Acyl chain orientational order in large unilamellar vesicles: comparison with multilamellar liposomes: a 2H and 31P nuclear magnetic resonance study

The effect of cholesterol on highly curved membranes measured by nanosecond Fluorescence Correlation Spectroscopy

Small lipid vesicles (with diameter ≤ 100nm) with their highly curved membranes comprise a special class of biological lipid bilayers. The mechanical properties of such membranes are critical for their function, e.g. exocytosis. Cholesterol is a near-universal regulator of membrane properties in animal cells. Yet measurements of the effect of cholesterol on the mechanical properties of membranes have remained challenging, and the interpretation of such measurements has remained a matter of debate. Here we show that nanosecond fluorescence correlation spectroscopy (FCS) can directly measure the ns-microsecond rotational correlation time (τr) of a lipid probe in high curvature vesicles with extraordinary sensitivity. Using a home-built 4-Pi fluorescence cross-correlation spectrometer containing polarization-modulating elements, we measure the rotational correlation time (τr) of Nile Red in neurotransmitter vesicle mimics. As the cholesterol mole fraction increases from 0 to 50 %, τr increases from 17 ± 1 to 112 ± 12 ns, indicating a viscosity change of nearly a factor of 7. These measurements are corroborated by solid-state NMR results, which show that the order parameter of the lipid acyl chains increases by about 50% for the same change in cholesterol concentration. Additionally, we measured the spectral parameters of polarity-sensitive fluorescence dyes, which provide an indirect measure of viscosity. The green/red ratio of Nile Red and the generalized polarization of Laurdan show consistent increases of 1.3x and 2.6x, respectively. Our results demonstrate that rotational FCS can directly measure the viscosity of highly curved membranes with higher sensitivity and wider dynamic range compared to other conventional techniques. Significantly, we observe that the viscosity of neurotransmitter vesicle mimics is remarkably sensitive to their cholesterol content.

Molecular Structure of Sphingomyelin in Fluid Phase Bilayers Determined by the Joint Analysis of Small-Angle Neutron and X-ray Scattering Data

We have determined the fluid bilayer structure of palmitoyl sphingomyelin (PSM) and stearoyl sphingomyelin (SSM) by simultaneously analyzing small-angle neutron and X-ray scattering data. Using a newly developed scattering density profile (SDP) model for sphingomyelin lipids, we report structural parameters including the area per lipid, total bilayer thickness, and hydrocarbon thickness, in addition to lipid volumes determined by densitometry. Unconstrained all-atom simulations of PSM bilayers at 55 °C using the C36 CHARMM force field produced a lipid area of 56 Ų, a value that is 10% lower than the one determined experimentally by SDP analysis (61.9 Ų). Furthermore, scattering form factors calculated from the unconstrained simulations were in poor agreement with experimental form factors, even though segmental order parameter (S_CD) profiles calculated from the simulations were in relatively good agreement with S_CD profiles obtained from NMR experiments. Conversely, constrained area simulations at 61.9 Ų resulted in good agreement between the simulation and experimental scattering form factors, but not with S_CD profiles from NMR. We discuss possible reasons for the discrepancies between these two types of data that are frequently used as validation metrics for molecular dynamics force fields.

On the characterization of pH-sensitive liposome/polymer complexes

A randomly alkylated copolymer of N-isopropylacrylamide, methacrylic acid and N-vinyl-2-pyrrolidone was characterized with regard to its pH- and temperature-triggered conformational change. It was then complexed to liposomes to produce pH-responsive vesicles. Light scattering and differential scanning calorimetry experiments performed at neutral pH revealed that the polymer underwent coil-to-globule phase transition over a wide range of temperatures. At 37 degrees C and pH 7.4, although the polymer was water-soluble, Fourier transform infrared spectroscopy analysis showed that it was partly dehydrated. At acidic pH, the decrease in the lower critical solution temperature was accompanied by an increase in cooperativity degree of the phase transition. Complexation of copolymer to liposomes did not substantially influence its phase transition. The liposome/copolymer complexes were stable at neutral pH but rapidly released their contents under acidic conditions. The copolymer slightly increased liposome circulation time following intravenous administration to rats. The addition of poly(ethylene glycol) to the formulation had a detrimental effect on pH-sensitivity but enhanced substantially the circulation time.

