension and area expansion, changes in surface pressure can be determined as a function of area change. The relationship between isotropic tension and surface area is called the “mechano-chemical” equation of state. We directly relate the internal equation of state, surface pressure versus area, to the mechano-chemical equation of state, isotropic tension versus area. Using this relationship, the thermodynamic changes produced in a closed, vesicular (or cellular) membrane by isotropic dilation or reduction in surface area are investigated and described in terms of experimentally determinable variables. These variables include the isothermal compressibility modulus, K T , the thermal area expansivity at constant or zero isotropic tension, ( ∂α ∂T ) T-0 , the temperature gradient in the area compressibility modulus, ( dK T dT ) , and the specific heat at constant or zero isotropic tension, C T . In conjunction with known equations of state, the thermoelastic change provides direct assessment of the temperature dependence of the interfacial free-energy density of hydrophobic interaction. In the absence of external forces, the surface pressure is determined by the interfacial free-energy density of hydrophobic interaction.