Monolayers dipalmitoyl lecithin

Dipalmitoyl Lecithin—Cholesterol Monolayers. The average area per molecule in dipalmitoyl lecithin-cholesterol monolayers shows deviation at low surface pressures, whereas at 30 dynes per cm. it follows the additivity rule (Figures 8 and 9) (42). The surface pressure—area curve of dipalmitoyl lecithin monolayers is liquid-expanded up to 30 dynes per cm., whereas above this surface pressure it is relatively incompressible (42). Figures 10b and c represent the structures of the dipalmitoyl... 

Figure 8. Average area per molecule of dipalmitoyl lecithin-cholesterol monolayers at various surface pressures...

The optimum condensation at molecular ratios of 3 to 1 and 1 to 3 in egg lecithin-cholesterol monolayers and 1 to 1 in dipalmitoyl lecithin-cholesterol monolayers (42) do not imply complex formation between lecithin and cholesterol but rather suggest average geometrical arrangements of these molecules. 

Data on emulsion film formation from insoluble surfactant monolayer are rather poor. It is known, however, that such films can be obtained when a bubble is blown at the surface of insoluble monolayers on an aqueous substrate [391,392]. Richter, Platikanov and Kretzschmar [393] have developed a technique for formation of black foam films which involves blowing a bubble at the interface of controlled monolayer (see Chapter 2). Experiments performed with monolayers from DL-Py-dipalmitoyl-lecithin on 510 3 mol dm 3 NaCl aqueous solution at 22°C gave two important results. Firstly, it was established that foam films, including black films, with a sufficiently long lifetime, formed only when the monolayer of lecithin molecules had penetrated into the bubble surface as well, i.e. there are monolayers at both film surfaces on the contrary a monolayer, however dense, formed only at one of the film surfaces could not stabilize it alone and the film ruptured at the instant of its formation. Secondly, relatively stable black films formed at rather high surface pressures of the monolayer at area less than 53A2 per molecule, i.e. the monolayer should be close-packed, which corresponds to the situation in black films stabilized with soluble surfactants. 

Figure 5. Dependence of A7r (O) and T (X) of l-14C-acetyU p-casein (Cpl = 10r4 %) on the iri of dipalmitoyl lecithin monolayers at 20.5°C. The film pressure at which the two-dimensional condensation starts is shown by the vertical dashed line. The other conditions were as described in Figure 2.

T ecithin, one of the principal lipids in cell membranes, controls many important biological processes. Nevertheless, little is known about the structure of its films (1, 2, 3, 4). Electron micrographs now show remarkable properties for the thin film or monolayer of dipalmitoyl lecithin transferred quantitatively from a water surface. In many respects the water corresponds to the aqueous phases that bound cell membranes. 

A typical effect related to surface relaxations is obtained in measurements of ti-A isotherms of insoluble monolayers. In most of the measurements with spread amphiphiles there are differences between the curve for compression and expansion of the surface films. Usually this characteristic behaviour is described as hysteresis. One experimental example of a spread dipalmitoyl lecithin is shown in Figs 3.12. This phenomenon corresponds to one or more of these surface relaxations. 

Tomoaia-Cotijel, M., Zsako, J. and Chifu, E., 1981, Dipalmitoyl lecithin and egg lecithin monolayers at an air/water interface, Ann. Chim. Rome, 71 189. 

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 ... 

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. 

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). 

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