Lecithin Lipid-water interface

Penetration of electrolytes into both the air-water interface and films of dipalmitoyl lecithin is accompanied by a relatively small surface potential increase, whereas hydrolysis of CaCl% produces accumulation of Ca(OH)t and related species at the interface (l). Although in the absence of ionic lipids a correlation between interfacial ionic populations of the electrolyte and the surface potential changes is not yet possible, the marked surface potential effects of CaCU accompanying the presence of small quantities of acidic phospholipids in dipalmitoyl lecithin films suggest that the acidic lipid contaminants are still the only certifiable species whose interaction with CaCl2 produces an appreciable surface potential increase. Surface radioactivity and IR absorption spectra of dipalmitoyl lecithin in the presence of CaCU produced no evidence of Ca -dipalmitoyl lecithin interaction. 

Figure 1. Schematic presentation of some polar lipids and their orientation at an air-water or oil-water interface, modified from Carey and Small (1970). (a) Lecithin, (b) lysolecithin, (c) ionized fatty acid, (d) bile salt, (e) cholesterol.

TiTuch of our understanding of the phase behavior of insoluble - monolayers of lipids at the air-water interface is derived from Adam s studies of fatty acid monolayers (I). It is now clear that the phase behavior of phospholipid monolayers (2) parallels that of the fatty acids we make use of these structure variations in our study of the interactions of phosphatidylcholine (lecithin) monolayers with proteins. Because of the biological significance of the interfacial behavior of lipids and proteins, there is a long history of studies on such systems. When Adam was studying lipid monolayers, other noted contemporary surface chemists were studying protein monolayers (3) and the interactions of proteins with lipid monolayers (4). The latter interaction has been studied by many so-called 4 penetration experiments where the protein is injected into the substrate below insoluble lipid monolayers that are spread on the... 

The effects of lecithin monolayers on Atr and r at different Cp s are shown in Figures 2 and 3. The lecithin monolayers were spread to initial film pressures (t ) of 10 dynes/cm so that the molecular areas for the dibehenoyl and egg yolk lecithins were about 50 and 88 A2/molecule. When Cp is less than 10 5 %, lecithin monolayers give larger values of Att than are observed at the clean air-water interface. In this region r is probably reduced, but the adsorption of protein is so limited that it approaches the limits of experimental error. However, when 10"5 < Cp < 4 X 10 3 %, the lecithin monolayers cause large reductions in Y and Att, and the condensed dibehenoyl lecithin film has the greater effect. The reduction in T is real and not an artifact caused by the lipid monolayer... 

Effect of Lipid Monolayers on Protein Adsorption. Originally, lipid-protein interaction at the interface was based on results obtained at low Cp s (< 5 X 10 6 % for /3-casein) where A7t is zero at the clean air-water interface but positive when an insoluble lipid monolayer is present. However, the data in Figures 2 and 3 for a large range of Cps indicate that under most conditions the presence of a lecithin monolayer inhibits protein adsorption rather than promoting it by the lipid-protein interaction. Because protein cannot occupy the same areas of the interface as the lipid molecples, the area of interface available to the protein is decreased (12). When this occurs at low T (Cp < 5 X 10"6 %), the resultant compression of the protein, rather than enhanced protein adsorption arising from lipid-protein interaction, leads to positive values of Att. 

The interaction of /3-casein molecules with lecithin monolayers is governed by the surface activity (hydrophobicity) of the macromolecule. Hydrophobicity does not guarantee interaction of a protein with lecithin in dispersion (22), but it favors penetration of proteins into monolayers at the air-water interface. The whole molecule seems to penetrate in the case of /3-casein (Figure 7) this leads to the compression of the lipid molecules and perhaps the formation of a layer of relatively restricted lecithin molecules around the periphery of the protein molecules. The situation is quite different for a very polar protein such as lysozyme where... 

J. Tinoco and D. J. McIntosh, Interactions Between Cholesterol and Lecithin in Monolayers at an Air-Water Interface, Chem. Phys. Lipids 4, 72-84 (1970). 

The absolute values of the interfacial tensions varied between different amphi-philes and solvents (Table 1). AOT, which is well known in the literature for the formation of microemulsions, showed the lowest surface tension at the interface of both solvents. The other nonionic snrfactants mentioned here. Span 80 and Brij 72 showed shghtly higher valnes. This was also observed for Lecithine, but this lipid precipitated partly during the spinning-drop measurements. Due to this phenomenon, it was not possible to measure accurate data for this emulsifying compound. The interfacial tension had also some influence on the mean size of the emulsion droplets and on the stability of the vesicles (Table 3). In addition to the stationary values of the surface tension, dynamic processes as the surfactant diffusion represented another important factor for the process of stimulated vesicle formation. If an aqueous droplet passed across the fluid interface it carried-over a thin layer of emulsifiers and thereby lowered the local surfactant concentration in the vicinity of the oil-water interface. In the short time span, before the next water droplet approached the interface, the surfactant films should entirely reform and this only occurred, if the surfactant diffusion was fast enough. 

One type of lipid that is dominant in biological interfaces is lecithin, and lecithin-water systems have therefore been examined extensively by different physical techniques. Small s binary system (3) for egg lecithin-water is presented in Figure 2. The lamellar phase is formed over a large composition range, and, at very low water content, the phase behavior is quite complex. Their structures as proposed by Luzzati and co-workers (4) are either lamellar with different hydrocarbon chain packings or based on rods both types are discussed below. 

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