Lipid monolayers phase separation

Two experimental observations led to the conclusion that these lipids were phase-separated in the monolayer condensed state. First, the mean molecular area (Am) varied linearly with the mole fraction of DSPE in the monolayer. Moreover, the data were fit well with a straight line which connected the Am values for the individual lipids, indicative of ideal behavior [34]. These data show that the lipids were either completely phase separated or ideally mixed. The... 

Mixing fatty acids with fatty bases can dissolve films as the resulting complexes become water-soluble however, in some cases the mixed Langmuir film is stabilized [128]. The application of an electric field to a mixed lipid monolayer can drive phase separation [129]. 

It has been shown by FM that the phase state of the lipid exerted a marked influence on S-layer protein crystallization [138]. When the l,2-dimyristoyl-OT-glycero-3-phospho-ethanolamine (DMPE) surface monolayer was in the phase-separated state between hquid-expanded and ordered, liquid-condensed phase, the S-layer protein of B. coagulans E38/vl was preferentially adsorbed at the boundary line between the two coexisting phases. The adsorption was dominated by hydrophobic and van der Waals interactions. The two-dimensional crystallization proceeded predominately underneath the liquid-condensed phase. Crystal growth was much slower under the liquid-expanded monolayer, and the entire interface was overgrown only after prolonged protein incubation. 

This review emphasizes an intriguing and potentially useful aspect of the polymerization of lipid assemblies, i.e. polymerization and domain formation within an ensemble of molecules that is usually composed of more than one amphiphile. General aspects of domain formation in binary lipid mixtures and the polymerization of lipid bilayers are discussed in Sects. 1.1 and 1.2, respectively. More detailed reviews of these topics are available as noted. The mutual interactions of lipid domains and lipid polymerization are described in the subsequent sections. Given the proper circumstances the polymerization of lipid monolayers or bilayers can lock in the phase separation of lipids, i.e. pre-existing lipid domains within the ensemble as described in Sect. 2. Section 3 reviews the evidence for the polymerization-initiated phase separation of polymeric domains from the unpolymerized lipids. 

Domain formation in binary mixtures of a polymerizable lipid and non-polymerizable lipid is well established for diacetylenic lipids. The rigid diacetylenic unit facilitates the formation of enriched domains in the condensed phase of monolayers or the solid-analogous phase of bilayers. Since diacetylenes polymerize most readily in solid-like states, most studies have focused on conditions that favor domain formation. Only in the case of a mixture of a charged diacetylenic lipid and a zwitterionic PC was phase separation not observed. Ringsdorf and coworkers first reported the polymerization of a phase-separated two-dimensional assembly in 1981 [33], Monolayer films were prepared from mixtures consisting of a diacetylenicPC (6) (Fig. 5) and a nonpolymerizable distearoyl PE (DSPE). 

The different behavior of 7 and 8 is probably due to the charged head group in 7. Phase separation to form enriched domains of this lipid in mixed monolayers would be inhibited by electrostatic repulsion. Interestingly, mono-layer films of 7 mixed with the biologically important molecule cholesterol did exhibit phase separation at all compositions provided the temperature was maintained below the Tm of 7. Presumably the significantly different shapes of the two molecules promotes the phase separation and overcomes the electrostatic barrier. 

The extensive studies of the behavior of mixed monolayers or bilayers of di-acetylenic lipids and other amphiphiles parallel to some degree the studies of dienoyl-substituted amphiphiles. Since the dienoyl lipids do not contain a rigid diacetylenic group in the middle of the hydrophobic chains, they tend to be miscible with other lipids over a wide range of temperatures and compositions. In order to decrease the lipid miscibility of certain dienoyl amphiphiles, Ringsdorf and coworkers utilized the well-known insolubility of hydrocarbons and fluorocarbons. Thus two amphiphiles were prepared, one with hydrocarbon chains and the other with fluorocarbon chains, in order to reduce their ability to mix with one another in the bilayer. Of course it is necessary to demonstrate that the lipids form a mixed lipid bilayer rather than independent structures. Elbert et al. used freeze fracture electron microscopy to demonstrate that a molar mixture of 95% DM PC and 5% of a fluorinated amphiphile formed phase-separated mixed bilayers [39]. Electron micrographs showed extensive regions of the ripple phase (Pb phase) of the DM PC and occasional smooth patches that were attributed to the fluorinated lipid. In some instances it is possible to... 

A detailed analysis of the effect of mixed monolayers of 15 and DMPC on the activity of phospholipase A2 was reported by Grainger et al. [53]. Monolayers composed of different ratios of DMPC and either 15 or primarily poly 5 were characterized by Langmuir isotherms and isobars. The phospholipse-A2-mediated hydrolysis of selected monolayer compositions was usefully employed to ascertain the effectiveness of the enzyme. Both 15 and polyl5 were resistant to hydrolysis. The DMPC hydrolysis was sensitive to its molecular environment in a manner that suggests the phase separation of the polyl5 from DMPC. Phospholipase A2 activity is known to be sensitive to the concentration of the hydrolytic products, i.e. the fatty acid and lysophospholipid. The effect of these reaction products of the activity of phospholipase A2 on mixed monolayers of nonpolymerizable lipids is the subject of a series of interesting studies which are beyond the scope of this review. Ahlers et al. reviewed some of this research [54],... 

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... 

Phase-separated monolayers and liposomes were characterized by R. Elbert1011 who synthesized saturated and polymerizable fluorocarbon amphiphiles (59, 60, 61) and investigated their mixing behavior with CH2-analogues and natural lipids. In these systems the fluorocarbon compounds are incompatible with hydrocarbon lipids in a wide range of compositions and tend to form domains of pure fluorocarbon and hydrocarbon amphiphiles. The domains can be visualized by freeze-fracture electron microscopy. 

Hydrophobically modified polybetaines combine the behavior of zwitterions and amphiphilic polymers. Due to the superposition of repulsive hydrophobic and attractive ionic interactions, they favor the formation of self-organized and (micro)phase-separated systems in solution, at interfaces as well as in the bulk phase. Thus, glasses with liquid-crystalline order, lyotropic mesophases, vesicles, monolayers, and micelles are formed. Particular efforts have been dedicated to hydrophobically modified polyphosphobetaines, as they can be considered as polymeric lipids [5,101,225-228]. One can emphasize that much of the research on polymeric phospholipids was not particularly focused on the betaine behavior, but rather on the understanding of the Upid membrane, and on biomimicking. So, often much was learnt about biology and the life sciences, but little on polybetaines as such. 

Shaikh, S.R., et ah. Lipid phase separation in phospholipid bilayers and monolayers modeling the plasma membrane, Biochim. Biophys. Acta, 2001, 1512(2), 317-328. 

Although most of the reported hexagonal phases are based on aggregates having a single curved surfactant layer (monolayers), a more complex type has been found in certain systems whose structure appears to be based on a hexagonal packing of cylinders formed by curved lipid bilayers that separate an inside and an outside of the same polarity [143]. 

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