The simulation of 31P NMR line shapes of lipid bilayers using an analytically soluble model

Van Faassen's method for obtaining an explicit solution to a first order stochastic differential equation is applied to the simulation of 31P NMR line shapes of unoriented phospholipid bilayers in the Lalpha phase and of oriented bilayers in both the Lalpha and Lbeta' phases. The effects of the two slowest motions on the density matrix are described by the stochastic Liouville equation (SLE) which is solved analytically using the method of van Faassen. These two slowest motions are assumed to be a rotational re-orientation about the long molecular axis and a uniform wobble of this axis within a conical volume with re-orientation rates characterised by correlation times tau(parallel) and tau(perpendicular) respectively. In the present work the Hamiltonian contains the intramolecular dipole-dipole interaction between the phosphorous nucleus and the four closest methylene protons of the choline headgroup, as well as the anisotropic chemical shielding interaction. Hence the contribution to relaxation from cross correlation between the dipole-dipole and anisotropic chemical shielding interactions is included. The reorientation of the headgroup is assumed to be a rotation sufficiently fast to lead to complete axially symmetric averaging of the Hamiltonian about the rotational axis (the P-O11 bond axis). Evaluation of the line shape in the present work involves only numerical integration and is therefore less computationally demanding than the large matrix inversions involved in the approaches of Campbell, Freed et al. The present theory also uses fewer parameters than that of Dufourc et al. but nevertheless results in good agreement with these authors' measurements on DMPC bilayers, using a fixed value of 10 for the ratio tau(perpendicular)/tau(parallel) in the case of the Lalpha phase. However, in contrast to Dufourc et al., we find that these correlation times are equal for the Lbeta' phase. Finally, we have simulated the decoupled powder line shapes obtained from the Lbeta' phase of DPPC by Campbell and Meirovitch. Again, we get good agreement providing tau(perpendicular)=tau(parallel).     

Distribution of merocyanine 540 in phospholipid membranes

The interaction of the fluorescent photosensitizer merocyanine 540 (MC-540) with model phospholipid membranes was studied. Two different-colored species (monomers and dimers) of MC-540 were registered in phospholipid liposomes. Variations in both phospholipid composition (DMPC, DPPC, POPC, egg PC) and temperature (15–60°C) resulted in changes in the MC-540 monomerdimer distribution. The values of the monomer-dimer equilibrium constant of MC-540 in egg PC (K=14.8 M), in POPC (K=26.7 M), and in DMPC (K=271.0 M) were determined at the temperature of 23±2°C. Suppression of MC-540 association with phospholipid bilayers was provoked by the addition of albumin to a liposome suspension. Albumin was observed to compete very successfully with lecithins containing saturated fatty acid chains (DPPC, DMPC), while only a weak competition of albumin with unsaturated lecithins (POPC, egg PC) for binding MC-540 molecules was registered.     

Investigating lipid interactions and the process of raft formation in cellular membranes using ToF-SIMS

There is an increased interest in how lipids interact with each other, especially in the lateral separation of lipids into coexisting liquid phases as this is believed to be an attribute of raft formation in cell membranes. ToF-SIMS has shown itself to be an excellent tool for investigating cellular and model membrane systems and will be perhaps the most powerful one for investigating raft formation. Results from our laboratory show the capability of ToF-SIMS at identifying unequivocally the content of coexisting liquid lipid phases. Using supported lipid monolayers we find that the inclusion of dipalmitoylphosphatidylethanolamine (DPPE) to a homogeneous dipalmitoyl-phosphatidylcholine (DPPC)/cholesterol phase results in the formation of cholesterol-rich domains [A.G. Sostarecz, C.M. McQuaw, A.G. Ewing, N. Winograd, J. Am. Chem. Soc. 126 (2004) 13882]. Also, for DPPE/cholesterol systems a single homogeneous DPPE/cholesterol phase is formed at ∼50 mol% cholesterol, whereas DPPC/cholesterol systems form a single phase at 30 mol% cholesterol [C.M. McQuaw, A. Sostarecz, L. Zheng, A.G. Ewing, N. Winograd, Langmuir 21 (2005) 807]. Currently we are exploring the incorporation of sphingomyelin into phospholipid-cholesterol mixtures in an effort to gain a better understanding of its role in raft formation.     

