Alkanethiol monolayers and alkanethiol|phospholipid bilayers supported by mercury: an electrochemical characterization

Enhanced adsorption of Ca-ATPase containing vesicles on a negatively charged solid-supported-membrane for the investigation of membrane transporters

A convenient model system for a biological membrane is a solid-supported membrane (SSM), which consists of a gold-supported alkanethiol|phospholipid bilayer. In combination with a concentration jump method, SSMs have been used for the investigation of several membrane transporters. Vesicles incorporating sarcoplasmic reticulum Ca-ATPase (SERCA) were adsorbed on a negatively charged SSM (octadecanethiol|phosphatidylserine bilayer). The current signal generated by the adsorbed vesicles following an ATP concentration jump was compared to that produced by SERCA-containing vesicles adsorbed on a conventional SSM (octadecanethiol|phosphatidylcholine bilayer). A significantly higher current amplitude was recorded on the serine-based SSM. The adsorption of SERCA-incorporating vesicles on the SSM was then characterized by surface plasmon resonance (SPR). The SPR measurements clearly indicate that in the presence of Ca(2+) and Mg(2+), the amount of adsorbed vesicles on the serine-based SSM is about twice that obtained using the conventional SSM, thereby demonstrating that the higher current amplitude recorded on the negatively charged SSM is correlated with a greater quantity of adsorbed vesicles. The enhanced adsorption of membrane vesicles on the PS-based SSM may be useful to study membrane preparations with a low concentration of transport protein generating small current signals, as in the case of various recombinantly expressed proteins.

Effect of headgroup on the physicochemical properties of phospholipid bilayers in electric fields: size matters

The effect of molecular structure on ensemble structure of phospholipid films has been investigated. Bilayers of dimyristoyl phosphatidylethanolamine (DMPE) were prepared on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaeffer deposition. Capacitance and charge density measurements were used to investigate the adsorption behavior and barrier properties of the lipid bilayers. In situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was employed to investigate the organization of the molecules within the bilayer. DMPE bilayers exhibit lower capacitance than bilayers formed from the related lipid, dimyristoyl phosphatidylcholine (DMPC). The infrared data show that these results can be explained by structural differences between the bilayers formed from each molecule. DMPE organizes into bilayers with hydrocarbon chains tilted at a smaller angle to the surface normal, which results in a thicker film. The hydrocarbon chains contain few conformational defects. Spectra in the carbonyl and phosphate stretching mode regions indicate low solvent content of DMPE films. Both of these effects combine to produce films with lower capacitance and enhanced barrier properties. The results are explained in terms of the differences in structure between the constituent molecules.

Building biomimetic membrane at a gold electrode surface

This article describes efforts to build a model biological membrane at a surface of a gold electrode. In this architecture, the membrane may be exposed to static electric fields on the order of 10(7) to 10(8) V m(-1). These fields are comparable in magnitude to the static electric field acting on a natural biological membrane. The field may be conveniently used to manipulate organic molecules within the membrane. By turning a knob on the control instrument one can deposit or lift the membrane from the gold surface. Electrochemical techniques can be used to control the physical state of the film while the infrared reflection absorption spectroscopy (IRRAS), surface imaging by STM and AFM and neutron scattering techniques can be employed to study conformational changes of organic molecules and their ordering within the membrane. This is shown on examples of membranes built of a simple zwitterionic phospholipid such as 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and a mixed membrane composed of DMPC and cholesterol. The results illustrate the tremendous effect of cholesterol on the membrane structure. Two methods of membrane deposition at the electrode surface, namely by unilamellar vesicles fusion and using the Langmuir-Blodgett technique, are compared. Applications of these model systems to study interactions of small antibiotic peptides with lipids are discussed.

An electrochemical investigation of sarcolipin reconstituted into a mercury-supported lipid bilayer

Sarcolipin was incorporated into a lipid bilayer anchored to a mercury electrode through a hydrophilic tetraethyleneoxy chain. The behavior of this tethered bilayer lipid membrane incorporating sarcolipin was investigated by electrochemical impedance spectroscopy and by recording charge versus time curves after potential jumps. When the transmembrane potential starts to become negative on the trans side, evidence is provided that sarcolipin aggregates into ion-conducting pores. Over the range of physiological transmembrane potentials, these pores are permeable to small inorganic anions such as chloride, phosphate, or sulfate but impermeable to inorganic cations such as Na+ and K+. Only at transmembrane potentials more negative than approximately -150 mV on the trans side do sarcolipin channels allow the translocation of the latter cations. A tentative relationship between this property of sarcolipin and its regulatory function on Ca-ATPase of sarcoplasmic reticulum is proposed.

