Crystalline phase monolayers, phospholipid

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]. 

An extensive analysis of the polymorphism in phospholipid monolayers has been reported by Albrecht et al. (1978) who established a phase diagram of dipal-mitoyl-lecithin monolayers involving four phases. Two of these have the liquid-type of disordered chains (liquid-expanded films). It was proposed that one has disordered chains oriented perpendicular to the water surface and the other has disordered chains in a tilted arrangement. Furthermore two crystalline phases were described, one with tilted chains and the other with perpendicular chains. 

The spectroscopic and thermodynamic data presented above suggest a correlation between molecular order, interfacial packing and the gel to liquid crystalline phase transition temperature. Aqueous phospholipid vesicles above their transition temperature readily form tightly packed, well-ordered monolayers at a water/CCU interface (e.g. DLPC). Vesicles below their transition temperature possess greater stability, forming monolayers which are considerably expanded and which show greater disorder (i.e. DPPC and DSPC). 

Vibrational sum frequency spectroscopy in conjunction with interfacial pressure measurements provide the first direct information about the structure of phospholipid monolayers composed of DLPC, DMPC, DPPC, and DSPC, adsorbed to the interface between two immiscible liquids. Temperature controlled experiments carried out with aqueous solutions of DSPC show the lipid bilayer gel to liquid crystalline phase transition temperature to play a pivotal role in determining interfacial monolayer concentration and alkyl chain structure. Even at equivalent interfacial concentrations longer chain phosphocho-line species form more disordered monolayers with a greater number of gauche defects than shorter chain phosphocholine species, as determined from relative intensities of vibrational bands in the CH stretching region. 

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. 

Up until 1977, the non-covalent polymeric assemblies found in biological membranes rarely attracted any interest in supramolecular organic chemistry. Pure phospholipids and glycolipids were only synthesized for biophysical chemists who required pure preparations of uniform vesicles, in order to investigate phase transitions, membrane stability and leakiness, and some other physical properties. Only very few attempts were made to deviate from natural membrane lipids and to develop defined artificial membrane systems. In 1977, T. Kunitake published a paper on A Totally Synthetic Bilayer Membrane in which didodecyl dimethylammonium bromide was shown to form stable vesicles. This opened the way to simple and modifiable membrane structures. Since then, organic chemists have prepared numerous monolayer and bilayer membrane structures with hitherto unknown properties and coupled them with redox-active dyes, porous domains and chiral surfaces. Recently, fluid bilayers found in spherical vesicles have also been complemented by crystalline mono-... 

Cholesterol crystallisation is thought to be the first step in the formation of gallstones in the human biliary system and the process of cholesterol nucleation remains incompletely understood. GIXD revealed a phase transition from a monolayer to a highly crystalline rectangular bilayer phase (165). The presence of the phospholipid DPPC in the cholesterol film inhibited cholesterol crystallisation [165]. AFM provided complementary information on the thickness and morphology of the cholesterol films transferred to a solid support The cholesterol monolayer thickness was 13 2 A and in the bilayer phase the presence of elongated faceted crystallites of pure cholesterol about 10 layers thick could be observed [165]. 

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