Lecithins, surface pressure area

Influence of subphase temperature, pH, and molecular structure of the lipids on their phase behavior can easily be studied by means of this method. The effect of chain length and structure of polymerizable and natural lecithins is illustrated in Figure 5. At 30°C distearoyllecithin is still fully in the condensed state (33), whereas butadiene lecithin (4), which carries the same numEer of C-atoms per alkyl chain, is already completely in the expanded state (34). Although diacetylene lecithin (6) bears 26 C-atoms per chain, it forms both an expanded and a condensed phase at 30°C. The reason for these marked differences is the disturbance of the packing of the hydrophobic side chains by the double and triple bonds of the polymerizable lipids. At 2°C, however, all three lecithins are in the condensed state. Chapman (27) reports about the surface pressure area isotherms of two homologs of (6) containing 23 and 25 C-atoms per chain. These compounds exhibit expanded phases even at subphase temperatures as low as 7°C. 

Figure 5. Surface pressure area isotherms of polymerizable and natural lecithins at 30°C (34j. Key ...

Figure 10. Surface pressure-area and surface potential-area plots for 1,2-dimyristol-3-lecithin, trilaurin, and a 1 to 1 molar mixture at 5°C.

In the present study various surface measurements were made on interfacial lipid films to analyze in more detail the nature of interactions of lipids, Ca2+, ATP, and the glycolate esters. Surface pressure-area and surface potential-area diagrams were obtained on surface films of stearic acid, lecithin, and a mixture of brain lipids with Ca2+, ATP, and the drugs present in the subsolution, individually or in combination. Using radioactive Ca2+, ATP, and drug, quantitative measurements were made on the surface adsorption of these substances to lipid films. In addition, electron microscopy was performed on brain lipid films formed in the presence or absence of Ca2+ and ATP. Our objective was to establish the existence of surface complexes involving these substances with the hope... 

Lecithin Monolayers. We have shown from surface pressure—area and surface potential-area curves of various lecithins that the molecular area increases and the interaction with metal ions decreases with increasing unsaturation of the fatty acyl chains (41, 43). 

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). 

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... 

In contrast to this, the system neutral lipid (2J)/DSPC shows considerably smaller deviations from the additivity rule and the surface pressure/area isotherms indicate two collapse points corresponding to those of the pure components62. Photopolymerization can be carried out down to low monomer concentrations and no rate change is observed. These facts prove that the system (23)/DSPC is immiscible to a great extent. The same holds true for mixed films of diacetylenic lecithin (18, n = 12) with DSPC, as well as for dioleoylphosphatidylcholine (DOPC) as natural component. 

Representative surface pressure/area per molecule isotherms from monolayers of distearoyl lecithin at the interface between 0.1M NaCl and cyclohexane, n-heptane, and isooctane at 20 °C and n-nonane and isooctane at 3°C are shown in Figure 1. Two completely independent isotherms which were actually determined some months apart for the n-heptane/O.lM NaCl interface are plotted to illustrate the precision and reproducibility of the method and the data. Quite clearly the area and surface pressure at which phase separation begins depend on the hydrocarbon component of the oil/water interfacial system. The areas and surface pressures at which phase separation occurs for these and the other solvents which have been investigated are summarized in Table I. 

Figure 8. Surface pressure-area, surface potential-area, and surface viscosity (At)-area curves of sphingomyelin ( ), galac-tocerebroside (O), and dipalmitoyl lecithin (A) on 0.I5M NaCl...

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

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

Egg Lecithin—Cholesterol Monolayers. The average area per molecule in egg lecithin-cholesterol monolayers shows deviation from the additivity rule at all surface pressures (42). The deviation in this case could be explained by the presence of molecular cavities caused by the kink in the oleoyl chain of egg lecithin, which would reduce the average area per molecule at low as well as high surface pressures (Figure lOg). 

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. 

Cholesterol and lecithin form completely miscible solutions in mono-layers at very low surface pressures, characterized by excess positive heats and excess negative areas of mixing. At elevated surface pressures, phase separation occurs. Since these solutions conform to regular solution theory, the hydrocarbon domain of the monolayer makes the major contribution to the heats of mixing. The polar region of the monolayer may... 

The mixed films, CTAB (1) + egg-lecithin (2), were treated as two-dimensional mixtures of known composition. From the average molecular areas at a given surface pressure, we deduced the partial molecular areas di and a2 at the same pressure (2) using the classical Bakhuis-Rooseboom method. The pressure of the mixed films as a function of the partial molecular areas is shown in Figure 2. Also shown are the isotherms of the pure components 1 and 2 and of the 1/1 mixed real film. 

Even at the highest pressure studied, 32 dynes/cm (Figure 2), the partial area of CTAB is much larger than the area of CTAB in the pure monolayer. Therefore, lecithin molecules, by interacting with CTAB molecules, determine their orientation or spreading and produce the variation An of the surface pressure. 

Monolayers of distearoyl lecithin at hydrocarbon/water interfaces undergo temperature and fatty acid chain length dependent phase separation. In addition to these variables, it is shown here that the area and surface pressure at which phase separation begins also depend upon the structure of the hydrocarbon solvent of the hydrocarbon oil/aqueous solution interfacial system. Although the two-dimensional heats of transition for these phase separations depend little on the structure of the hydrocarbon solvent, the work of compression required to bring the monomolecular film to the state at which phase separation begins depends markedly upon the hydrocarbon solvent. Clearly any model for the behavior of phospholipid monolayers at hydrocarbon/water interfaces must account not only for the structure of the phospholipid but also for the influence of the medium in which the phospholipid hydrocarbon chains are immersed. 

Table I. Area and Surface Pressure at which Phase Separation Begins in Distearoyl Lecithin Films at Various Hydrocarbon /Aqueous NaCl Interfaces...

Figure 1. Surface pressure (ir)- molecular area (A) isotherms for dibehenoyl lecithin (A), dipalmitoyl lecithin (B), and egg yolk lecithin (C) on phosphate buffer (pH 7,1 = 0.1) at room temperature...

It can be assumed that the area per molecule represents the area of a square at the interface. Thus, the square root of the area per molecule gives the length of one side of the square, which represents the intermolecular distance. Figure 2 schematically illustrates the area per molecule and intermolecular distance in these four lecithins. The corresponding intermolecular distances were calculated to be 9.5, 8.8, 7.1, and 6.5 A, respectively, at a surface pressure of 20 mN/m [2]. Thus, one can conclude that a change in the saturation of the fatty acid chains produces subangstrom changes in the intermolecular distance in the monolayer. 

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