Diffusion measurements in oriented phospholipid bilayers by 1H-NMR in a static fringe field gradient

Molecular dynamics insights on temperature and pressure effects on electroporation

Electroporation is a cell-level phenomenon caused by an ionic imbalance in the membrane, being of great relevance in various fields of knowledge. A dependence of the pore formation kinetics on the environmental conditions (temperature and pressure) of the cell membrane has already been reported, but further clarification regarding how these variables affect the pore formation/resealing dynamics and the transport of molecules through the membrane is still lacking. The objective of the present study was to investigate the temperature (288-348 K) and pressure (1-5000 atm) effects on the electroporation kinetics using coarse-grained molecular dynamics simulations. Results shown that the time for pore formation and resealing increased with pressure and decreased with temperature, whereas the maximum pore radius increased with temperature and decreased with pressure. This behavior influenced the ion migration through the bilayer, and the higher ionic mobility was obtained in the 288 K/1000 atm simulations, i.e., a combination of low temperature and (not excessively) high pressure. These results were used to discuss some experimental observations regarding the extraction of intracellular compounds applying this technique. This study contributes to a better understanding of electroporation under different thermodynamic conditions and to an optimal selection of processing parameters in practical applications which exploit this phenomenon.

Attempts at the Characterization of In-Cell Biophysical Processes Non-Invasively-Quantitative NMR Diffusometry of a Model Cellular System

In the literature, diffusion studies of cell systems are usually limited to two water pools that are associated with the extracellular space and the entire interior of the cell. Therefore, the time-dependent diffusion coefficient contains information about the geometry of these two water regions and the water exchange through their boundary. This approach is due to the fact that most of these studies use pulse techniques and relatively low gradients, which prevents the achievement of high b-values. As a consequence, it is not possible to register the signal coming from proton populations with a very low bulk or apparent self-diffusion coefficient, such as cell organelles. The purpose of this work was to obtain information on the geometry and dynamics of water at a level lower than the cell size, i.e., in cellular structures, using the time-dependent diffusion coefficient method. The model of the cell system was made of baker’s yeast (Saccharomyces cerevisiae) since that is commonly available and well-characterized. We measured characteristic fresh yeast properties with the application of a compact Nuclear Magnetic Resonance (NMR)-Magritek Mobile Universal Surface Explorer (MoUSE) device with a very high, constant gradient (~24 T/m), which enabled us to obtain a sufficient stimulated echo attenuation even for very short diffusion times (0.2–40 ms) and to apply very short diffusion encoding times. In this work, due to a very large diffusion weighting (b-values), splitting the signal into three components was possible, among which one was associated only with cellular structures. Time-dependent diffusion coefficient analysis allowed us to determine the self-diffusion coefficients of extracellular fluid, cytoplasm and cellular organelles, as well as compartment sizes. Cellular organelles contributing to each compartment were identified based on the random walk simulations and approximate volumes of water pools calculated using theoretical sizes or molar fractions. Information about different cell structures is contained in different compartments depending on the diffusion regime, which is inherent in studies applying extremely high gradients.

Coupling of Membrane Nanodomain Formation and Enhanced Electroporation near Phase Transition

Biological cells are enveloped by a heterogeneous lipid bilayer that prevents the uncontrolled exchange of substances between the cell interior and its environment. In particular, membranes act as a continuous barrier for salt and macromolecules to ensure proper physiological functions within the cell. However, it has been shown that membrane permeability strongly depends on temperature and, for phospholipid bilayers, displays a maximum at the transition between the gel and fluid phase. Here, extensive molecular dynamics simulations of dipalmitoylphosphatidylcholine bilayers were employed to characterize the membrane structure and dynamics close to phase transition, as well as its stability with respect to an external electric field. Atomistic simulations revealed the dynamic appearance and disappearance of spatially related nanometer-sized thick ordered and thin interdigitating domains in a fluid-like bilayer close to the phase transition temperature (Tm). These structures likely represent metastable precursors of the ripple phase that vanished at increased temperatures. Similarly, a two-phase bilayer with coexisting gel and fluid domains featured a thickness minimum at the interface because of splaying and interdigitating lipids. For all systems, application of an external electric field revealed a reduced bilayer stability with respect to pore formation for temperatures close to Tm. Pore formation occurred exclusively in thin interdigitating membrane nanodomains. These findings provide a link between the increased membrane permeability and the structural heterogeneity close to phase transition.

