Membrane phospholipid monolayers

Folded membranes were generated out of two phospholipid monolayers at the air/subphase interface [158]. A thin Teflon septum separated the two half-cells of the setup which was also made of Teflon. This septum, with an orifice 50-200 jim in diameter, was... 

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

A BLM can even be prepared from phospholipid monolayers at the water-air interface (Fig. 6.10B) and often does not then contain unfavourable organic solvent impurities. An asymmetric BLM can even be prepared containing different phospholipids on the two sides of the membrane. A method used for preparation of tiny segments of biological membranes (patch-clamp) is also applied to BLM preparation (Fig. 6.10C). 

Since its first important flowering in the hands of purely physical chemists, interest in monolayers at the air-water interface has waxed and waned with a frequency of roughly 25 years. The first resurrection of interest came from biochemistry, primarily during the 1955-65 decade as phospholipid monolayers were studied as models for the cell membrane (see, for example, Chapman, 1968). This is still a very productive field of biophysical research. 

Studies of Drug-Membrane Interactions using Phospholipid Monolayers... 

Several studies have been undertaken to measure the retention times of compounds on chromatography columns comprising immobilized liposomes or artificial membranes. Liposomes comprise aqueous compartments surrounded by lipid bilayers, whereas the artificial membranes comprise monolayers of phospholipids (Lundahl and Beigi, 1997). Interaction between the compound and the phospholipids present mimics the interaction of compounds with cell membranes. Retention... 

A very suitable method for measurement of the lateral diffusion of molecules adsorbed at the foam film surfaces is Fluorescence Recovery after Photobleaching (FRAP) ([491-496], see also Chapter 2). Measurements of the lateral diffusion in phospholipid microscopic foam films, including black foam films, are of particular interest as they provide an alternative model system for the study of molecular mobility in biological membranes in addition to phospholipid monolayers at the air/water interface, BLMs, single unilamellar vesicles, and multilamellar vesicles. 

In vivo, the folding process may be supported by a periplasmic chaperone called Skp. Skp is a 17 kDa protein associated with the plasma membrane that, together with peptidyl prolyl isomerases and disulfideexchanging enzymes, helps folding freshly synthesized proteins in the periplasm (Schafer et al., 1999). Skp binds to partially unfolded polypeptides. Depending on the presence of phospholipids, lipopolysaccharides, and bivalent cations, Skp exists in two conformations, one of which is protease-sensitive (DeCock et al., 1999). Moreover, it was shown that Skp binds to unfolded periplasmic proteins and inserts into phospholipid monolayers, corroborating its putative role as helper in folding and membrane insertion. 

In the study described here, VSFS spectra of a series of phospholipid monolayers at the CCU/water interface were acquired and the effect of a common inhaled anesthetic, halothane (CFsCHClBr), on their conformation was examined [62]. Phospholipid mono-layers at an organic/aqueous interface are a useful model system for the study of cell membranes because they provide a realistic model of the hydrophilic and hydrophobic environments commonly found in vivo. In addition, the understanding and thermodynamics of these monolayer systems has been extensively described and rigorous theoretical analyses have been reported [63]. In these studies, the CCI4/D2O interface was used for experimental convenience, and although this interface is non-biological, it does mimic the hydrophobic/aqueous interface found in many biological systems [64]. 

Ion transfer across phospholipid monolayers at liquid-liquid interfaces has been studied with the aim of elucidating the mechanism and kinetics of ion transport across a bilayer lipid membrane (BLM). The main advantage of using these systems is in the possibility of controlling the interfacial potential difference, which in the case of the BLM has to be inferred indirectly [141]. 

Fig. 1. General oil-droplet model of lipoproteins is presented for chylomicron, very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) structures. Apolipoproteins in the outer phospholipid membrane, designated by letters, are defined in Table II. The major differences between the lipoproteins are the size of the neutral lipid (triglyceride and esterified cholesterol) core, liquid composition in the core, and apolipoprotein composition. (E) Triglycerides, ( Q ) phospholipids, and ( -) esterified cholesterol are shown. Although not shown, unesterified cholesterol is found predominantly in the phospholipid monolayer.

These results show that the ability of these signal peptides to interact with phospholipid monolayers indeed correlates with their in vivo activity. The pressure increases due to the functional signal peptides (8—11 dyn/cm) are in the same range as those caused by proteins known to insert into monolayers (Bougis et al., 1981). In contrast, prothrombin, which binds only to the membrane surface, causes a pressure increase in a phospholipid monolayer of 1.9-2.3 dyn/cm (Mayer et al., 1983). These values are almost identical to those obtained for perturbation of the monolayer by the deletion-mutant signal peptide. 

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