Phospholipid black lipid membrane

The multilamellar bilayer structures that form spontaneously on adding water to solid- or liquid-phase phospholipids can be dispersed to form vesicular structures called liposomes. These are often employed in studies of bilayer properties and may be combined with membrane proteins to reconstitute functional membrane systems. A valuable technique for studying the properties of proteins inserted into bilayers employs a single bilayer lamella, also termed a black lipid membrane, formed across a small aperture in a thin partition between two aqueous compartments. Because pristine lipid bilayers have very low ion conductivities, the modifications of ion-conducting... 

Often when phospholipids such as (I) through (IV) are dispersed in water, they form so-called liposomes, which have a large number (e.g., 10 or more) of concentric bilayer shells. When phospholipids are dissolved in detergents and the detergents are removed slowly by dialysis, the phospholipids form relatively large vesicles containing one or only a few bilayer shells. Lipid bilayers have also been studied as supported by black lipid membranes [Fettiplace et al. (1975)]. 

The ability of PHBs and OHBs to conduct salts across phospholipids bilayers was examined in a planar bilayer voltage-clamp setup.69,70 In this system, a bilayer is formed between two aqueous solutions by painting a decane solution of phospholipids across a small aperture (ca. 0.2 mm in diameter) in a partition separating two chambers containing aqueous salt solutions. The hydrocarbon drains away and the phospholipids spontaneously arrange themselves into a bilayer (black lipid membrane or BLM). The aqueous solution on one side (cis side) of the bilayer... 

Similar inhibitory activity was observed for cobra venom phospholipase A2 and the mixture of bacterial alkylresorcinols in lecithin black lipid membrane and phospholipid emulsion systems. Almost complete inhibition (95%) of the enzyme studied was observed at a concentration of about 8 mM of resorcinolic lipids [351]. 

In black lipid membranes containing phospholipid In combination with hopanold, the mobility of a potassium Ion complex can he measured [48]. The mobility of this complex Is decreased by Increasing the molecular fraction of the hopanold. These experiments demonstrate viscosity enhancing property of hopanolds. 

For decades, colloid and surface scientists have known that amphiphilic molecules such as phospholipids can self-assemble or self-organize themselves into supramolecular structures of bilayer lipid membranes (planar BLMs and spherical liposomes), emulsions, and micelles [2-4]. As a matter of fact, our current understanding of the structure and function of biomembranes can be traced to the studies of these experimental systems such as soap films and Langmuir monolayers, which have evolved as a direct consequence of applications of classical principles of colloid and interfacial chemistry. As already mentioned in Section I, the seminal work on the self-assembly of planar lipid bilayers and bilayer or black lipid membranes was carried out in 1959-1963. The idea started while one of the authors was reading a paperback edition of Soap Bubbles by C. 

A biological membrane is a structure particularly suitable for study by the LB technique. The eukaryotic cell membrane is a barrier that serves as a highway and controls the transfer of important molecules in and out of the cell (Roth etal., 2000). The cell membrane consists of a bilayer or a two-layer LB film (Tien etal, 1998). Lipid bilayers are composed of a variety of amphiphilic molecules, mainly phospholipids and sterols which in turn consist of a hydrophobic tail, and a hydrophilic headgroup. The complexity of the biomembrane is such that frequently simpler systems are used as models for physical investigations. They are based on the spontaneous self-organization of the amphiphilic lipid molecules when brought in contact with an aqueous medium. The three most frequently used model systems are monolayers, black lipid membranes, and vesicles or liposomes. 

Mueller et al. [6] discovered in 1962 that when a small quantity of a phospholipid (2% wt/vol alkane solution) was carefully placed over a small hole (0.5 mm) in a thin sheet of Teflon or polyethylene (10-25 pm thick), a thin film gradually forms at the center of the hole, with excess lipid flowing towards the perimeter (forming a Plateau-Gibbs border ). Eventually, the central film turns optically black as a single (5 nm-thick) bilayer lipid membrane (BLM) forms over the hole. Suitable lipids for the formation of a BLM are mostly isolated from natural sources, e.g.,... 

The widespread interest in transport across membranes of living cells has led to studies of diffusion in lyotropic liquid crystals. Biological membranes are generally thought to contain single bimolecular leaflets of phospholipid material, leaflets which are like the large, flat micelles of lamellar liquid crystals. No effort is made here to review the literature on transport either across actual cell membranes or across single bimolecular leaflets (black lipid films) which have often been used recently as model systems for membrane studies. Instead, experiments where lamellar liquid crystals have been used as model systems are discussed. 