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.     

Temperature effect on thin lipid film elasticity and phase separation: insights from Langmuir monolayer and fluorescence microscopy techniques

Langmuir monolayer pressure isotherms and compressibility modulus measurements of phospholipid mixtures in several Langmuir monolayer systems at the air/water interface were investigated in this study. The ultimate aim was to carry out a comparison of the elasticity modulus for monolayers with different mixtures of l,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and chicken egg yolk sphingomyelin (eSM), in the presence/absence of cholesterol (Chol). In particular, we were able to propose that the leading force beyond the phase separation into liquid expanded (LE-) and liquid condensed (LC-) phases emerges from the increasing barrier to incorporate DOPC molecules into a highly ordered LC-phase. In addition, our findings suggest that DOPC lipid molecules have a priority to incorporate in a disordered LE-phase, while DPPC and eSM prefer the ordered one. Also, Chol seems to split almost equally into both phases, indicating that Chol has no priority for either phase and there are no particular interactions between Chol and saturated lipid molecules.     

NMR and FT-Raman Studies on the Interaction of Lanthanide Ions with Sphingomyelin Bilayers

The interactions of lanthanide ions with sphingomyelin bilayers have been studied by using 2D NOESY spectroscopy and FT-Raman spectroscopy methods. The results indicate that lanthanide ions, as well as divalent calcium, combine mainly to the phosphate group in the polar headgroup and do not change the conformation of O-C-C-N⁺ backbone in the choline group of sphingomyelin bilayers. The polar headgroup is still extending parallel to the bilayer surface and O-C-C-N⁺ group is still in its gauche conformer.     

Solid-state NMR spectroscopy of a membrane protein in biphenyl phospholipid bicelles with the bilayer normal parallel to the magnetic field

Bicelles composed of the long-chain biphenyl phospholipid TBBPC (1-tetradecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-PC) and the short-chain phospholipid DHPC align with their bilayer normals parallel to the direction of the magnetic field. In contrast, in typical bicelles the long-chain phospholipid is DMPC or DPPC, and the bilayers align with their normals perpendicular to the field. Samples of the membrane-bound form of the major coat protein of Pf1 bacteriophage in TBBPC bicelles are stable for several months, align magnetically over a wide range of temperatures, and yield well-resolved solid-state NMR spectra similar to those obtained from samples aligned mechanically on glass plates or in DMPC bicelle samples "flipped" with lanthanide ions so that their bilayer normals are parallel to the field. The order parameter of the TBBPC bicelle sample decreases from approximately 0.9 to 0.8 upon increasing the temperature from 20 degrees C to 60 degrees C. Since the frequency spans of the chemical shift and dipolar coupling interactions are twice as large as those obtained from proteins in DMPC bicelles without lanthanide ions, TBBPC bicelles provide an opportunity for structural studies with higher spectral resolution of the metal-binding membrane proteins without the risk of chemical or spectroscopic interference from the added lanthanide ions. In addition, the large temperature range of these samples is advantageous for the studies of membrane proteins that are unstable at elevated temperatures and for experiments requiring measurements as a function of temperature.     

Miscibility phase diagrams of giant vesicles containing sphingomyelin

Saturated sphingomyelin (SM) lipids are implicated in lipid rafts in cell plasma membranes. Here we use fluorescence microscopy to observe coexisting liquid domains in vesicles containing SM, an unsaturated phosphatidylcholine lipid (either DOPC or POPC), and cholesterol. We note similar phase behavior in a model membrane mixture without SM (DOPC/DPPC/Chol), but find no micron-scale liquid domains in membranes of POPC/PSM/Chol. We delineate the onset of solid phases below the miscibility transition temperature, and detail indirect evidence for a three-phase coexistence of one solid and two liquid phases.     