Asymmetrical membranes and surface tension

The (31)P-nuclear magnetic resonance chemical shift of phosphatidic acid in a membrane is sensitive to the lipid head group packing and can report qualitatively on membrane lateral compression near the aqueous interface. We have used high-resolution (31)P-nuclear magnetic resonance to evaluate the lateral compression on each side of asymmetrical lipid vesicles. When monooleoylphosphatidylcholine was added to the external monolayer of sonicated vesicles containing dioleoylphosphatidylcholine and dioleoylphosphatidic acid, the variation of (31)P chemical shift of phosphatidic acid indicated a lateral compression in the external monolayer. Simultaneously, a slight dilation was observed in the inner monolayer. In large unilamellar vesicles on the other hand the lateral pressure increased in both monolayers after asymmetrical insertion of monooleoylphosphatidylcholine. This can be explained by assuming that when monooleoylphosphatidylcholine is added to large unilamellar vesicles, the membrane bends until the strain is the same in both monolayers. In the case of sonicated vesicles, a change of curvature is not possible, and therefore differential packing in the two layers remains. We infer that a variation of lipid asymmetry by generating a lateral strain in the membrane can be a physiological way of modulating the conformation of membrane proteins.

Characterization of permeability and morphological perturbations induced by nisin on phosphatidylcholine membranes

Nisin is an antimicrobial peptide used as food preservative. To gain some insights into the hypothesis that its bactericidal activity is due to the perturbation of the lipid fraction of the bacterial plasmic membrane, we have investigated the effect of nisin on model phosphatidylcholine (PC) membranes. We show that nisin affects the PC membrane permeability, and this perturbation is modulated by the lipid composition. Nisin-induced leakage from PC vesicles is inhibited by the presence of cholesterol. This inhibition is associated with the formation of a liquid ordered phase in the presence of cholesterol, which most likely reduces nisin affinity for the membrane. Conversely, phosphatidylglycerol (PG), an anionic lipid, promotes nisin-induced leakage, and this promotion is associated with an increased affinity of the peptide for the bilayer because nisin is a cationic peptide. When the electrostatic interactions are encouraged by the presence of 70 mol% PG in PC, the inhibitory effect of cholesterol is not observed anymore. Nisin drastically modifies the morphology of the dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multilamellar dispersion without causing a significant change in the gel-to-liquid crystalline phase transition of the lipid. The morphological changes are observed from (31)P and (2)H NMR and cryo-electron microscopy. From the NMR point of view, the interactions giving rise to a broad signal (quadrupolar interactions and chemical shift anisotropy for (2)H NMR and (31)P NMR, respectively) are partly averaged out in the presence of nisin. This phenomenon is interpreted by the formation of curved lipid planes that lead to the lipid lateral diffusion occurring in the intermediate motional regime. By cryo-electron microscopy, large amorphous aggregates containing small dense globular particles are observed for samples quenched from 25 and 50 degrees C. Long thread-like structures are also observed in the fluid phase. A structural description of DPPC/nisin complex, consistent with the experimental observation, is proposed. The presence of 30 mol% cholesterol in DPPC completely inhibits the morphological changes induced by nisin. Therefore, it is concluded that nisin can significantly perturb PC bilayers from both the permeability and the structural points of view, and these perturbations are modulated by the lipidic species in the bilayer.

Spontaneous liposome formation induced by grafted poly(ethylene oxide) layers: theoretical prediction and experimental verification

Spontaneous liposome formation is predicted in binary mixtures of fluid phase phospholipids and poly(n)ethylene oxide (PEO)-bearing lipids by using single chain mean field theory. The range of stability of the spontaneous liposomes is determined as a function of percentage of PEO-conjugated lipids and polymer molecular weight. These predictions were tested by using cast films of 1, 2-diacyl-sn-glycerophosphocholines (e.g., egg L-alpha-lecithin, 1, 2-dimyristoyl-sn-glycero-3-phosphocholine, 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine, and 1, 2-distearoyl-sn-glycero-3-phosphocholine) and 1, 2-dipalmitoyl-sn-glycerophosphatidylethanolamine-PEO conjugates (i.e. , 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxypoly(e thylen e glycol)2000]carboxamide and 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxypoly(e thylen e oxide)5000]carboxamide) that were hydrated above their gel-liquid crystal phase transition temperatures. Particle sizes of the resulting dispersions, analyzed by quasielastic light scattering, solute retention, 31P NMR, and freeze-fracture electron microscopy measurements, confirmed the single chain mean field predictions. These data indicate that thermodynamically stable, unilamellar liposomes are formed spontaneously by simple hydration of fluid phase phospholipid bilayer films containing low molar ratios of PEO-based amphiphiles. They further suggest that the equilibrium size and colloidal properties of fluid phase, PEO-modified liposomes can be predicted by using this theoretical approach. The implication of these results on the design and processing of sterically stabilized liposomes used in drug delivery applications also is described.