Efforts to stabilize BLMs by the use of polymerizable lipids have been successful, but the electrochemical properties of these membranes were greatly compromised and ion channel phenomena could not be observed [21]. Microfiltration and polycarbonate filters, polyimide mesh, and hydrated gels have been used successfully as stabilizing supports for the formation of black lipid films [22-25] and these systems were observed to retain their electrical and permeability characteristics [24]. Poly(octadec-l-ene-maleic anhydride) (PA-18) was found to be an excellent intermediate layer for interfacing phospholipids onto solid substrates, and is sufficiently hydrophilic to retain water for unimpeded ion transfer at the electrode-PA-18 interface [26]. Hydrostatic stabilization of solventless BLMs has been achieved by the transfer of two lipid monolayers onto the aperture of a closed cell compartment however, the use of a system for automatic digital control of the transmembrane pressure difference was necessary [27]. 

To make these membranes, a suitable phospholipid, lipid or mixture of lipids is dissolved in an organic solvent (say n-decane) the mixture is gently brushed across a circular orifice in a piece of machined teflon, which itself forms a partition between the two sides of a chamber filled with suitable electrolyte (say 0.1 M NaCl). The diameter of the orifice is usually 1 or 2 mm, and over a few seconds the lipid-containing solution thins down, the excess lipid remaining as a torus around the hole, until a black lipid bilayer remains covering the orifice and separating two salt solutions. 

It turned out to be true that proteinoids, without any lipids, also form bimolecular membranes.Despite the fact that black proteinoid membranes are not as long-lived in the ultrathin state as phospholipid membranes, they last long enough to be examined. Those rich in hydrocarbon-rich amino acid side chains mostly display properties characteristic for BLMs. The same polymers are among those that most readily combine with lecithin. 

Fig. 5. A mode for the role of MTP in VLDL assembly. As newly synthesized apo B (soild black line) translocates across the ER membrane, a small, lipid-poor apo B-containing particle (small gray circle surrounded by solid line) is formed by the addition of some lipid, particularly phospholipid and unesterified cholesterol a role has been proposed for MTP in this process. Large, fully lipidated VLDL particles are formed by addition of more TG to the small apo B-containing particles in an MTP-independent process. The TG used in this step is proposed to be derived in an unidentified process from an apo B-free TG droplet that resides in the ER lumen. MTP is required for transfer of TG to this lumenal TG droplet, likely from the cytosolic TG pool.

Artificial lipid bilayer membranes can be made [22,23] either by coating an orifice separating two compartments with a thin layer of dissolved lipid (which afterwards drains to form a bilayered structure—the so-called black film ) or by merely shaking a suspension of phospholipid in water until an emulsion of submicroscopic particles is obtained—the so-called liposome . Treatment of such an emulsion by sonication can convert it from a collection of concentric multilayers to single-walled bilayers. Bilayers may also be blown at the end of a capillary tube. Such bilayer preparations have been very heavily studied as models for cell membranes. They have the advantage that their composition can be controlled and the effect of various phospholipid components and of cholesterol on membrane properties can be examined. Such preparations focus attention on the lipid components of the membrane for investigation, without the complication of protein carriers or pore-forming molecules. Finally, the solutions at the two membrane interfaces can readily be manipulated. Many, but not all, of the studies on artificial membranes support the view developed in the previous sections of this chapter that membranes behave in terms of their permeability properties as fairly structured and by no means extremely non-polar sheets of barrier molecules. 

Figure 12 Schematic view of a membrane. Proteins (shaded) are embedded in the membrane, which consists of the layers of lipids. Most of these, the phospholipids have a negatively charged phosphate group (black) in contact with water inside and outside the cell, and attached to two long hydrocarbon chains. The membrane proteins may consist of a sin e a-helix in the lipid, with external and internal domains, or more often, of a series of a-helices, joined by loops, as shown on the left.

From these results, one may wonder which role(s) acyl lipids may play in the outer monolayer of thylakoid membranes. Two lines of research have been recently pursued. In the first one, phospholipid or galactolipid depletion in the outer leaflet was found to alter markedly both the binding and the inhibitory properties of DCMU and atrazine in thylakoid membranes isolated from susceptible and resistant biotypes of Black Nightshade (Solanum nigrum). 178... 

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