Influence of nanotopography on phospholipid bilayer formation on silicon dioxide

We have investigated the effect of well-defined nanoscale topography on the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid vesicle adsorption and supported phospholipid bilayer (SPB) formation on SiO2 surfaces using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). Unilamellar lipid vesicles with two different sizes, 30 and 100 nm, were adsorbed on pitted surfaces with two different pit diameters, 110 and 190 nm, as produced by colloidal lithography, and the behavior was compared to results obtained on flat surfaces. In all cases, complete bilayer formation was observed after a critical coverage of adsorbed vesicles had been reached. However, the kinetics of the vesicle-to-bilayer transformation, including the critical coverage, was significantly altered by surface topography for both vesicle sizes. Surface topography hampered the overall bilayer formation kinetics for the smaller vesicles, but promoted SPB formation for the larger vesicles. Depending on vesicle size, we propose two modifications of the precursor-mediated vesicle-to-bilayer transformation mechanism used to describe supported lipid bilayer formation on the corresponding flat surface. Our results may have important implications for various lipid-membrane-based applications using rough or topographically structured surfaces.     

Phospholipase D-mediated aggregation, fusion, and precipitation of phospholipid vesicles

Large unilamellar vesicles with a diameter of 100 nm were prepared from the zwitterionic phospholipid POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) at pH 8.0. After addition to these vesicles of the enzyme phospholipase D (PLD) from Streptomyces sp. AA586 at 40 degrees C, the terminal phosphate ester bond of POPC was hydrolyzed, yielding the negatively charged POPA (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid) and the positively charged choline. While the reaction yield in the presence of 1 mM Ca2+ reached 100%, the yield was only approximately 68% in the absence of Ca2+. Furthermore, in the absence of Ca2+, the size of the vesicles did not change significantly with time upon PLD addition, as judged from turbidity, dynamic light scattering, and electron microscopy measurements. In the presence of 1 mM Ca2+, however, PLD addition resulted in vesicle aggregation, fusion, and precipitation, originating from the interaction of Ca2+ ions with the negatively charged phospholipids formed in the membranes. Vesicle fusion was monitored by using a novel fusion assay system involving vesicles containing entrapped trypsin and vesicles containing entrapped chymotrypsinogen A. After vesicle fusion, chymotrypsinogen A transformed into a-chymotrypsin, catalyzed by trypsin inside the fused vesicles. The alpha-chymotrypsin formed could be detected with benzoyl-L-Tyr-p-nitroanilide as a membrane permeable chymotrypsin substrate. The observed vesicle precipitation occurring after vesicle fusion in the presence of 1 mM Ca2+ was correlated with an increase of the main phase transition temperature, Tm, of POPA to values above 40 degrees C.     

Fluorescence microscopic investigation on morphological changes of giant multilamellar vesicles induced by amphiphilic additives

Adding an artificial bolaamphiphile to a dispersion of giant multilamellar vesicles (GMVs) made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) induced a cup-shaped deformation in GMVs accompanied by partial extrusion of the inner vesicle; thereafter, the deformed vesicles returned to their original shape. On the other hand, when the artificial bolaamphiphile together with a surfactant was added to the vesicular dispersion, these deformation and reformation dynamics were transmitted from the outer membranes in GMVs to the inner membranes until an intact inner vesicle was extruded out of the outer membrane. The microscopic aspects of these processes were investigated using amphiphiles tagged with individual fluorophores.     

Specific adsorption of cytochrome C on cardiolipin-glycerophospholipid monolayers and bilayers

In this study, we examined the adsorption of cytochrome c (cyt c) on monolayers and liposomes formed from (i) pure 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), or cardiolipin (CL) and on (ii) the more thermodynamically stable binary mixtures of POPE/CL (0.8:0.2 mol/mol) and POPC/CL (0.6:0.4 mol/mol). Constant surface pressure experiments showed that the maximum and minimum interactions occurred in the pure CL (anionic phospholipid) and the pure POPE (zwitterion) monolayers, respectively. Observation by atomic force microscopy (AFM) of the images of Langmuir-Blodgett (LB) films extracted at 30 mN m-1 suggests that the different interactions of cyt c with POPE/CL and the POPC/CL monolayers could be due to lateral phase separation occurring in the POPE/CL mixture. The competition between 8-anilino-1-naphthalene sulfonate (ANS) and cyt c for the same binding sites in liposomes that have identical nominal compositions with respect to those of the monolayers was used to obtain binding parameters. In agreement with the monolayer experiments, the most binding was observed in POPE/CL liposomes. All of our observations strongly support the existence of selective adsorption of cyt c on CL, which is modulated differently by different neutral phospholipids (POPE and POPC).     