鞘磷脂与胆固醇相互作用的拉曼光谱研究

利用ＦＴ Ｒａｍａｎ光谱研究了不同浓度的胆固醇对鞘磷脂质脂体的作用。结果表明,胆固醇没有改变鞘磷脂的Ｏ Ｃ Ｃ Ｎ＋骨架构象,极性头仍然平行膜表面。Ｃ Ｃ骨架振动强度比Ｉ１０８６／Ｉ１０６４说明,随着胆固醇浓度上升,鞘磷酸分子内部旁式构象也逐渐增多。从Ｉ２８４７／Ｉ２８８０强度比看,胆固醇具有使鞘磷脂脂双层有序性上升,流动性降低的作用,但这种作用随胆固醇浓度上升而逐渐减弱。     

Pegylation of magnetically oriented lipid bilayers

We report NMR data for magnetically oriented phospholipid bilayers which have been doped with a lipid derivatized with a polyethylene glycol polymer headgroup to stabilize samples against aggregation. (13)C, (31)P, and (2)H NMR data indicate that the incorporation of PEG2000-PE (1% molar to DMPC) does not interfere with the orientation properties of bicelles prepared at 25% w/v with or without the presence of lanthanide. Bicelles prepared at 10% w/v are also shown to orient when PEG2000-PE is added. The addition of PEG2000-PE to cholesterol-containing, lanthanide-flipped bicelles is shown to inhibit sample phase separation and improve spectral quality. Furthermore, the addition of PEG2000-PE to high w/v bicelles (40% w/v) is demonstrated to lead to an increase in overall sample order.     

Multi-dimensional 1H-13C HETCOR and FSLG-HETCOR NMR study of sphingomyelin bilayers containing cholesterol in the gel and liquid crystalline states

(13)C cross polarization magic angle spinning (CP-MAS) and (1)H MAS NMR spectra were collected on egg sphingomyelin (SM) bilayers containing cholesterol above and below the liquid crystalline phase transition temperature (T(m)). Two-dimensional (2D) dipolar heteronuclear correlation (HETCOR) spectra were obtained on SM bilayers in the liquid crystalline (L(alpha)) state for the first time and display improved resolution and chemical shift dispersion compared to the individual (1)H and (13)C spectra and significantly aid in spectral assignment. In the gel (L(beta)) state, the (1)H dimension suffers from line broadening due to the (1)H-(1)H homonuclear dipolar coupling that is not completely averaged by the combination of lipid mobility and MAS. This line broadening is significantly suppressed by implementing frequency switched Lee-Goldburg (FSLG) homonuclear (1)H decoupling during the evolution period. In the liquid crystalline (L(alpha)) phase, no improvement in line width is observed when FSLG is employed. All of the observed resonances are assignable to cholesterol and SM environments. This study demonstrates the ability to obtain 2D heteronuclear correlation experiments in the gel state for biomembranes, expands on previous SM assignments, and presents a comprehensive (1)H/(13)C NMR assignment of SM bilayers containing cholesterol. Comparisons are made to a previous report on cholesterol chemical shifts in dimyristoylphosphatidylcholine (DMPC) bilayers. A number of similarities and some differences are observed and discussed.     

Role of dislocation loops on the elastic constants of lyotropic lamellar phases

We study the role of dislocation loops defects on the elasticity of lamellar phases by investigating the variation of the lamellar elastic constants, ˉ and K, induced by the proliferation of these defects. We focus our interest on one particular lamellar phase made up of a mixture of C₁₂E₅ and DMPC in water, which is already well-characterised. This lamellar phase undergoes a second-order (or weakly first-order) lamellar-to-nematic phase transition at about 19^°C and dislocation loops are seen to proliferate within the lamellar structure when temperature is decreased below 30^°C. The values of both elastic constants of this given lamellar phase are measured as a function of temperature, approaching the lamellar-to-nematic transition, with the help of Quasi-Elastic Light Scattering (QELS) on oriented lamellar phases. Very surprisingly we observe a strong and rapid increase in both ˉ and K as the lamellar-to-nematic transition temperature is approached. These increases are seen to start as soon as dislocation loops can be observed in the lamellar phase. We interpret our results as being the consequence of the appearance and proliferation of dislocation loops within the lamellar structure. According to a simple model we developped we show that ˉ and K are proportional to the density of dislocation loops in the lamellar phase.     

