Bimolecular thick membranes

The phenomena of association colloids in which the limiting structure of a lamellar micelle may be pictured as composed of a bimolecular leaflet are well known. The isolated existence of such a limiting structure as black lipid membranes (BLM) of about two molecules in thickness has been established. The bifacial tension (yh) on several BLM has been measured. Typical values lie slightly above zero to about 6 dynes per cm. The growth of the concept of the bimolecular leaflet membrane model with adsorbed protein monolayers is traceable to the initial experiments at the cell-solution interface. The results of interfacial tension measurements which were essential to the development of the paucimolecular membrane model are discussed in the light of the present bifacial tension data on BLM. 

The bimolecular lipid membrane (BLM) produced in aqueous solution described in this paper is of considerable interest for two main reasons. First, the BLM is a new type of interfacial film of ultrathinness. The limiting thickness of BLM is 40-130 A. as estimated from various measurements. The values obtained by optical methods are probably most reliable, indicating that the thickness of the BLM is equal to about twice the length of the lipid molecules. The environment in which the BLM is formed and the molecular orientation at the biface lends itself as a promising tool in understanding some outstanding problems in colloid and interfacial chemistry such as Van der Waals attraction and... 

Chemical characterization of the bimolecular thickness of solventless BLMs can be derived from the action of gramicidin at micromolar concentration levels. This peptide can transport cations through the membrane by forming ion channels. Gramicidin has no electrochemical effect on membranes that are thicker than one bilayer [1,7,11], since the length of the gramicidin molecule is not sufficient to span a distance greater than that associated with the hydrocarbon zone of a BLM. 

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

A solution of brain lipids was brushed across a small hole in a 5-ml. polyethylene pH cup immersed in an electrolyte solution. As observed under low power magnification, the thick lipid film initially formed exhibited intense interference colors. Finally, after thinning, black spots of poor reflectivity suddenly appeared in the film. The black spots grew rapidly and evenutally extended to the limit of the opening (5, 10). The black membranes have a thickness ranging from 60-90 A. under the electron microscope. Optical and electrical capacitance measurements have also demonstrated that the membrane, when in the final black state, corresponds closely to a bimolecular leaflet structure. Hence, these membranous structures are known as bimolecular, black, or bilayer lipid membranes (abbreviated as BLM). The transverse electrical and transport properties of BLM have been studied usually by forming such a structure interposed between two aqueous phases (10, 17). 

BLM bimolecular, bilayer, or black lipid membrane (or film). Not all BLM are bimolecular in thickness, however. 

The bulk constituent of cells is water (H20). The cell membrane or plasma membrane (PM) that encloses the living cell is basically composed of a phospholipid bilayer, a 0.01 micrometre ( xm) (10 nm) thick bimolecular layer of hydrophobic (or water repelling) fatty molecules. In eukaryotes (organisms having a nucleus) there is a phospholipid bilayer PM enclosing the cell. Similar membranes bound specialized intracellular organelles, namely the endoplasmic reticulum (ER), ER-associated Golgi vesicles, lysosomes, vacuoles, peroxisomes, nucleus and mitochondria (and, additionally, the chloroplasts in plant cells). 

The primary structure of the cell membrane, shown in Fig. 3 is a 5-nm thick bimolecular lipid film that separates intracellular and extracellular fluids. The lipid is composed mainly of the phospholipids phos-phatidylserine and phosphatidylinositol, and contains saturated and unsaturated fatty acids and sterols. The bilayer exhibits high permeability to hydrophobic molecules and low permeability to hydrophilic molecules. 

As already mentioned, the generally accepted bimolecular leaflet model of the plasma membrane is that first proposed by Gorter and Grendel in 1925, which has dominated our thinking ever since [2]. Until then, our knowledge or the properties of bimolecular lipid layers was derived entirely from indirect experimental evidence. In 1961, the reconstitution of membranous structures from lipids of bovine brain was finally achieved [1-4]. These reconstituted membranes not only had a thickness ranging from 6 to 9 nm. 

FIGURE 1 Ion channels in the plasma membrane. The membrane phospholipids are arranged in a bimolecular layers with their polar heads on the outside and their hydrophobic tails inside. The bilayer is about 30 A thick. Sitting in it are various intrinsic proteins, including the channels shown here. 

The presence of mutually repulsive electrostatic forces between bimo-lecukr suface membranes is established. However, weak van der Waals forces arising between neutral atoms in the bimolecular membrane are attractive. Of interest in this respect, is the distance relationship that exists between van der Waals and electrostatic forces between two adjacent membranes. It is known that the van der Waals forces will fall oflF as the inverse square of the distance, while the electrostatic force will vary exponentially with the distance between the membranes. The electrostatic double layer (a repulsive force) is of definite thickness and is dependent on ionic strength. At low surface potentials, this thickness is represented by the Debye-Hiickel equation ... 

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