高效液相色谱-荧光检测法测定敬钊毒素-I的磷脂膜结合活性

敬钊毒素-I(JZTX-I)是一种能够抑制心肌钠通道失活的新型蜘蛛神经毒素,该文结合高效液相色谱与色氨酸荧光测定技术研究了JZTX-I的磷脂膜结合活性。脂质体共沉淀实验表明,JZTX-I具有不依赖于带负电荷磷脂组成的生物膜结合活性。当加入由酸性或中性磷脂构成的脂质体后,JZTX-I能够分别产生6.4和4.7 nm的蓝移以及7.4和8.0 nm的红移激发漂移,显示JZTX-I能够插入磷脂膜,同时该分子疏水表面的色氨酸残基处于一个运动受限的界面区域。荧光淬灭实验进一步证实,与脂质体结合能够减少该毒素分子表面色氨酸残基的溶剂暴露。该研究结果为阐明JZTX-I的离子通道门控调节机制提供了新的信息。     

Shrink-wrap vesicles

We describe a simple approach to the controlled removal of molecules from the membrane of large unilamellar vesicles made of fatty acids. Such vesicles shrink dramatically upon mixing with micelles composed of a mixture of fatty acid and a phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)), as fatty acid molecules leave the vesicle membrane and accumulate within the mixed micelles. Vesicle shrinkage was confirmed by dynamic light scattering, fluorescence recovery after photobleaching of labeled vesicles, and fluorescence resonance energy transfer between lipid dyes incorporated into the vesicle membrane. Most of the encapsulated impermeable solute is retained during shrinkage, becoming concentrated by a factor of at least 50-fold in the final small vesicles. This unprecedented combination of vesicle shrinkage with retention of contents allows for the preparation of small vesicles containing high solute concentrations, and may find applications in liposomal drug delivery.     

高效液相色谱-荧光检测法测定敬钊毒素-I的磷脂膜结合活性

敬钊毒素-I(JZTX-I)是一种能够抑制心肌钠通道失活的新型蜘蛛神经毒素,该文结合高效液相色谱与色氨酸荧光测定技术研究了JZTX-I的磷脂膜结合活性。脂质体共沉淀实验表明,JZTX-I具有不依赖于带负电荷磷脂组成的生物膜结合活性。当加入由酸性或中性磷脂构成的脂质体后,JZTX-I能够分别产生6.4和4.7nm的蓝移以及7.4和8.0nm的红移激发漂移,显示JZTX-I能够插入磷脂膜,同时该分子疏水表面的色氨酸残基处于一个运动受限的界面区域。荧光淬灭实验进一步证实,与脂质体结合能够减少该毒素分子表面色氨酸残基的溶剂暴露。该研究结果为阐明JZTX-I的离子通道门控调节机制提供了新的信息。     

Tethered bilayer lipid membranes on mixed self-assembled monolayers of a novel anchoring thiol: impact of the anchoring thiol density on bilayer formation

Tethered bilayer lipid membranes (tBLMs) are designed on mixed self-assembled monolayers (SAMs) of a novel synthetic anchoring thiol, 2,3-di-o-palmitoylglycerol-1-tetraethylene glycol mercaptopropanoic acid ester (TEG-DP), and a new short dilution thiol molecule, tetraethylene glycol mercaptopropanoic acid ester (TEG). tBLM formation was accomplished by self-directed fusion of small unilamellar vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. The influence of the dilution of the anchoring thiol molecule in the SAM on the vesicle fusion process and on the properties of the resulting tBLMs is studied. It is observed by quartz crystal microbalance that vesicle fusion is a one-step process for a pure TEG-DP SAM as well as for mixed SAMs containing a high concentration of the anchoring thiol. However, upon dilution of the anchoring thiol to moderate concentrations, this process is decelerated and possibly follows a pathway different from that observed on a pure TEG-DP SAM. Electrochemical impedance spectroscopy is used to qualitatively correlate the composition of the SAM to the electrical properties of the tBLM. In this paper we also delineate the necessity of a critical concentration of this anchoring TEG-DP thiol as a requisite for inducing the fusion of vesicles to form a tBLM.     