Lipid Bilayers: The Effect of Force Field on Ordering and Dynamics

The sensitivity of the structure and dynamics of a fully hydrated pure bilayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in molecular dynamics simulations to changes in force-field and simulation parameters has been assessed. Three related force fields (the Gromos 54A7 force field, a Gromos 53A6-derived parameter set and a variant of the Berger parameters) in combination with either particle-mesh Ewald (PME) or a reaction field (RF) were compared. Structural properties such as the area per lipid, carbon-deuterium order parameters, electron density profile and bilayer thicknesses, are reproduced by all the parameter sets within the uncertainty of the available experimental data. However, there are clear differences in the ordering of the glycerol backbone and choline headgroup, and the orientation of the headgroup dipole. In some cases, the degree of ordering was reminiscent of a liquid-ordered phase. It is also shown that, although the lateral diffusion of the lipids in the plane of the bilayer is often used to validate lipid force fields, because of the uncertainty in the experimental measurements and the fact that the lateral diffusion is dependent on the choice of the simulation conditions, it should not be employed as a measure of quality. Finally, the simulations show that the effect of small changes in force-field parameters on the structure and dynamics of a bilayer is more significant than the treatment of the long-range electrostatic interactions using RF or PME. Overall, the Gromos 54A7 best reproduced the range of experimental data examined.

Cholesterol-dependent nanomechanical stability of phase-segregated multicomponent lipid bilayers

Cholesterol is involved in endocytosis, exocytosis, and the assembly of sphingolipid/cholesterol-enriched domains, as has been demonstrated in both model membranes and living cells. In this work, we explored the influence of different cholesterol levels (5-40 mol%) on the morphology and nanomechanical stability of phase-segregated lipid bilayers consisting of dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/SM/Chol) by means of atomic force microscopy (AFM) imaging and force mapping. Breakthrough forces were consistently higher in the SM/Chol-enriched liquid-ordered domains (Lo) than in the DOPC-enriched fluid-disordered phase (Ld) at a series of loading rates. We also report the activation energies (DeltaEa) for the formation of an AFM-tip-induced fracture, calculated by a model for the rupture of molecular thin films. The obtained DeltaEa values agree remarkably well with reported values for fusion-related processes using other techniques. Furthermore, we observed that within the Chol range studied, the lateral organization of bilayers can be categorized into three distinct groups. The results are rationalized by fracture nanomechanics of a ternary phospholipid/sphingolipid/cholesterol mixture using correlated AFM-based imaging and force mapping, which demonstrates the influence of a wide range of cholesterol content on the morphology and nanomechanical stability of model bilayers. This provides fundamental insights into the role of cholesterol in the formation and stability of sphingolipid/cholesterol-enriched domains, as well as in membrane fusion.

Lipid transfer between charged supported lipid bilayers and oppositely charged vesicles