Patterning silver nanocubes in monolayers using phase separated lipids as templates

Strategies for assembling silver nanocubes (NCs) into distinct 2D patterns on Langmuir–Blodgett (LB) films are demonstrated using two different lipid mixtures as vehicles: (1) raft mixtures containing 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), sphingomyelin (SPM), and cholesterol in different mole ratios (2:2:1 and 1:1:1) and (2) 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) at a 1:3 mol ratio. Atomic force microscopy was employed to unveil the mechanisms of such pattern formation in the LB film. The results demonstrate that aggregation of NCs into round-like pattern is governed by preferential localization of NCs within the liquid condensed (LC) domains of DOPC/SPM/Cholesterol mixture. Cholesterol was found to govern the size and shape of the rounded islands. On the other hand, incorporation of NCs within the liquid expanded (LE) phase of DPPC/DLPC mixture produced linear-branched chains, oriented normal to the Langmuir film transfer direction. The as engineered patterns of silver NCs exhibited characteristic plasmonic signatures. Our results reveal the potential in assembling plasmonic metal nanoparticles into diverse patterns on solid substrates by exploiting their preferential localization either in LC or LE phase of appropriate lipid mixture in Langmuir film.     

Diamagnetic persistent current in diffusive normal-metal rings

We have measured a diamagnetic persistent current with flux periodicities of both h/e and h/2e in an array of thirty diffusive mesoscopic gold rings. At the lowest temperatures, the magnitudes of the currents per ring corresponding to the h/e- and h/2e-periodic responses are both comparable to the Thouless energy E(c) identical with Planck's over 2pi/tau(D), where tau(D) is the diffusion time. Taken in conjunction with earlier experiments, our results strongly challenge the conventional theories of persistent current. We consider a new approach associated with the saturation of the phase coherence time tau(phi).     

Packing of phospholipid vesicles studied by oxygen quenching of Laurdan fluorescence

Steady-state fluorescence oxygen quenching experiments were performed on phospholipid vesicles where 2-dimethylamino-6-lauroylnaphthalene (Laurdan) was inserted. The quenching efficiency was found to be much higher in vesicles in the liquid-crystalline phase with respect to the gel phase, by a factor of about 50. Since the oxygen solubility in the two phospholipid phases can differ at most by a factor of 4 based on literature values, we concluded that oxygen diffusion must be responsible for the great difference in the quenching efficiency. A relatively high quenching efficiency was also found in vesicles composed of equimolar gel and liquid-crystalline phospholipids. Simulations were performed using the linear superposition of the properties of the pure phases to demonstrate that, in the case of vesicles composed of coexisting phases, the diffusional properties of oxygen in each phase are largely modified by the presence of the other. The addition of 10 mol% cholesterol to the gel phase rendered Laurdan fluorescence approximately as quenchable as in the equimolar mixture of the two phases. This result points out that molecules such as cholesterol, which introduce packing defects in the bilayer, favor oxygen diffusion. From the oxygen quenching experiments and using the properties of generalized polarization, the rate of Laurdan dipolar relaxation can be estimated.Abbreviations used Laudran 2-dimethylamino-6-lauroylnapthalene - DLPC dilauroylphosphatidylcholine - DMPC dimyristoylphosphatidylcholine - DPH 1,6-diphenyl-1,3,5-hexatriene - DPPC dipalmitoylphosphatidylcholine - TNS p-tofuidinyl-6-naphthalene sulfonic acid - PBS phosphate-buffered saline solution - GP generalized polarization - NMR nuclear magnetic resonance - EPR electron paramagnetic resonance     

Dipolar bosons in a planar array of one-dimensional tubes

We investigate bosonic atoms or molecules interacting via dipolar interactions in a planar array of one-dimensional tubes. We consider the situation in which the dipoles are oriented perpendicular to the tubes by an external field. We find various quantum phases reaching from a "sliding Luttinger liquid" phase to a two-dimensional charge density wave ordered phase. Two different kinds of charge density wave order occur: a stripe phase in which the bosons in different tubes are aligned and a checkerboard phase. We further point out how to distinguish the occurring phases experimentally.     