Water permeability of polyunsaturated lipid membranes measured by 17O NMR

Diffusion-controlled water permeation across bilayers of polyunsaturated phospholipids was measured by 17O nuclear magnetic resonance. In 100-nm extruded liposomes containing 50 mM MnCl2, water exchange between internal and external solutions was monitored via changes in the linewidth of the 17O water resonance of external water. Liposome size and shape were characterized by light scattering methods and determination of liposome trapped volume. At 25 degrees C, the following water permeability coefficients were determined: 18:0-18:1n-9 PC, 155 +/- 24 microns/s; 18:0-18:3n-3 PC, 330 +/- 88 microns/s; and 18:0-22:6n-3 PC, 412 +/- 91 microns/s. The addition of 1 M ethanol reduced permeability coefficients to 66 +/- 15 microns/s for 18:0-18:1n-9 PC and to 239 +/- 67 microns/s for 18:0-22:6n-3 PC. Furthermore, the addition of 50 mol% 18:1n-9-18:1n-9 PE reduced the water permeability from 122 +/- 21 microns/s for pure 18:1n-9-18:1n-9 PC to 74 +/- 15 microns/s for the mixture. The significant increase in water permeation for membranes with polyunsaturated hydrocarbon chains correlates with looser packing of polyunsaturated lipids at the lipid-water interface and the suggested deeper penetration of water into these bilayers. Ethanol may block water diffusion pathways by occupying points of water entry into bilayers at the interface. The addition of dioleoylphosphatidylethanolamine increases lipid packing density and, consequently, reduces permeation rates.

Permeability of acetic acid across gel and liquid-crystalline lipid bilayers conforms to free-surface-area theory

Solubility-diffusion theory, which treats the lipid bilayer membrane as a bulk lipid solvent into which permeants must partition and diffuse across, fails to account for the effects of lipid bilayer chain order on the permeability coefficient of any given permeant. This study addresses the scaling factor that must be applied to predictions from solubility-diffusion theory to correct for chain ordering. The effects of bilayer chemical composition, temperature, and phase structure on the permeability coefficient (Pm) of acetic acid were investigated in large unilamellar vesicles by a combined method of NMR line broadening and dynamic light scattering. Permeability values were obtained in distearoylphosphatidylcholine, dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dilauroylphosphatidylcholine bilayers, and their mixtures with cholesterol, at various temperatures both above and below the gel-->liquid-crystalline phase transition temperatures (Tm). A new scaling factor, the permeability decrement f, is introduced to account for the decrease in permeability coefficient from that predicted by solubility-diffusion theory owing to chain ordering in lipid bilayers. Values of f were obtained by division of the observed Pm by the permeability coefficient predicted from a bulk solubility-diffusion model. In liquid-crystalline phases, a strong correlation (r = 0.94) between f and the normalized surface density sigma was obtained: in f = 5.3 - 10.6 sigma. Activation energies (Ea) for the permeability of acetic acid decreased with decreasing phospholipid chain length and correlated with the sensitivity of chain ordering to temperature, [symbol: see text] sigma/[symbol: see text](1/T), as chain length was varied. Pm values decreased abruptly at temperatures below the main phase transition temperatures in pure dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine bilayers (30-60-fold) and below the pretransition in dipalmitoylphosphatidylcholine bilayers (8-fold), and the linear relationship between in f and sigma established for liquid-crystalline bilayers was no longer followed. However, in both gel and liquid-crystalline phases in f was found to exhibit an inverse correlation with free surface area (in f = -0.31 - 29.1/af, where af is the average free area (in square angstroms) per lipid molecule). Thus, the lipid bilayer permeability of acetic acid can be predicted from the relevant chain-packing properties in the bilayer (free surface area), regardless of whether chain ordering is varied by changes in temperature, lipid chain length, cholesterol concentration, or bilayer phase structure, provided that temperature effects on permeant dehydration and diffusion and the chain-length effects on bilayer barrier thickness are properly taken into account.

