Determination of lipid phase transition temperatures in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the lipid monolayer in hybrid bilayer membranes (HBMs) was investigated using vibrational sum frequency spectroscopy (VSFS). In the gel phase, the acyl chains of the lipid molecules assume an ordered, all-trans configuration, whereas in the LC phase, the acyl chains exhibit a significant number of disordered gauche conformers. VSFS has unique sensitivity to the order/disorder transitions in the acyl chains and was used to determine T(m) for a series of saturated phosphatidylcholine lipids on octadecanethiolate self-assembled monolayers (SAMs). The values obtained for T(m) for all lipids studied are significantly higher than for the corresponding lipids in vesicles in solution. Additionally, the transition widths are broader for the lipids in HBMs. The underlying SAM clearly influences the phase behavior of the overlying lipid monolayer.     

Characterization of solvation properties of lipid bilayer membranes in liposome electrokinetic chromatography

The nature of solute interactions with biomembrane-like liposomes, made of naturally occurring phospholipids and cholesterol, was characterized using electrokinetic chromatography (EKC). Liposomes were used as a pseudo-stationary phase in EKC that provided sites of interactions for uncharged solutes. The retention factors of uncharged solutes in liposome EKC are directly proportional to their liposome-water partition coefficients. Linear solvation energy relationship (LSER) models were developed to unravel the contributions from various types of interactions for solute partitioning into liposomes. Size and hydrogen bond acceptor strength of solutes are the main factors that determine partitioning into lipid bilayers. This falls within the general behavior of solute partitioning from an aqueous into organic phases such as octanol and micelles. However, there exist subtle differences in the solvation properties of liposomes as compared to those of octanol and various micellar pseudo-phases such as aggregates of sodium dodecyl sulfate (SDS), sodium cholate (SC), and tetradecylammonium bromide (TTAB). Among these phases, the SDS micelles are the least similar to the liposomes, while octanol, SC, and TTAB micelles exhibit closer solvation properties. Subsequently, higher correlations are observed between partitioning into liposomes and the latter three phases than that into SDS.     

Bias-dependent admittance in hybrid bilayer membranes

Artificial bilayer membranes provide a platform for bioelectronic devices based on their structural, sensing, and transport functions. In this letter, we report on the impedance response of an engineered membrane with a lower leaflet of octadecanethiol on gold and an outer leaflet of dioleoylphosphatidylcholine with the monomeric channel protein gramicidin. This hybrid bilayer exhibits an electrical response analogous to a solid-state diode: the admittance is very low (<10(-)(7) Omega(-)(1) cm(-)(2)) over a wide potential range but increases exponentially at negative potentials.     

Supramolecular chemistry in lipid bilayer membranes

Lipid bilayer membranes form compartments requisite for life. Interfacing supramolecular systems, including receptors, catalysts, signal transducers and ion transporters, enables the function of the membrane to be controlled in artificial and living cellular compartments. In this perspective, we take stock of the current state of the art of this rapidly expanding field, and discuss prospects for the future in both fundamental science and applications in biology and medicine.

This perspective provides an overview of the current state of the art in supramolecular chemistry in lipid bilayer membranes, including receptors, signal transducers, catalysts and transporters, and highlights prospects for the future.     

Interfacial tension of bilayer lipid membranes

Interfacial tension is an important characteristic of a biological membrane because it determines its rigidity, thus affecting its stability. It is affected by factors such as medium pH and by the presence of certain substances, for example cholesterol, other lipids, fatty acids, amines, amino acids, or proteins, incorporated in the lipid bilayer. Here, the effects of various parameters to on interfacial tension values of bilayer lipid membranes are discussed.     

Electronic processes and photosensitization in bilayer lipid membranes

Abstract— In part one of this paper, evidence for electronic processes in experimental and biological membranes are reviewed. The membrane under consideration, be it experimental or biological, is understood to mean an ultrathin bamer separating two aqueous phases. The question ‘can electronic processes occur in/across such a structure immersed in an aqueous environment?’ is answered affirmatively. In the second part of this paper, photosensitization by dyes and photoelectric effects in experimental bilayer lipid membranes observed recently are described.     

