Lipid adsorbed monolayer

Figure 9.4 A schematic diagram showing the process of self-assembly for a lipid bilayer on a freshly cleaved metal surface step 1, a metal wire being cut under a lipid droplet with a sharp blade forming an adsorbed monolayer of lipid step 2, upon immersion of the lipid-coated wire tip into aqueous solution, a self-assembled BLM is formed (see [10], [11], [28], and [29] for details).

The molecular alignment of liquid crystals on solid surfaces is not only of fundamental interest in physics [1] but is also relevant for practical applications, for example in optoelectronic devices. In liquid crystal displays the molecules are confined between two surfaces. To minimize the number of defects, surfaces are favored that induce a high degree of orientation of the molecules. Different surface treatments are used to induce and control the orientation of the molecules. A homeotropic (perpendicular) alignment is favored on hydrophobic surfaces that are rough on a molecular level. This is observed in adsorbed monolayers of a surface-active compound such as lipids or surfactant molecules on glass, both for energetic and sterical reasons. Surface modifications can alter the positional order and molecular orientation of liquid crystalline... 

It has been shown by FM that the phase state of the lipid exerted a marked influence on S-layer protein crystallization [138]. When the l,2-dimyristoyl-OT-glycero-3-phospho-ethanolamine (DMPE) surface monolayer was in the phase-separated state between hquid-expanded and ordered, liquid-condensed phase, the S-layer protein of B. coagulans E38/vl was preferentially adsorbed at the boundary line between the two coexisting phases. The adsorption was dominated by hydrophobic and van der Waals interactions. The two-dimensional crystallization proceeded predominately underneath the liquid-condensed phase. Crystal growth was much slower under the liquid-expanded monolayer, and the entire interface was overgrown only after prolonged protein incubation. 

Step 2 - A patch pipet is removed from the solution, the polar head groups of the monolayer lipids are adsorbed to the interface while the fatty acid hydrophobic tails are exposed to the air ... 

An analogous apparatus to that of Ref. 9 was used to follow the effect of the lipid monolayer on the rate of electron transfer (ET). In this setup [47], an organic phase droplet (1,2-DCE) is continuously expanded into the aqueous phase, and the resulting current transient was monitored in the absence and presence of the adsorbed lipid mono-layer. The rate of ET was decreased as a function of the lipid concentration. 

In order to verify that the adsorbed lipid membrane indeed forms a bilayer film, another experiment is conducted with an aim to detect the formation of a monolayer lipid. It starts with a piranha-cleaned micro-tube treated with silane to render its inner surface hydrophobic. POPC liposome is then injected into the microtube. It is known that POPC lipid will form a monolayer to such a surface by orienting their hydrophobic tails toward the hydrophobic wall. The experimental results using a mode with similar sensitivity as the previous experiment are shown in Fig. 8.39. The resonance shift in this case is 22 pm, which is about half of that observed for the adsorption of a lipid bilayer. These two experiments suggest that the microtube resonator is capable of accurately determining an adsorbed biomolecular layer down to a few nm thicknesses. 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

Since Upids are known to associate with DNA with high affinity, the adsorption of ssDNA at lipid membranes as a medium for DNA incorporation on a GC surface was extensively studied [60]. Exploiting DNA-Upid interactions, various approaches were designed for the incorporation of ssDNA [61] and dsDNA [62] at a modified bilayer lipid membrane (BLM) GC surface, such as (1) the formation of self-assembled BLMs over ssDNA previously adsorbed on GC, (2) the direct adsorption of ss- and dsDNA [62] into a previously BLM-modified GC and, (3) formation of a BLM with incorporated ssDNA at the GC surface using the monolayer folding technique [61]. 

Cationic amphiphiles 2Ci8-glu-N spread on pure water, in the solution of 10 xM DNA containing 10 xM intercalating dyes (proflavine). The dye-intercalated DNA anions were expected to adsorb to the cationic lipid mono-layer due to electrostatic interactions and was transferred to a hydrophobized glass plate at a surface pressure of 35 mN m at 20 °C. From a moving area of a barrier, two layers of the monolayer were confirmed to be transferred in each one cycle (Y-type deposition). When the QCM plate was employed as a transfer plate, the transferred mass could be calculated from frequency decreases (mass increase on the QCM) [29-31]. It was confirmed that 203 10 ng of two lipid monolayers and 74 5 ng of DNA strands were transferred on to the substrate per dipping cycle, which means ca. 95% of the monolayer area was covered by DNA molecules. 

It has long been established that all cell membranes in the body are composed of a fundamental structure called plasma membrane. This boundary surrounds single cells such as epithelial cells. More complex membranes such as intestinal epithelium and skin, are composed of multiples of this fundamental structure, which has been visualized as a bimolecular layer of lipid molecules with a monolayer of protein adsorbed into each surface. Cell membranes are further interspersed with small pores that can be protein line channels through the lipid layer or, simply, spaces between the lipid molecules. In membranes composed of many cells, the spaces between the cells constimte another kind of membrane pores (2). 

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. 

To gain insight into the effect of physical state and/or molecular organization on lipid oxidation, a variety of model systems have been used. These include dispersions, liposomes or vesicles (37,38), monolayers adsorbed on silica (39,40,41), and red blood cell ghosts (42). In most of these studies, oxidation was conducted at relatively low temperatures, i.e., 20 - 40°C. Very little information is available on the effects of physical state on high temperature oxidative reactions or interactions of lipids. 

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

An even more striking comparison can be made between the wild-type signal peptide s conformation when adsorbed to the monolayer and its conformation in aqueous solution. In both of these environments, the peptide should be solvated by water, but its conformations are very different. The peptide is 100% )3 structure when adsorbed to the mono-layer, while it is 80% random in aqueous buffer. Thus, it appears that contact with the lipid surface induces substantial amounts of secondary structure in a molecule that takes on little structure in an aqueous environment. This finding implies that the initial binding of a signal sequence to a membrane may induce a particular structure, which may be important to the mechanism of signal-sequence function. 

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