Chain interactions, phospholipid monolayers

Since the solidity or fluidity of the bilayer membrane is likely to depend on the alkyl chain interactions and consequently their length, an understanding of the relationship between chain order and chain length for tightly packed monolayers of phospholipids are important. As an example of how VSFS can be employed to study phospholipids at a liquid surface, a series of saturated symmetric chain phosphatidylcholines (PCs) were examined at the air/water and CCfr/water interfaces [49]. At the air/D20 interface, chain order within the monolayer was found to increase as the length of the chains increased (Figure 2.9a) under conditions of constant phospholipid head group area. 

Fig. 2. A model for lipoprotein structure based on the interactions between apolipopro-teins and lipid constituents. The surface monolayer is composed of phospholipids and apolipoproteins. The apoproteins contain helical regions which are amphipathic. The hydrophobic surface of the amphipathic helix interacts with the fatty acyl chains of phospholipids, and the hydrophilic surface is exposed to the aqueous environment of the polar head groups and the plasma. Adapted from Pownall et al.. (1981).

Physical Methods. H.p.l.c. procedures for the separation and assay of ubiquinone and homologues278-280 and of menaquinone cis- and fra/ts-isomers, 2,3-epoxides, and chain-length homologues281 282 have been described. A XH n.m.r. study has been reported283 of the location and motion of ubiquinones in perdeuteriated phosphatidylcholine bilayers. Other aspects of the interaction of ubiquinone with phospholipid monolayers have been studied.284... 

It has been mentioned previously that sterols were readily incorporated into lecithin or mixed phospholipid monolayers [194—196]. The area of sterol-containing films was smaller than that calculated for the separate components, indicating some kind of interaction between the two molecules [221,222]. Sterols have also been shown to reduce the phase transitions of pure lecithin dispersions [214]. It has been suggested that cholesterol modifies the fluidity of the hydrocarbon chains of the phospholipid molecules by disrupting the crystalline chain lattice of the gel phase and by inhibiting the flexing of the chains in the dispersed liquid-crystalline phase [221—226]. 

The amino groups of the DAB residues in the cyclic polypeptide portion of the polymyxin molecule are necessary for the antimicrobial activity, which is reduced on acetylation [142]. Removal of the side chain from the molecule also reduces the activity by about 70% [143]. Various techniques have been used to probe the molecular interaction between polymyxin and the membrane components. Few [144] showed that polymyxin interacts with monolayers prepared from lipids and phospholipids of bacterial origin. Penetration of the mono-layers was followed by an increase in surface pressure, and polymyxin E was... 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

At the nitrobenzene-water interface a dipalmitoylphosphatidylcholine monolayer undergoes the phase transition at 13 °C, while at the air-water interface the phase transition temperature is 43 °C. This difference demonstrates weaker lateral interaction between phospholipid side chains at the nitrobenzene-water interface and suggests the penetration of nitrobenzene molecules in the hydrocarbon chain part of the monolayer (Fig. 9) [99, 100]. 

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