The bidirectional transfer of phospholipids between a charged, supported lipid bilayer (SLB) on SiO(2) and oppositely charged, unilamellar vesicles was studied by means of quartz crystal microbalance with dissipation (QCM-D) and optical reflectometry techniques. SLBs and vesicles were prepared from binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) mixed with different fractions of either 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine] (POPS) (negatively charged) or 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (POEPC) (positively charged). The interaction process consists of an attachment-transfer-detachment (ATD) sequence, where added vesicles first attach to and interact with the SLB, after which they detach, leaving behind a compositionally modified SLB and ditto vesicles. When the process is complete, there is no net addition or reduction of total lipid mass in the SLB, but lipid exchange has occurred. The time scale of the process varies from a few to many tens of minutes depending on the type of charged lipid molecule and the relative concentration of charged lipids in the two membranes. Electrostatically symmetric cases, where only the charge sign (but not the fraction of charged lipid) was reversed between the SLB and the vesicles, produce qualitatively similar but quantitatively different kinetics. The time scale of the interaction varies significantly between the two cases, which is attributed to a combination of the differences in the molecular structure of the lipid headgroup for the positively and the negatively charged lipids used, and to nonsymmetric distribution of charged lipids in the lipid membranes. The maximum amounts of attached vesicles during the ATD process were estimated to be 25-40% of a full monolayer of vesicles, with the precise amount depending on the actual charge fractions in the vesicles and the SLB. Interrupted vesicle exposure experiments, and experiments where the bulk concentration of vesicles was varied, show that vesicles in some cases may be trapped irreversibly on the SLB, when only partial transfer of lipid molecules has occurred. Additional supply of vesicles and further transfer induces detachment, when a sufficient amount of oppositely charged lipids has been transferred to the SLB, so that the latter becomes repulsive to the attached vesicles. Possible mechanistic scenarios, including monomer insertion and hemifusion models, are discussed. The observed phenomena and the actual SLB preparation process form a platform both for studies of various intermembrane molecular transfer processes and for modifying the composition of SLBs in a controlled way, for example, for biosensor and cell culture applications.

Perturbation of DPPC/POPG bilayers by the N-terminal helix of lung surfactant protein SP-B: a (2)H NMR study

SP-B(8-25) is a synthetic peptide comprising the N-terminal helix of the essential lung surfactant protein SP-B. Rat lung oxygenation studies have shown that SP-B(8-25) retains some of the function of full-length SP-B. We have used deuterium nuclear magnetic resonance ((2)H-NMR) to examine the influence of SP-B(8-25) on the mixing properties of saturated PC and unsaturated PG lipids in model mixed lipid bilayers containing dipalmitoylphosphatidylcholine (DPPC) and palmitoyl-oleoyl-phosphatidylglycerol (POPG), in a molar ratio of 7:3. In the absence of the peptide, (2)H-NMR spectra of DPPC/POPG mixtures, with one or the other lipid component deuterated, indicate coexistence of large liquid crystal and gel domains over a range of about 10 degrees C through the liquid crystal to gel transition of the bilayer. Addition of SP-B(8-25) has little effect on the width of the transition but the spectra through the transition range cannot be resolved into distinct liquid crystal and gel spectral components suggesting that the peptide interferes with the tendency of the DPPC and POPG lipid components in this mixture to phase separate near the bilayer transition temperature. Quadrupole echo decay observations suggest that the peptide may also reduce differences in the correlation times for local reorientation of the two lipids. These observations suggest that SP-B(8-25) promotes a more thorough mixing of saturated PC and unsaturated PG components and may be relevant to understanding the behaviour of lung surfactant material under conditions of lateral compression which might be expected to enhance the propensity for saturated and unsaturated surfactant lipid components to segregate.

Diffusion in supported lipid bilayers: influence of substrate and preparation technique on the internal dynamics

The diffusion law of DMPC and DPPC in Supported Lipid Bilayers (SLB), on different substrates, has been investigated in details by Fluorescence Recovery After Patterned Photobleaching (FRAPP). Over micrometer length scales, we demonstrate the validity of a purely Brownian diffusive law both in the gel and the fluid phases of the lipids. Measuring the diffusion coefficient as a function of temperature, we characterize the gel-to-liquid phase transition of DMPC and DPPC. It is shown that, depending on the type of substrate and the method used for bilayer preparation, completely different behaviours can be observed. On glass substrates, using the Langmuir-Blodgett deposition technique, both leaflets of the bilayer have the same dynamics. On mica, the dynamics of the proximal leaflet is slower than the dynamics of the distal leaflet, although the transition temperature is the same for both layers. Preparing bilayers from vesicle fusion in same conditions leads to more random behaviours and shifted transition temperatures.