Curvature-modulated phase separation in lipid bilayer membranes

Cellular membranes exhibit a variety of controlled curvatures, with filopodia, microvilli, and mitotic cleavage furrows being only a few of many examples. Coupling between local curvature and chemical composition in membranes could provide a means of mechanically controlling the spatial organization of membrane components. Although this concept has surfaced repeatedly over the years, controlled experimental investigations have proven elusive. Here, we introduce an experimental platform, in which microfabricated surfaces impose specific curvature patterns onto lipid bilayers, that allows quantification of mechanochemical couplings in membranes. We find that, beyond a critical curvature value, membrane geometry governs the spatial ordering of phase-separated domain structures in membranes composed of cholesterol and phospholipids. The curvature-controlled ordering, a consequence of the distinct mechanical properties of the lipid phases, makes possible a determination of the bending rigidity difference between cholesterol-rich and cholesterol-poor lipid domains. These observations point to a strong coupling between mechanical bending and chemical organization that should have wide-reaching consequences for biological membranes. Curvature-mediated patterning may also be useful in controlling complex fluids other than biomembranes.     

Reconstituted olfactory receptors in bilayer lipid membranes

Bullfrog olfactory receptors were reconstituted in bilayer lipid membranes (BLMs). Three odorants were presented to the reconstituted system. The three structurally related odorants were diethylsulfide (DES), thiophene (THP) and diethanolsulfide (DOS). The ordorants were presented in pairs. DOS induced a response in the presence of either of the other two odorants. DES and THP did not induce a response in the presence of either of the other two odorants. These observations suggest that there are two substructures, one common to the three odorants and one that is unique to DOS. The results support the notion that olfactory receptors detect certain molecular segments of odorants.     

Protein deformation of lipid hybrid bilayer membranes studied by sum frequency generation vibrational spectroscopy

Structural deformations of lipid hybrid bilayer membranes induced by signal peptideless (SPL) proteins have been studied for the first time using the inherently surface specific nonlinear optical technique of sum frequency generation vibrational spectroscopy. Specifically, deformations of 1,2-distearoylphosphatidylglycerol(DSPG) membranes induced by interaction with FGF-1, a SPL protein which is released asa function of cellular stress through a nonclassical pathway, have been investigated. FGF-1 was found to induce lipid alkyl chain deformations in previously highly ordered DSPG membranes at the extremely low concentration of 1 nM at 60 degrees C. The deformation process was shown to exhibit a degree of reversibility upon removal of the protein by rinsing with buffer solution.     

Diffusion of liquid domains in lipid bilayer membranes

We report diffusion coefficients of micron-scale liquid domains in giant unilamellar vesicles of phospholipids and cholesterol. The trajectory of each domain is tracked, and the mean square displacement grows linearly in time, as expected for Brownian motion. We study domain diffusion as a function of composition and temperature and measure how diffusion depends on domain size. We find mechanisms of domain diffusion which are consistent with membrane-dominated drag in viscous L(o) phases and bulk-dominated drag for less viscous L(alpha) phases. Where applicable, we obtain the membrane viscosity and report activation energies of diffusion.     

Cholesterol/phospholipid interactions in hybrid bilayer membranes

The interactions between cholesterol and saturated phospholipids in hybrid bilayer membranes (HBMs) were investigated using the interface-sensitive technique of vibrational sum frequency spectroscopy (VSFS). The unique sensitivity of VSFS to order/disorder transitions of the lipid acyl chains was used to determine the main gel to liquid crystal phase transition temperature, Tm, for HBMs of binary cholesterol/phospholipid mixtures on octadecanethiolate self-assembled monolayers. The phase transition temperature and the breadth of the transition were shown to increase with cholesterol content, and the phase boundaries observed in the cholesterol/phospholipid HBMs were comparable to the published phase diagrams of binary cholesterol/phospholipid vesicles. A thermodynamic assessment of the cooperative units of the HBM phase transitions revealed the presence of <10 nm diameter domains that were independent of the cholesterol composition.     