RNA selectively interacts with vesicles depending on their size

RNA and vesicles are two important molecular classes in the origin of life and early evolution, but they are not generally considered as interacting partners. The present paper reports about the interaction between tRNA (Esherichia coli) and vesicles made of the zwitterionic surfactant POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), partially positively charged with small molar fractions (max 10%) of the single-chained CTAB (cetyltrimethylammonium bromide). CTAB is capable to insert efficiently in POPC vesicles (as determined by zeta-potential measurements), and the binding of tRNA to such charged vesicles operates a strong selection being critically dependent upon the vesicle size. The binding of tRNA to the vesicles is size-selective as it induces a strongly pronounced process of aggregation of large vesicles (ca. 160-nm diameter) but not of small ones (ca. 80-nm diameter) that are stable against vesicle aggregation (as followed by dynamic light-scattering and optical density measurements). The aggregation of the large vesicles is fully reversible upon the addition of RNase A. The selective behavior of tRNA with respect to differently sized vesicles is observable in separated samples as well as in a mixture of both populations. In the latter case, the fraction of large vesicles readily aggregates in the presence of the small ones that remain unaltered in the mixture. This kind of discrimination capability of RNA might have been of importance in the early phases of the formation of the protocells.     

Probing the kinetics of membrane-mediated helix folding

The kinetics of peptide-membrane association have been studied previously using stopped-flow tryptophan fluorescence; however, such experiments do not directly report the coil-to-helix transition process, which is a hallmark of peptide-membrane interaction. Herein, we report a new method for directly assessing the kinetics of the helix formation accompanied by the peptide-membrane association. This method is based on the technique of fluorescence resonance energy transfer (FRET) and an amino acid FRET pair, p-cyano-L-phenylalanine and tryptophan. To demonstrate the utility of this method, we have studied the membrane-mediated helix folding dynamics of a mutant of magainin 2, an antibiotic peptide found in the skin of the African clawed frog, Xenopus laevis. Our results indicate that the coil-to-helix transition occurs during the binding of the peptide to the lipid vesicle (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], 3:1, wt/wt) but prior to the full insertion of the peptide into the hydrophobic region of the lipid bilayers.     

Surface plasmon resonance study of vesicle rupture by virus-mimetic attack

Frank and coworkers [N. J. Cho, S. J. Cho, K. H. Cheong, J. S. Glenn and C. W. Frank, J. Am. Chem. Soc., 2007, 129, 10050] investigated what happens when lipid vesicles made of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), which serves as a mimic for cell membranes, are exposed to the amphipathic helix peptide, PEP1, which is of the same type found in hepatitis C virus. Using atomic force field microscopy and quartz crystal microbalance measurements they presented evidence that the vesicle is transformed into a lipid bilayer. We use surface plasmon resonance (SPR) microscopy to follow this process in real time. We find an induction period (intermediate state) of approximately 10-min duration between the time of membrane binding and membrane rupture. The SPR data support the interpretation that a lipid bilayer is formed and allow us to put forward a mechanism for the vesicle-rupture event. As a side benefit, we demonstrate how to build two-dimensional lipid patterns on a gold surface using this vesicle-rupture process.     

Lipid nanotube formation from streptavidin-membrane binding

A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upon the binding of streptavidin to biotinylated membranes. Unlike membrane budding and tubulation processes caused by proteins involved with endocytosis and vesicle fusion, streptavidin is known to crystallize at near the isoelectric point (pI 5 to 6) into planar sheets against biotinylated films. We have found, however, that at neutral pH membranes of low bending rigidity (<10kT), such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), spontaneously produce tubular structures with widths ranging from micrometers to below the diffraction limit (<250 nm) and lengths spanning up to hundreds of micrometers. The nanotubes were typically held taut between surface-bound vesicles suggesting high membrane tension, yet the lipid nanotubes exhibited a fluidic nature that enabled the transport of entrained vesicles. Confocal microscopy confirmed the uniform coating of streptavidin over the vesicles and nanotubes. Giant vesicles composed of lipid membranes of higher bending energy exhibited only aggregation in the presence of streptavidin. Routes toward the development of these highly curved membrane structures are discussed in terms of general protein-membrane interactions.     

