Lecithins, surface pressure area isotherms

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 ...

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. 

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. 

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...

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