Tethered bilayer lipid membranes with giga-ohm resistances

The successful reconstitution of a tethered BLM on μ-electrodes ranging from 4000 μm to 8 μm is shown in this article. The increase in membrane resistance with decreasing electrode size and the dependency of the membrane capacitance on the electrode size was studied. Furthermore the functional incorporation of α-hemolysin from Staphylococcus aureus into a tBLM situated on μ-electrodes was achieved.     

Examination of bilayer lipid membranes for 'pin-hole' character

BLM prepared on electrode substrates by supporting or tethering were tested for 'pin-hole' character, comparing data from cyclic voltammetry (CV), surface plasmon resonance (SPR) and rotating disc electrodes (RDE). 1-hexadecylamine tethered BLMs on SAM modified gold electrodes were compared with BLMs assembled on modified polyHEMA or sol-gel layers. BLM formation followed by SPR showed that the initial phase of the assembly was complete in 5-20 minutes and produced layers of thickness >5 nm, compared with the expected final BLM thickness of approximately 3 nm. The CVs of the K(3)[Fe(CN)(6)] couple were significantly suppressed irrespective of the method of BLM assembly, without major differences emerging for the different methods. However, data from the RDE distinguished the 'pin-hole' character of the different preparations. The data were consistent with incomplete initial (<1 h, SPR estimated BLM thickness >5 nm) vesicle fusion leaving 'pin-holes' of approximately 2 microm (HDA-11-mercaptoundecanoic acid (MUA) tethered BLM) to approximately 3 microm (tetraethylorthosilicate sol-gel supported BLM) followed by a slow maturation (>15 h; impedance spectroscopy estimated thickness approximately 3 nm) and lateral spreading and fusion, resulting in loss of 'pin-hole' character (<1 microm). The BLM could be used in conjunction with potentiometric measurement to observe the incorporation of nystatin into the BLM and the rate of incorporation adjusted according to original permeability of the BLM. The 'pin-hole-free' BLM construction with lowest permeability (TEOS supported, 4 x 10(-10) cm s(-1) compared with HDA-MUA, 3 x 10(-9) cm s(-1)) gave a potentiometric signal independent of bulk ion-concentration across 5 decades change in concentration. Formed on an ion-selective electrode, nystatin incorporation could be followed as a change in potential, over >2 h, whereas the TEOS supported BLM with permeability 1 x 10(-9) cm s(-1) shows nystatin incorporation within 1 h. In this instance, addition of ConA reduced the potential to the same value as prior to nystatin incorporation, consistent with nystatin channel closure.     

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.     

Amphiphile bilayer films from DPPC: bilayer lipid membranes and Newton black films

Amphiphile bilayer films are obtained from 1,2 dipalmitoyl-glycero-3-phosphocholine (DPPC): bilayer lipid membranes (BLM) and Newton black films (NBF), through thinning of the respective thin liquid films, thus allowing for a very precise determination of the moment of their formation. Stability (or rupture) and formation of BLM and NBF are considered from a unified point of view with the microscopic theory of Kashchiev–Exerowa [J. Colloid Interface Sci., 77 (1980) 501–511], based on the formation of nanoscopic holes in them. BLM and NBF are obtained and studied with the microinterferometric method of Scheludko–Exerowa in its contemporary version. The equivalent thickness of both BLM (in benzene solution between two water phases with 0.1 M NaCl) and NBF in aqueous DPPC solution (in the presence of 0.1 M NaCl) is determined as being h_w = 7.0 nm for BLM and h_w = 7.8 nm for NBF. By means of the dependences: BLM lifetime versus DPPC concentration and probability for BLM formation versus DPPC concentration, it is established that there exist metastable BLM and stable NBF. The good fit between the experimental results of τ(C) dependence and theory in the case of BLM allow to determine the three constants: pre-exponential factor A = 1.5 × 10⁻³ s, related to the process kinetics; constant B = 20.2 ± 0.2, related to the specific hole energy γ = 1.7 × 10⁻¹¹ J/m and the equilibrium concentration C_e = 6 × 10⁻⁴ ± 7.2 × 10⁻⁶ m/l. The specific hole linear energy γ = 1.7 × 10⁻¹¹ J/m determined as well as the binding energy Q between first neighbor molecules in the bilayers Q = 1.48 × 10⁻¹⁹ J (36 kT) are lower than the ones determined for DPPC foam bilayer in gel state γ = 9.1 × 10⁻¹¹ J/m and Q = 55 kT. This means that interaction is weaker in the case of BLM. The critical concentration C_c at which bilayer formation starts is: for BLM C_c = 30 μg/ml and for NBF C_c = 70 μg/ml. This concentration characterizes quantitatively the formation of the amphiphile bilayer and is a very useful parameter that can be used for various purposes.     