Self-Assembled Monolayers on Mercury Probed in a Modified Surface Force Apparatus

Self-assembled monolayers (SAMs) of three thiol compounds formed on mercury are investigated by a combination of cyclic voltammetry, electrocapillary curves, and a novel method of measuring electrical double-layer properties. The last method involves a modified surface force apparatus in which a flat mica surface is pressed down toward a fixed mercury drop held beneath it, while both are immersed in aqueous electrolyte solution. Optical interference measurements are made of the mica-mercury separation as a function of electrical potential applied to the mercury, which yields information on the double-layer interaction between the two surfaces. Mercury is decorated by SAMs of 11-mercapto-1-undecanoic acid, which is shown to bring negative charge to the mercury/aqueous interface due to dissociation of the carboxylic acid groups; 11-mercapto-1-undecanol, which although it is uncharged changes the dipole potential of the interface; and 1-undecanethiol, which likewise changes the dipole potential, but by a different amount. The difference between the changes in dipole potential (90 mV) can be related to the different terminal groups of these two SAMs, -CH3 compared to -OH, that are in contact with the aqueous phase.

Studying electron transfer through alkanethiol self-assembled monolayers on a hanging mercury drop electrode using potentiometric measurements

A new approach based on measuring the change of the open-circuit potential (OCP) of a hanging mercury drop electrode (HMDE), modified with alkanethiols of different chain length conducted in a solution containing a mixture of Ru(NH3)6(2+) and Ru(NH3)6(3+) is used for studying electron transfer across the monolayer. Following the time dependence of the OCP allowed the extraction of the kinetic parameters, such as the charge transfer resistance (R(ct)) and the electron transfer rate constant (k(et)), for different alkanethiol monolayers. An electron tunneling coefficient, beta, of 0.9 A(-1) was calculated for the monolayers on Hg.

Electrochemical and PM-IRRAS studies of the effect of the static electric field on the structure of the DMPC bilayer supported at a Au(111) electrode surface

Differential capacity, charge density measurements, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) were employed to study the fusion of small unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) on a Au(111) electrode surface. The differential capacity and charge density data showed that the vesicles fuse onto the gold surface at charge densities between -10 microC/cm(2) < sigma(M) < 10 microC/cm(2) to form a bilayer. When sigma(M) < -10 microC/cm(2), the film is detached from the surface but it remains in close proximity to the surface. PM-IRRAS experiments provided IR spectra for the bilayer in the adsorbed and the desorbed state. Ab initio normal coordinate calculations were performed to assist interpretation of the IR spectra. The IR bands were analyzed quantitatively, and this analysis provided information concerning the conformation and orientation of the acyl chains and the polar head region of the DMPC molecule. The orientation of the chains, hydration, and conformation of the headgroup of the DMPC molecule strongly depend on the electrode potential.

Tethered bilayer lipid membranes self-assembled on mercury electrodes

In order to incorporate integral proteins in a functionally active state, metal-supported lipid bilayers must have a hydrophilic region interposed between the bilayer and the metal. This region is realized with a hydrophilic molecule terminating at one end with a sulfhydryl or disulfide group that anchors this "hydrophilic spacer" to the surface of a metal, such as gold or mercury. The other end of the hydrophilic spacer may be covalently linked to the polar head of a phospholipid molecule, giving rise to a supramolecule called "thiolipid" (TL). With respect to gold, mercury has the advantage of providing a defect-free and fluid surface to the self-assembling spacer. Hydrophilic spacers consisting of a polyethyleneoxy or a hexapeptide chain, as well as thiolipids derived from these spacers, were employed to fabricate mercury-supported lipid bilayers. The formation of a lipid bilayer on top of a self-assembled monolayer of a hydrophilic spacer, or of a single-lipid monolayer on top of a self-assembled monolayer of a thiolipid, was realized by simply immersing the coated mercury electrode into an aqueous solution across a lipid film previously spread on its surface at its spreading pressure. Particularly stable mercury-supported lipid bilayers were obtained by using thiolipids. The biomimetic properties of these lipid bilayers were tested by incorporating channel-forming polypeptides (gramicidin and melittin) and proteins (OmpF porin). The effect of the transmembrane potential on the function of these channels was estimated by using a simple electrostatic model of the mercury-solution interphase.

Electrochemical and photon polarization modulation infrared reflection absorption spectroscopy study of the electric field driven transformations of a phospholipid bilayer supported at a gold electrode surface

Electrochemistry and polarization modulation Fourier transform infrared reflection absorption spectroscopy (PM-FTIRRAS) was employed to investigate fusion of small unilamellar vesicles of 1,2dioyl-sn-glycero-3-phosphatidyl choline (DOPC) onto the Au(111) electrode. Electrochemical studies demonstrated that the DOPC vesicles fuse and spread onto the gold electrode surface at small charge densities -8 microC cm(-2)<sigmaM<0 microC cm(-2)(if the static electric field is <2 x 10(8) V/m) to form a bilayer. At sigmaM<-8 microC cm(-2), the film is detached from the electrode surface; however, the film remains in close proximity to the surface. The PM-FTIRRAS experiments demonstrated that the field-driven transformation of the film involves changes in hydration, orientation, and conformation in the polar headgroup region and that changes in the packing and tilt of the acyl chains are consequences of the headgroup rearrangements.

