Dioleoyl lecithin

The electrical potential across a LB film of dioleoyl-lecithin deposited onto a fine-pore membrane, imposed between equimolar aqueous solutions of NaCl and KC1, was reported to exhibit rhythmic and sustained pulsing or oscillations of electrical potential between the two solutions. These oscillations were attributed to the change of permeability of Na+ and K+ ions across the membrane, which originated from the phase transition of lecithin. 

On the basis of the interaction of metal ions and AV-pH and AV-log C plots (41, 43), we propose ionic structures for dioleoyl, egg, and di-palmitoyl lecithin monolayers represented in Figure 1. Schematically shown in Figure 1A is the internal salt linkage between the phosphate and trimethylammonium groups in dioleoyl lecithin, preventing the inter-... 

A Dioleoyl lecithin shows internal salt linkage between phosphate and trimethyl-ammonium groups. Broken lines in egg lecithin diagram represents weak interactions of phosphate with Ca2+ and trimethylammonium group. Solid line between Ca + and phosphate group in dipalmitoyl lecithin diagram represents strong interaction... 

Influence of Intermolecular Spacing on Enzymic Hydrolysis of Lecithin Monolayers. When snake venom phospholipase A is injected under a lecithin monolayer, it splits lecithin into lysolecithin and free fatty acid. The change in polar groups of the monolayer results in a change of surface potential. However, if prior to injection of enzyme into the subsolution, a lecithin monolayer is compressed to such a surface pressure that the active site of the enzyme is unable to penetrate the monolayer, hydrolysis does not proceed. For monolayers of dipalmitoyl, egg, soybean, and dioleoyl lecithins the threshold surface pressure values at which hydrolysis does not proceed are 20, 30, 37, and 45 dynes per cm., respectively (40). This is also the same order for area per molecule in their surface pressure-area curves, indicating that enzymic hydrolysis of lecithin monolayers is influenced by the unsaturation of the fatty acyl chains and hence the intermolecular spacing in monolayers (40). 

Similar reasoning accounts for the condensation of monolayers of dioleoyl lecithin by cholesterol reported by Van Deenen (48) (Figure lOh). 

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. 

Fig. 8. Thermograms of bilayers containing dioleoyl lecithin (DOL) and sphingomyelin (SM). There is clear phase separation between DOL (low melting point) and SM (high melting point). As the cholesterol content of the bilayers is increased step-wise to a 0.3 molar ratio, the sphingomyelin transition is suppressed in preference to the DOL transition.

Mercury electrodes coated with dioleoyl lecithin was used for the determination of polynuclear aromatic hydrocarbons (PAHs) and other micropollutants in aqueous solutions which penetrate the lecithin monolayer. The response is quantitatively related to the concentration of PAH in solution. The monolayer appears highly sensitive to the solution behavior of PAHs because only soluble unbound PAHs penetrate the monolayer. The PAH studied were anthracene, phenanthrene, pyrene, benzanthracene, fluroanthrene, and perylene. The penetration was mgnitored by measuring the differential capacitance of the mono-layer the uptake of PAH causes a potential shift (up to —0.25 V) in the cathodic capacitance peaks. This occurs without displacement of the lipid from the mercury [128]. 

During the past quarter century, considerable studies have been carried out on the reactions in monomolecular films of surfactant, or monolayers. Figure 1 shows the surface pressure-area curves for dioleoyl, soybean, egg, and dipalmitoyl lecithins [1]. For these four lecithins, the fatty acid composition was determined by gas chromatography. The dioleoyl lecithin has both chains unsaturated, soybean lecithin has polyunsaturated fatty acid chains, egg lecithin has 50% saturated and 50% unsatmated chains, and dipalmitoyl lecithin has both chains fully saturated. It is evident that, at any fixed surface pressure, the area per molecule is in the following order ... 

Dioleoyl lecithin > soybean lecithin > egg lecithin > dipalmitoyl lecithin... 

FIG. 1 Surface pressure-area curves of dipalmitoyl, egg, soybean, and dioleoyl lecithins. 

Monoolein will also form the cubic phase together with lecithin (e.g. dioleoyl phosphatidylcholine, see Figure 1), but above about 50% (w/w) lecithin the cubic phase is transformed into the lamellar phase (2). Moreover, water may be replaced by glycerol, completely or partly, in the cubic phase. Vegetable oils, e.g. sesame oil, can be incorporated to some extent (a few percent) in the cubic phase, and the same holds for bile salts. 

Figure I. Schematic of interaction of calcium ion with dioleoyl, egg, and di-palmitoyl lecithins, and of egg lecithin-cholesterol monolayers...

The infrared absorption spectra of unsaturated phospholipids, such as 2,3-dioleoyl-DL-phosphatidylethanolamines, at room temperature are similar to those observed with a liquid. Cooling unsaturated phospholipids, such as 2-oleoyl-3-stearoylphosphatidylcholine (lecithin), to lower temperatures enables a crystalline-type spectra to be obtained. This is in agreement with the DTA results, which show that the transition temperature for crystalline to liquid crystalline is lower with the unsaturated than with the saturated phospholipids. When this particular phospholipid is heated to about 40°C., the spectrum is replaced by one in which all the fine structure vanishes, and the spectrum is again typical of that observed with a liquid (5). 

FIG. 2 Schematic representation of the area per molecule and intermolecular distance in dioleoyl, soybean, egg, and dipalmitoyl lecithin monolayers based on the data plotted in Fig. 1. 

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