Absence of field anisotropy in the intrinsic ferromagnetic signals of highly oriented pyrolytic graphite

We have measured the magnetization of bulk samples of highly oriented pyrolytic graphite (HOPG) at magnetic fields applied parallel and perpendicular to the graphene layers. Within experimental error the intrinsic ferromagnetic signals of the samples show similar magnetic moments at saturation for the two magnetic field directions, in contrast to recently published data (J. ?ervenka et al., Nat. Phys. 5 (2009) 840). To check that the SQUID device provides correctly the small ferromagnetic signals obtained after subtracting the 100 times larger diamagnetic background, we have prepared a sample with a superconducting Pb-film deposited on one of the HOPG surfaces. We show that the field dependence of the measured magnetic moment and after the background subtraction is highly reliable even in the sub-μ emu range providing the real magnetic properties of the embedded small ferromagnetic and superconducting signals.     

拉曼光谱研究人参皂苷Rb1与DPPC双层膜的作用

为深入了解人参皂苷的分子药理学特性,阐明人参皂苷与细胞膜的作用机制,利用拉曼光谱从分子水平研究了不同浓度人参皂苷Rb1与DPPC(二棕榈酰磷脂酸胆碱)双层膜的作用.结果表明,人参皂苷Rb1没有改变DPPC的极性头部O-C-C-N+的稳定构象,极性头仍然平行于膜表面.并且,拉曼峰值比I1096/I1126/1096/I1062和I2848/I288/0随着药物浓度的增加而相应的变大,说明Rbl增加了烃链的无序度,增强了双层膜的流动性.由此推测该药物与DPPC的作用可能由于皂苷分子内及分子间的氢键与磷脂双层膜的极性头部相作用而停留在膜的表面.     

Methods for studying transmembrane peptides in bicelles: consequences of hydrophobic mismatch and peptide sequence

We have shown that bicelles prepared from dilauryl phosphatidylcholine (DLPC) and dipalmitoyl phosphatidylcholine (DPPC) align in a magnetic field under conditions similar to the more common dimyristoyl phosphatidylcholine (DMPC) bicelles. In addition, a model transmembrane peptide, P16, with a hydrophobic stretch of 24 A, and specific alanine-d(3) labels, was incorporated into all of the different bicelles. The long-chain phospholipid (DLPC, DMPC, or DPPC) remained unperturbed upon incorporation of the peptide while the quadrupolar splitting of the short-chain phospholipid along the bicelle rim increased by varying degrees in the different bicelle systems. The change in quadrupolar splitting of the short-chain phospholipids was attributed to changes in either fluidity of the planar region of the bicelle or differences in overall lipid packing. When the hydrophobic stretch of the bilayer was 22.8 (DMPC) or 26.3 A (DPPC), the peptide tilt was found to be transmembrane (33-35 degrees with respect to the bicelle normal). When the hydrophobic stretch of the bilayer was 19.5 A (DLPC), the peptide quadrupolar splittings suggested a loss of transmembrane orientation. When tryptophan was incorporated in the middle of the transmembrane region, the transmembrane orientation was also lost.     

Photoelectron angular distributions from O K shell of oriented CO molecules: A critical comparison between theory and experiment

The dynamical information (ten dipole matrix elements and eight phase differences) has been deduced from the measured angular distributions of photoelectrons from O K shell of oriented CO molecules near the ionization threshold in the region of a sigma(*) shape resonance. Light polarization parallel and perpendicular to the molecular axis has been used. An important contribution of six lsigma partial waves with 0相似文献