Proton permeation into single vesicles occurs via a sequential two-step mechanism and is heterogeneous

This article describes the first single-vesicle study of proton permeability across the lipid membrane of small (approximately 100 nm) uni- and multilamellar vesicles, which were composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). To follow proton permeation into the internal volume of each vesicle, we encapsulated carboxyfluorescein, a pH-sensitive dye whose fluorescence was quenched in the presence of excess protons. A microfluidic platform was used for easy exchange of high- and low-pH solutions, and fluorescence quenching of single vesicles was detected with single-molecule total internal reflection fluorescence (TIRF) microscopy. Upon solution exchange and acidification of the extravesicular solution (from pH 9 to 3.5), we observed for each vesicle a biphasic decay in fluorescence. Through single-vesicle analysis, we found that rate constants for the first decay followed a Poisson distribution, whereas rate constants for the second decay followed a normal distribution. We propose that proton permeation into each vesicle first arose from formation of transient pores and then transitioned into the second decay phase, which occurred by the solubility-diffusion mechanism. Furthermore, for the bulk population of vesicles, the decay rate constant and vesicle intensity (dependent on size) correlated to give an average permeability coefficient; however, for individual vesicles, we found little correlation, which suggested that proton permeability among single vesicles was heterogeneous in our experiments.     

Quantitative analysis of tethered vesicle assemblies by quartz crystal microbalance with dissipation monitoring: Binding dynamics and bound water content

To implement the molecular recognition properties of membrane proteins for applications including biosensors and diagnostic arrays, the construction of a biomimetic platform capable of maintaining protein structure and function is required. In this paper, we describe a tethered phospholipid vesicle assembly that overcomes the major limitations of planar supported lipid bilayers and alternative biomimetic membrane platforms and characterize it using quartz crystal microbalance with dissipation monitoring (QCM-D) and fluorescence microscopy. We provide evidence of a one-step mechanism for bilayer formation and monitor the subsequent adsorption and binding of streptavidin, vesicles, and streptavidin-coated microspheres. For all three species, we identify a critical surface density above which a significant amount of coupled interstitial water contributes to the response of the quartz resonator in a phenomenon similar to dynamic coupling due to surface roughness. A Sauerbrey-type analysis is sufficient to accurately interpret the QCM-D results for streptavidin binding if water is treated as an additional inertial mass, but viscoelastic models must be invoked for vesicle and microsphere binding. Additionally, we present evidence of vesicle flattening, possibly enhanced by a biotin-mediated membrane-membrane interaction.     

Molecular umbrella-assisted transport of glutathione across a phospholipid membrane

A di-walled molecular umbrella (1a) has been synthesized by acylation of the terminal amino groups of spermidine with cholic acid, followed by condensation with bis(3-O-[N-1,2,3-benzotriazin-4(3H)-one]yl)-5,5'-dithiobis-2-nitrobenzoate (BDTNB), and displacement with glutathione (gamma-Glu-Cys-Gly, GSH). Replacement of the sterol hydroxyls with sulfate groups, prior to displacement with GSH, afforded a hexasulfate analogue 1b. Both conjugates have been found to enter large unilamellar vesicles (200 nm diameter, extrusion) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and to react with entrapped GSH to form oxidized glutathione (GSSG). Evidence for vesicular entry has come from the formation of oxidized glutathione (GSSG) within the interior of the vesicle, the appearance of the thiol form of the umbrella (USH), and the absence of release of GSH into the external aqueous phase. Results that have been obtained from monolayer experiments, together with the fact that the heavily sulfated conjugate is able to cross the phospholipid bilayer, have yielded strong inferential evidence for an "umbrella-like" action of these molecules as they cross the lipid bilayer.     

Composition and asymmetry in supported membranes formed by vesicle fusion

The structure and formation of supported membranes at silica surfaces by vesicle fusion was investigated by neutron reflectivity and quartz crystal microbalance (QCM-D) measurements. The structure of equimolar phospholipid mixtures of DLPC-DPPC, DMPC-DPPC, and DOPC-DPPC depends intricately on the vesicle deposition conditions. The supported bilayer membranes exhibit varying degrees of compositional asymmetry between the monolayer leaflets, which can be modified by the deposition temperature as well as the salt concentration of the vesicle solution. The total lipid composition of the supported bilayers differs from the composition of the vesicles in solution, and the monolayer proximal to the silica surface is always enriched in DPPC compared to the distal monolayer. The results, which show unambiguougsly that some exchange and rearrangement of lipids occur during vesicle deposition, can be rationalized by considering the effects of salt screening and temperature on the rates of lipid exchange, rearrangement, and vesicle adsorption, but there is also an intricate dependence on the lipid-lipid interactions. Thus, although both symmetric and asymmetric supported bilayers can be prepared from vesicles, the optimal conditions are sensitive to the lipid composition of the system.     