NMR investigation of the dynamics of banana shaped molecules in the isotropic phase: a comparison with calamitic mesogens behaviour

The first translational self-diffusion NMR measurements in the isotropic phase of banana-shaped liquid crystals are reported. In this paper, two banana-shaped mesogens, having a similar molecular structure and showing a nematic phase, have been investigated by means of translational self-diffusion NMR, (2)H NMR spin-spin and (1)H NMR spin-lattice relaxation measurements in the isotropic phase. While (1)H diffusion and (2)H relaxation times reveal a peculiar slow dynamic behaviour of banana-shaped mesogens compared with calamitic mesogens, the (1)H relaxation times seem to be affected by fast dynamics only. The origin of these dynamic features is discussed in terms of overall and internal molecular motions, in the frame of recent speculations concerning the formation of molecular clusters or aggregates in the isotropic phase of banana-shaped liquid crystals.

Diffusion measurements of water, ubiquinone and lipid bilayer inside a cylindrical nanoporous support: A stimulated echo pulsed-field gradient MAS-NMR investigation

Stimulated echo pulsed-field gradient 1H magic angle spinning NMR has been used to investigate the mobility of water, ubiquinone and tethered phospholipids, components of a biomimetic model membrane. The diffusion constant of water corresponds to an isotropic motion in a cylinder. When the lipid bilayer is obtained after the fusion of small unilamellar vesicles, the extracted value of lipid diffusion indicates unrestricted motion. The cylindrical arrangement of the lipids permits a simplification of data analysis since the normal bilayer is perpendicular to the gradient axis. This feature leads to a linear relation between the logarithm of the attenuation of the signal intensity and a factor depending on the gradient strength, for lipids covering the inner wall of aluminium oxide nanopores as well as for lipids adsorbed on a polymer sheet rolled into a cylinder. The effect of the bilayer formation on water diffusion has also been observed. The lateral diffusion coefficient of ubiquinone is in the same order of magnitude as the lipid lateral diffusion coefficient, in agreement with its localization within the bilayer.

A study of anisotropic water self-diffusion and defects in the lamellar mesophase

The correlation of molecular diffusion coefficients obtained via a novel two-dimensional pulsed gradient spin-echo (PGSE) NMR method has been shown to reveal detailed structural information on the mesophases of lyotropic liquid crystals. A four-component system containing both nonionic (pentaethylene glycol monododecyl ether) and ionic (sodium dodecyl sulfate) surfactants, water, and decane was prepared and left to equilibrate. In the temperature region around 309 K, a lamellar mesophase forms. A two-dimensional Laplace inverse transformation was performed on the (gammadeltag)2(delta - delta/3) domain data to separate any multiexponential behavior that resulted from local anisotropy. The results of the double PGSE experiment with contiguous gradient pulse pairs, applied both collinearly and orthogonally, clearly show the presence of local anisotropic self-diffusion of the water molecules and suggest a preferred orientation of the lamellae. Information about defects/domain size was obtained by the insertion of a mixing time (t(m)') between the successive gradient pulse pairs. This work highlights the value of this new NMR correlation method in the study of surfactant systems.

Rupture of molecular thin films observed in atomic force microscopy. II. Experiment

In atomic force microscope studies of thin films often a defined jump of the tip through the film is observed once a certain threshold force has been exceeded. In particular, on lipid bilayers this is regularly observed. In a previous paper [H.-J. Butt and V. Franz, Phys. Rev. E 66, 031601 (2002)] we presented two complementary models to describe film rupture. The aim of this study was to verify these models. Experiments were done with solid supported bilayers consisting of dioleoyloxypropyl-trimethylammonium chloride (DOTAP) and dioleoylphosphatidylserine (DOPS) in aqueous solutions and with propanol. Both models describe experimental results adequately. In particular, a narrow distribution of yield forces and an increase of the mean yield force with increasing loading rate is correctly predicted. For the lipid bilayers spreading pressures of roughly 20 mN/m (DOTAP) and 5 mN/m (DOPS) were measured. Line tensions for the edge of a lipid bilayer ranged between 3 (DOTAP) and 6 pN (DOPS).