硫醇－磷脂混合双层膜的电化学性质及其与蜂毒素的相互 作用研究

报道了硫醇－磷脂混合双层膜的循环伏安和电化学交流阻抗行为研究,并用电化学方法考察了蜂毒素与其相互作用,实验中通过冷冻表面沾有磷脂溶液的硫醇单层膜制备混合双层膜,研究表明双层膜在电极表面形成致密的绝缘层,阻碍了电极表面的电子传递,在双层膜体系上引入的蜂毒素可在膜表面上形成孔洞,破坏膜的绝缘性,降低膜电阻,增加膜电容,使带负电的探针Fe(CN)6^3-的氧化还原反应速度加快。     

Melting and mechanical properties of polymer grafted lipid bilayer membranes

The influence of polymer grafting on the phase behavior and elastic properties of two tail lipid bilayers have been investigated using dissipative particle dynamics simulations. For the range of polymer lengths studied, the L(c) to L(α) transition temperature is not significantly affected for grafting fractions, G(f) between 0.16 and 0.25. A decrease in the transition temperature is observed at a relatively high grafting fraction, G(f) = 0.36. At low temperatures, a small increase in the area per head group, a(h), at high G(f) leads to an increase in the chain tilt, inducing order in the bilayer and the solvent. The onset of the phase transition occurs with the nucleation of small patches of thinned membrane which grow and form continuous domains as the temperature increases. This region is the co-existence region between the L(β)(thick) and the L(α)(thin) phases. The simulation results for the membrane area expansion as a function of the grafting density conform extremely well to the scalings predicted by self-consistent mean field theories. We find that the bending modulus shows a small decrease for short polymers (number of beads, N(p) = 10) and low G(f), where the influence of polymer is reduced when compared to the effect of the increased a(h). For longer polymers (N(p) > 15), the bending modulus increases monotonically with increase in grafted polymer. Using the results from mean field theory, we partition the contributions to the bending modulus from the membrane and the polymer and show that the dominant contribution to the increased bending modulus arises from the grafted polymer.     

Specific binding structures of dendrimers on lipid bilayer membranes

Dissipative particle dynamics simulations are used to study the specific binding structures of polyamidoamine (PAMAM) dendrimers on amphiphilic membranes and the permeation mechanisms. Mutually consistent coarse-grained (CG) models both for PAMAM dendrimers and for dimyristoylphosphatidylcholine (DMPC) lipid molecules are constructed. The PAMAM CG model describes correctly the conformational behavior of the dendrimers, and the DMPC CG model can properly give the surface tension of the amphiphilic membrane. A series of systematic simulations is performed to investigate the binding structures of the dendrimers on membranes with varied length of the hydrophobic tails of amphiphiles. The permeability of dendrimers across membranes is enhanced upon increasing the dendrimer size (generation). The length of the hydrophobic tails of amphiphiles in turn affects the dendrimer conformation, as well as the binding structure of the dendrimer-membrane complexes. The negative curvature of the membrane formed in the dendrimer-membrane complexes is related to dendrimer concentration. Higher dendrimer concentration together with increased dendrimer generation is observed to enhance the permeability of dendrimers across the amphiphilic membranes.