Elucidating driving forces for liposome rupture: external perturbations and chemical affinity

Atomic force microscopy (AFM) studies under aqueous buffer probed the role of chemical affinity between liposomes, consisting of large unilamellar vesicles, and substrate surfaces in driving vesicle rupture and tethered lipid bilayer membrane (tLBM) formation on Au surfaces. 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-pyridyldithio) propionate] (DSPE-PEG-PDP) was added to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles to promote interactions via Au-thiolate bond formation. Forces induced by an AFM tip leading to vesicle rupture on Au were quantified as a function of DSPE-PEG-PDP composition with and without osmotic pressure. The critical forces needed to initiate rupture of vesicles with 2.5, 5, and 10 mol % DSPE-PEG-PDP are approximately 1.1, 0.8, and 0.5 nN, respectively. The critical force needed for tLBM formation decreases from 1.1 nN (without osmotic pressure) to 0.6 nN (with an osmotic pressure due to 5 mM of CaCl(2)) for vesicles having 2.5 mol % DSPE-PEG-PDP. Forces as high as 5 nN did not lead to LBM formation from pure POPC vesicles on Au. DSPE-PEG-PDP appears to be important to anchor and deform vesicles on Au surfaces. This study demonstrates how functional lipids can be used to tune vesicle-surface interactions and elucidates the role of vesicle-substrate interactions in vesicle rupture.     

Influence of the molecular architecture on the adsorption onto solid surfaces: comb-like polymers

The processes of adsorption of grafted copolymers onto negatively charged surfaces were studied using a dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The control parameters in the study of the adsorption are the existence or absence on the molecular architecture of grafted polyethyleneglycol (PEG) chains with different lengths and the chemical nature of the main chain, poly(allylamine) (PAH) or poly(L-lysine) (PLL). It was found out that the adsorption kinetics of the polymers showed a complex behavior. The total adsorbed amount depends on the architecture of the polymer chains (length of the PEG chains), on the polymer concentration and on the chemical nature of the main chain. The comparison of the thicknesses of the adsorbed layers obtained from D-QCM and from ellipsometry allowed calculation of the water content of the layers that is intimately related to the grafting length. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. It is shown that the adsorption of polymers with a charged backbone is not driven exclusively by the electrostatic interactions, but the entropic contributions as a result of the trapping of water in the layer structure are of fundamental importance.     

Calcium oxalate monohydrate precipitation at membrane lipid rafts

The precipitation of calcium oxalate monohydrate (COM) was monitored at a Langmuir monolayer containing lipid raft domains. The raft-forming monolayer consists of a 2:1:1 mixture of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/sphingomyelin/dihydrocholesterol, where the raft liquid ordered phase is enriched in sphingomyelin and the sterol. COM crystals, monitored by Brewster angle microscopy, appear at the phase boundary between the raft domains and the expanded phase.     

Shape transformation of giant phospholipid vesicles at high concentrations of C12E8

Giant unilamellar phospholipid vesicles were prepared by the method of electroformation from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC). We studied the influence of different concentrations of the surfactant octaethyleneglycol dodecylether (C(12)E(8)) on the spontaneous shape transformations of POPC vesicles at room temperature. In accordance with previous results, we observed that low concentration of C(12)E(8) increased the speed of the characteristic vesicle shape transformation, starting from the initial shape with thin tubular protrusion, through beaded protrusion where the number of beads gradually decreased, to final spherical shapes with invagination, whereby the average mean curvature of the vesicle membrane monotonously decreased. In contrast, higher concentration of C(12)E(8) initially induced the shape transformation in the "opposite direction": in the protrusion, the number of beads gradually increased and eventually a tube was formed whereby the average mean curvature of the vesicle membrane gradually increased. However, at a certain point, an abrupt shape change took place to yield the vesicle with invagination. In this transition, the average mean curvature of the vesicle membrane discontinuously decreased. After this transition, the vesicle began to shrink and finally disappeared. We discuss possible mechanisms involved in the observed transformations.