A Model System To Study the Insertion of Cholesterol into a Phospholipid Monolayer

Supported lipid bilayer membrane arrays on micro-patterned ITO electrodes

Lipid bilayer arrays were formed on micropatterned ITO electrodes. With this bilayer array platform both the fluorescence microscopy and electrochemical detection can be realized to explore the biophysical properties of cell membrane.

Photosynthetic Proteins in Supported Lipid Bilayers: Towards a Biokleptic Approach for Energy Capture

In nature, plants and some bacteria have evolved an ability to convert solar energy into chemical energy usable by the organism. This process involves several proteins and the creation of a chemical gradient across the cell membrane. To transfer this process to a laboratory environment, several conditions have to be met: i) proteins need to be reconstituted into a lipid membrane, ii) the proteins need to be correctly oriented and functional and, finally, iii) the lipid membrane should be capable of maintaining chemical and electrical gradients. Investigating the processes of photosynthesis and energy generation in vivo is a difficult task due to the complexity of the membrane and its associated proteins. Solid, supported lipid bilayers provide a good model system for the systematic investigation of the different components involved in the photosynthetic pathway. In this review, the progress made to date in the development of supported lipid bilayer systems suitable for the investigation of membrane proteins is described; in particular, there is a focus on those used for the reconstitution of proteins involved in light capture.

Modulating the lateral tension of solvent-free pore-spanning membranes

The plasma membrane of animal cells is attached to the cytoskeleton, which significantly contributes to the lateral tension of the membrane. Lateral membrane tension has been shown to be an important physical regulator of cellular processes such as cell motility and morphology as well as exo- and endocytosis. Here, we report on lipid bilayers spanning highly ordered pore arrays, where we can control the lateral membrane tension by chemically varying the surface functionalization of the porous substrate. Surface functionalization was achieved by a gold coating on top of the pore rims of the hexagonal array of pores in silicon nitride substrates with pore radii of 600 nm followed by subsequent incubation with various n-propanolic mixtures of 6-mercapto-1-hexanol (6MH) and O-cholesteryl N-(8'-mercapto-3',6'-dioxaoctyl)carbamate (CPEO3). Pore-spanning membranes composed of 1,2-diphytanoyl-sn-glycero-3-phosphocholine were prepared by spreading giant unilamellar vesicles on these functionalized porous silicon nitride substrates. Different mixtures of 6MH and CPEO3 provided self-assembled monolayers (SAMs) with different compositions as analyzed by contact angle and PM-IRRAS measurements. Site specific force-indentation experiments on the pore-spanning membranes attached to the different SAMs revealed a clear dependence of the amount of CPEO3 in the monolayer on the lateral membrane tension. While bilayers on pure 6MH monolayers show an average lateral membrane tension of 1.4 mN m(-1), a mixed monolayer of CPEO3 and 6MH obtained from a solution with 9.1 mol % CPEO3 exhibits a lateral tension of 5.0 mN m(-1). From contact angle and PM-IRRAS results, the mole fraction of CPEO3 in solution can be roughly translated into a CPEO3 surface concentration of 40 mol %. Our results clearly demonstrate that the free energy difference between the supported and freestanding part of the membrane depends on the chemical composition of the SAM, which controls the lateral membrane tension.

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.

PEGylated phospholipid membrane on polymer cushion and its interaction with cholesterol

By employing poly(ethylene glycol) (PEG) shielding and a polymer cushion to achieve air stability of the lipid membrane, we have analyzed PEG influence on dried membranes and the interaction with cholesterol. Small unilamellar vesicles (SUVs) formed by the mixture of 1,2-dimyristoylphosphatidylcholine (DMPC) with different molar fraction of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG(2000)) adsorb and fuse into membranes on different polymer-modified silicon dioxide surfaces, including chitosan, poly(L-lysine) (PLL), and hyaluronic acid. Dried membranes are further examined by ellipsometer and atomic force microscopy (AFM). Only chitosan can support a visible and uniform lipid array. The thickness of dry PEGylated lipid membrane is reduced gradually as the molar fraction of PEG increases. AFM scanning confirms the lipid membrane stacking for vesicles containing low PEG, and only a proper amount of PEG can maintain a single lipid bilayer; however, the air stability of the membrane will be destroyed if overloading PEG. Cholesterol incorporation can greatly improve the structural stability of lipid membrane, especially for those containing high molar fraction of PEG. Different amounts of cholesterol influence the thickness and surface morphology of dried membrane.

Native E. coli inner membrane incorporation in solid-supported lipid bilayer membranes

Solid-supported bilayer lipid membranes (SBLMs) containing membrane protein have been generated through a simple lipid dilution technique. SBLM formation from mixtures of native Escherichia coli bacterial inner membrane (IM) vesicles diluted with egg phosphatidylcholine (egg PC) vesicles has been explored with dissipation enhanced quartz crystal microbalance (QCM-D), atomic force microscopy (AFM), attenuated total internal-reflection Fourier-transform infrared spectroscopy (ATR-FTIR), and fluorescence recovery after photobleaching (FRAP). QCM-D studies reveal that SBLM formation from vesicle mixtures ranging between 0% and 100% IM can be divided into two regimes. Samples with < or = 40% IM form SBLMs, while samples of greater IM fractions are dominated by vesicle adsorption. FRAP experiments showed that the bilayers formed from mixed vesicles with < or = 40% IM were fluid, and comprised a mixture of both egg PC and IM. ATR-FTIR measurements on SBLMs membranes formed with 30% IM confirm that protein is present. SBLM formation was also explored as a function of temperature by QCM-D and FRAP. For samples of 30% IM, QCM-D data show a decreased mass and viscoelasticity at elevated temperatures, and an increased fluidity is observed by FRAP measurements. These results suggest improved biomimetic characteristics can be obtained by forming and maintaining the system at, or close to, 37 degrees C.