Spectroelectrochemical studies of bilayers of phospholipids in gel and liquid state on Au(111) electrode surface

Differential capacity, chronocoulometry and Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy (PM FTIRRAS) were employed to investigate spreading of small unilamellar vesicles (SUVs) of DOPC and DMPC onto a Au(111) electrode surface. The electrochemical experiments demonstrated that vesicles fuse onto the electrode surface and at E>-0.5V (SSCE) or at charge densities -10<sigma(M)<5 microC cm(-2) to form defected bilayers, directly in contact with the metal surface. The analysis of PM FTIRRA spectra in the CH stretching region showed that the DOPC bilayer is in the liquid crystalline state while the bilayer of DMPC is in the gel or in the ripple phase. The spectra were also used to determine the tilt of the acyl chains in the bilayer of DMPC as a function of the electrode potential. At E<-0.5 V (SSCE), where the bilayer is detached from the metal surface, the chains are tilted at approximately 29 degrees with respect to surface normal. When the bilayer is adsorbed at the metal surface at E>-0.5 V (SSCE), the tilt angle increases to approximately 42 degrees. The increase of the tilt angle is discussed in terms of a change in the packing of the polar head of the phospholipids molecules in the bilayer adsorbed at the electrode surface.

Electric field-driven transformations of a supported model biological membrane--an electrochemical and neutron reflectivity study

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the support's surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the support's surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the support's surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.

Two Photobioelectrochemical Applications of Self-Assembled Films on Mercury

The homogeneous, defect-free surface of a hanging mercury drop electrode was used to self-assemble films apt for the investigation of two photobioelectrochemical systems. Monolayers of straight-chain C12-C18alkane-1-thiols were anchored to a hanging mercury drop electrode and a film of chlorophyll was self-assembled on the top of them. The dependence of the photocurrents generated by illumination of the chlorophyll film with red light, on the thickness of the alkane-1-thiol monolayer and the applied potential is discussed. The photocurrents of purple membrane fragments, adsorbed on a mixed hexadecane-1-thiol/ dioleoylphosphatidylcholine bilayer self-assembled on mercury, were investigated in the presence of sodium perchlorate, chloride and acetate. The effect of the anions on the kinetics of the light-driven proton transport by bacteriorhodopsin has been determined.

DC photoelectric signals from bacteriorhodopsin adsorbed on lipid monolayers and thiol/lipid bilayers supported by mercury

Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer and on a mixed alkanethiol/DOPC bilayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The light-on and light-off capacitive currents on an alkanethiol/DOPC bilayer at pH 6.4 were interpreted on the basis of a simple equivalent circuit. The pH dependence of the biphasic decay kinetics of the light-on currents was analyzed to estimate the pK(a) values for the transitions releasing protons to, and taking up protons from, the solution. The linear dependence of the stationary light-on current of bR on a DOPC monolayer self-assembled on mercury upon the applied potential was interpreted on the basis of an equivalent circuit.

Electrochemical studies of blocking properties of solid supported tethered lipid membranes on gold

The insulating properties of self-assembled thiolipid monolayers and tethered lipid bilayers on polycrystalline gold electrodes were studied by means of cyclic voltammetry (CV). These films were formed by two-step self-assembly processes. Electrochemical measurements of the heterogeneous electron transfer rate constant of different redox couples such as potassium ferrocyanide (K(4)[Fe(CN)(6)]) and dopamine (DP) were used to examine the molecular integrity and structural defects and pinholes within the monolayers. We demonstrate by means of cyclic voltammetry that the bilayer lipid membranes tethered to the gold surface are blocking, stable, yet retaining their dynamic properties and can be used as a model of the cell membrane.

Hybrid polar compounds produce a positive shift in the surface dipole potential of self-assembled phospholipid monolayers

Hybrid polar compounds (HPCs) are powerful inducers of terminal differentiation of various types of tumors, including Friend murine erythroleukemia cells (MELCs). They are known to act synergistically with an increase in the extracellular concentration of cations, which causes a positive shift in the negative value of the ionic surface potential. Two HPCs, hexamethylenebisacetamide (HMBA) and suberoylanilide hydroxamic acid (SAHA), were adsorbed on self-assembled phospholipid monolayers supported on a mercury drop and the shift in the surface dipole potential chi of the lipid film due to their adsorption was estimated from charge measurements. At their optimal concentrations for inducing MELC terminal differentiation (5 mM for HMBA and 2.6 microM for SAHA), these HPCs cause a chi shift of about 15-20 mV, positive toward the hydrocarbon tails, both on neutral phosphatidylcholine films and on negatively or positively charged phosphatidylserine films. This strongly suggests that the nonspecific effect of HPCs of different structure in inducing cancer cells to rescue their differentiation program is related to a positive chi shift on the extracellular side of the cell membrane.