Phosphatidylethanol as a 13C-NMR probe for reporting packing constraints in phospholipid membranes

13CH2-ethyl labeled phosphatidylethanol (PEth), a rare naturally occurring anionic phospholipid, was used to probe the interleaflet packing density difference in small and large unilamellar phospholipid vesicles (SUVs and LUVs, respectively). The intrinsically tighter lipid packing in the inner leaflet of the SUVs resulted in the splitting of the CH2-ethyl 13C-resonance into two distinct components originating from PEth molecules residing in the inner and outer leaflets. The splitting of the 13C-NMR signal from the PEth headgroup appears to be unique among naturally occurring phospholipids. We present data suggesting that the splitting of the PEth signal reports on transleaflet packing density difference modulated by unequal electrostatic interactions and structured water on the inner and outer surfaces of the SUV. The PEth resonance splitting was insensitive to pH changes over the range 5.3-8.6 and cannot be accounted for by differences in the pKa of PEth in the inner and outer monolayers of the SUV. In 13C-NMR spectra of LUVs, where packing constraints in both monolayers are approximately similar, only a single, narrow symmetrical CH2-ethyl signal was observed, which was shifted downfield at higher PEth concentrations. The carbonyl and C3-glycerol backbone PEth resonances were shifted upfield compared to those of phosphatidylcholine or phosphatidylglycerol, suggesting a more tightly packed/hydrophobic environment for these segments of the PEth molecule in the membrane. We conclude that the unique splitting of the PEth 13C-resonance reported here can be used to characterize the lipid packing conditions in various membranes and to monitor the transbilayer distribution/movement of PEth.

Effect of lipid molecular structure and gramicidin A on the core of lipid vesicle bilayers. A time-resolved fluorescence depolarization study

We have investigated the molecular orientational order and reorientational dynamics of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in the core of the membrane bilayer. Vesicles of lipids of varying unsaturation and headgroup (POPC, DOPC, DLPC, DLLPC, EGGPG, DOPG, DGDG, and SQDG) were studied using the time-resolved fluorescence anisotropy of DPH. Generally, values of the second order parameter <P2> for DPH are found to be very small. However, this should not be interpreted as DPH having low orientational order as witnessed by large values of the next relevant order parameter <P4>. This implies considerable transverse populations of DPH molecules within the bilayer. In phosphatidylcholines with an acyl chain of 18 carbon atoms, the value of <P2> for DPH decreases with increasing lipid unsaturation and even attains negative values. No effect of the lipid headgroup on the order and dynamics of DPH is detected. Furthermore, we study the peptide-lipid interaction of the hydrophobic antibiotic gramicidin A (gA) in DOPC vesicles using DPH. The nonchannel conformation has an ordering effect on DPH in the bilayer core, which the channel confirmation lacks. This can be understood in terms of the geometrical shape of the gA dimer, as shown previously with the probes TMA-DPH and DPHPC [Muller, J. M., et al. (1995) Biochemistry 34, 3092]. We find that for DPH data the conventional Brownian rotational diffusion (BRD) model and the compound motion model (CMM) give equivalent fits. In this respect, DPH differs from TMA-DPH and DPHPC, for which probes only the CMM allowed a consistent interpretation of the molecular orientation.

Phospholipid surface density determines the partitioning and permeability of acetic acid in DMPC:cholesterol bilayers

Relationships between the permeability coefficient (PHA) and partition coefficient (K m/w) of acetic acid and the surface density of DMPC:cholesterol bilayers have been investigated. Permeability coefficients were measured in large unilamellar vesicles by NMR line broadening. Bilayer surface density, sigma, was varied over a range of 0.5-0.9 by changing cholesterol concentration and temperature. The temperature dependence of PHA for acetic acid exhibits Arrhenius behavior with an average apparent activation energy (Ea) of 22 +/- 3 kcal/mole over a cholesterol mole fraction range of 0.00-0.40. This value is much greater than the enthalpy change for acetic acid partitioning between bulk decane and water (delta H degree = 4.8 +/- 0.8 kcal/mole) and the calculated Ea (= 8.0 kcal/mole) assuming a "bulk phase" permeability model which includes the enthalpy of transfer from water to decane and the temperature dependence of acetic acid's diffusion coefficient in decane. These results suggest that dehydration, previously considered to be a dominant component, is a minor factor in determining Ea. Values of 1n PHA decrease linearly with the normalized phospholipid surface density with a slope of kappa = -12.4 +/- 1.1 (r = 0.90). Correction of PHA for those temperature effects considered to be independent of lipid chain order (i.e., enthalpy of transfer from water to decane and activation energy for diffusion in bulk hydrocarbon) yielded an improved correlation (kappa = -11.7 +/- 0.5 (r = 0.96)). The temperature dependence of Km/w is substantially smaller than that for PHA and dependent on cholesterol composition. Values of 1n K m/w decrease linearly with the surface density with a slope of kappa = -4.6 +/- 0.3 (r = 0.95), which is 2.7-fold smaller than the slope of the plot of 1n PHA vs. sigma. Thus, chain ordering is a major determinant for molecular partitioning into and transport across lipid bilayers, regardless of whether it is varied by lipid composition or temperature.