Manipulation and charge determination of proteins in photopatterned solid supported bilayers

This work demonstrates the use of deep UV micropatterned chlorotrimethylsilane (TMS) monolayers to support lipid membranes on SiO(2) surfaces. After immersing such a patterned surface into a solution containing small unilamellar vesicles of egg PC, supported bilayer lipid membranes were formed on the hydrophilic, photolyzed regions and lipid monolayer over the hydrophobic, non-photolyzed regions. A barrier between the lipid monolayer and bilayer regions served to stop charged lipids migrating between the two. This allows the system to be used to separate charged lipids or proteins by electrophoresis. Either oppositely charged fluorescence labeled lipids [Texas Red DHPE (negative charge) and D291 (positive charge)] or lipids with different charge numbers [Texas Red DHPE (one negative charge) and NBD PS (two negative charges)] can be separated. We have also studied the migration of streptavidin attached to a biotinylated lipid. Negatively charged streptavidin responds to the applied electric field by moving in the direction of electroosmotic flow, i.e. towards the negative electrode. However the direction of streptavidin movement can be controlled by altering the difference in zeta potential between that of the streptavidin (zeta(1)) and the lipid membrane (zeta(2)). If zeta(1) > zeta(2), streptavidin moves to the negative electrode, while if zeta(1) < zeta(2), streptavidin moves to the positive electrode. This balance was manipulated by adding positively charged lipid DOTAP to the membrane. After measuring the average drift velocity of streptavidin as a function of DOTAP concentration, the point where zeta(1) approximately zeta(2) was found. At this point zeta(1) was calculated to be -9.8 mV which is in good agreement with the value of -13 mV from force measurements and corresponds to a charge of -2e per streptavidin, thus demonstrating the applicability of this method for determining protein charge.

Quantitative investigation by atomic force microscopy of supported phospholipid layers and nanostructures on cholesterol-functionalized glass surfaces

Understanding the interaction mechanisms of phospholipids with surfaces is crucial for the exploitation of lipid bilayers as models of the cell membrane as well as templates for biosensors. Moreover, controlling and manipulating lipid nanoparticles for the investigation of their properties by means of single-particle sensitive surface techniques require the ability to tailor the chemical properties of surfaces to achieve a stable and sparse binding of lipid particles, while keeping them from aggregating, or denaturing. Here we present a quantitative morphological and structural investigation by atomic force microscopy of supported phospholipid layers and nanostructures on cholesterol-functionalized glass surfaces, in comparison with other surfaces with different interfacial properties. We show that the functionalization of glass coverslips with cholesterol groups is a viable route for the production of optically transparent, scanning probe microscopy-compatible clean substrates for the effective immobilization of both extended single lipid bilayers and lipid nanoparticles.

Probing the effects of cholesterol on pyrene-functionalized interfacial adlayers

We have synthesized a novel set of pyrene-functionalized, covalently bound surface adlayers with and without cholesterol derivatives coadded to the adlayer. We have deposited these adlayers on quartz, oxidized silicon wafers, and indium-doped tin oxide coated substrates. The addition of tethered cholesterol to the adlayer creates a hydrophobic, likely disordered, microenvironment in which the surface-bound pyrene resides. X-ray photoelectron spectroscopy measurements demonstrate the covalent attachment of both cholesterol and pyrene in our adlayers. The presence of the cholesterol moieties gives rise to a reduction in film thickness, as measured ellipsometrically, and contact angle data indicate significant surface heterogeneity. Steady-state fluorescence data show that the presence of cholesterol moieties reduces the extent of pyrene excimer formation and provides a less polar environment for the chromophore. Fluorescence lifetime measurements on surface-bound pyrene were biexponential, consistent with multiple local environments, regardless of whether tethered cholesterol was present or not. Cyclic voltammetry reveals competition between the pyrene and cholesterol moieties for binding to available surface sites on the epoxide-terminated surface-binding layer we use.

Titration of ionizable monolayers by measurement of the electric double-layer force

We have determined the pKa of surface-bound primary amine groups by determining the surface potential as a function of solution pH from the magnitude of the electric double-layer force. Using colloid-probe atomic force microscopy (AFM), we measured the force as a function of separation between a particle of radius R = 10 microm and a planar surface, each coated with a self-assembled monolayer of HS(CH2)2CONH((CH2)2O)8(CH2)2NH2. The force was measured from pH 3 to 7, and the surface potential was determined by fitting the results to solutions of the nonlinear Poisson-Boltzmann equation. The surface pKa of the primary amine group was found to be 5.0 +/- 0.2, in agreement with the results of contact-angle and chemical-force titrations on similar surfaces with primary amine groups. The surface charge density indicates that less than 1% of the NH2 groups are dissociated at pH 3, suggesting that ionization is very unfavorable in the local environment of the ethylene oxide chains. This noncontact method should be of general applicability to surfaces with ionizable groups and avoids the possible complications of large contact forces on the surfaces under study.