Low-frequency collective chain dynamics in a model biomembrane

Proton NMR was employed as a probe for the collective hydrocarbon chain dynamics in decylammonium chloride (C10H21NH3Cl), a model biomembrane undergoing an irreversible structural phase transition sequence. Our rotating frame spin-lattice relaxation measurements revealed a low-frequency critical collective chain dynamics in the kHz regime, which is associated with the interdigitated to noninterdigitated chain configurational phase transition.

Modeling the static fringe field of superconducting magnets

The resonance frequency-space and the frequency gradient-space relations are evaluated analytically for the static fringe magnetic field of superconducting magnets used in the NMR diffusion measurements. The model takes into account the actual design of the high-homogeneity magnet coil system that consists of the main coil and the cryoshim coils and enables a precise calibration of the on-axis magnetic field gradient and the resonance frequency inside and outside of the superconducting coil.

NMR study of translational and rotational dynamics in monoolein-water mesophases: obstruction and hydration effects

Using a variety of NMR methods (magnetic field gradient echo decays, 2H one- and two-dimensional spectra, spin-lattice relaxation rates), both translational and rotational dynamics of the constituents of monoolein MO/H(2)O mesophases have been studied. The experiments lead to the following conclusions. The translational dynamics of the lipid molecules is essentially dominated by obstruction effects due to the topologies of the diverse mesophases. On the other hand, water dynamics-in the regime of small water concentrations-is strongly influenced by hydration of the lipid head groups. Hydration is seen in diffusion data, in spectra and in spin-lattice relaxation of the water molecules. This work represents an involved extension of our recently published work [Chem. Phys. Lipids 106, 115 (2000)].

Relating structure and translational dynamics in aqueous dispersions of monoolein

The temperature dependence of the molecular diffusion in monoolein/water systems is investigated at several levels of hydration. Using the proton/deuteron selectivity, field gradient NMR allows the simultaneous determination of the diffusion constants of both, lipid and water molecules in the various lamellar and non-lamellar phases. Due to the mesoscopic structure of the monoolein/water phases, the diffusion coefficients are interpreted as 'reduced' or 'effective' diffusion coefficients, and are related to the microscopic molecular displacements by a so-called 'obstruction factor'. Changes in the microscopic structure at the phase transition from the bicontinuous cubic phases to the inverse hexagonal phase are reflected in the obstruction factor of the monoolein diffusion coefficients. The reduction of the water diffusion coefficients is too high to be explained by an obstruction factor only, implying a mechanism of molecular motion, which strongly differs from that of bulk water. Experiments on samples prepared with isotopic labeled water (2H(2)O and H(2)(17)O) indicate a chemical exchange of protons between the water molecules and the lipid headgroups on a millisecond timescale.

Measurement of the lateral diffusion of dipalmitoylphosphatidylcholine adsorbed on silica beads in the absence and presence of melittin: a 31P two-dimensional exchange solid-state NMR study

31P two-dimensional exchange solid-state NMR spectroscopy was used to measure the lateral diffusion, D(L), in the fluid phase of dipalmitoylphosphatidylcholine (DPPC) in the presence and absence of melittin. The use of a spherical solid support with a radius of 320 +/- 20 nm, on which lipids and peptides are adsorbed together, and a novel way of analyzing the two-dimensional exchange patterns afforded a narrow distribution of D(L) centered at a value of (8.8 +/- 0.5) x 10(-8) cm2/s for the pure lipid system and a large distribution of D(L) spanning 1 x 10(-8) to 10 x 10(-8) cm2/s for the lipids in the presence of melittin. In addition, the determination of D(L) for nonsupported DPPC multilamellar vesicles (MLVs) suggests that the support does not slow down the lipid diffusion and that the radii of the bilayers vary from 300 to 800 nm. Finally, the DPPC-melittin complex is stabilized at the surface of the silica beads in the gel phase, opening the way to further study of the interaction between melittin and DPPC.