Natural phospholipid molecules

New concepts for making blood compatible polymer materials have been proposed based on the characteristics of natural phospholipid molecules in plasma. It was considered that if a polymer surface possesses a phospholipid-like structure, a large amount of natural phospholipids in plasma can be adsorbed on the surface by their self-assembling character. Based on this idea, a methacrylate monomer with a phospholipid polar group, 2-methacryloyloxy ethyl phosphorylcholine (MFC), was designed and synthesized The polymers, composed of MFC and various alkyl methacrylates (as shown in Fig. 1) or styrene derivatives were prepared and their blood compatibility carefully evaluated ". Flatelet adhesion and activation were significantly suppressed on the surface of the MFC polymers when the MFC composition was above 30 mol%. These excellent... 

That vesicles and liposomes form at all is a consequence of the amphi-pathic nature of the phospholipid molecule. Ionic interactions between the... 

In order to determine whether these surfactant vesicles were of polymerized vesicle forms, a 25% V/V ethanol (standard grade) was added to the three year old sample solution. Alcohols are known (34) to destroy surfactant vesicles derived from natural phospholipids, however, synthetically prepared polymerized vesicles are stable in as much as 25% (V/V) alcohol addition. Photomicrographs shown in Figures 7c and 7d indicate that these vesicles partially retain their stability (being mesomorphic) and therefore are suspected to be polymerized surfactants. Whether surfactant molecules of these vesicles are single or multipla bonds in tail, or in head groups remains to be seen. 

For pure phosphatidylcholine bilayers, the orientation of the headgroup has been well characterized showing that headgroups are aligned approximately parallel to the bilayer surface. Because only one phosphorus with 100% natural abundance is contained in the phospholipid molecule, NMR has become an important tool to study the phospholipid headgroup structure and dynamics. ... 

The interactions obviously differed between the lipid bilayers and the natural membranes. Furthermore, cholesterol slightly hinders the drug partitioning into the liquid-crystalline bilayers, in agreement with several previous reports, and the drug molecules interact electrostatically with membrane proteins at the hydrophilic interface adjacent to the polar headgroups of the phospholipid molecules (7). 

Liposomes are characteristic hollow spherical aggregates or vesicles that form spontaneously when phospholipids are dispersed in water. This is a function of their low solubilities in both oil and water and results from the hydrophobic nature of the twin acyl tails and the strongly hydrophilic polar head group which are the main characteristics of phospholipids (Figure 9.1). As a result, phospholipid molecules, when dispersed in water form double layers where the phospholipids align themselves, tail-to-tail and head-to-head (Figure 9.5)... 

A decrease in occupied area of the head group results in an increase in packing density of the molecules (45) exhibits only an expanded phase, (46) both a liquid and a solid-like phase, and (47) forms only a condensed film. Monolayer properties of many natural phospholipids and synthetic amphiphiles are described in the literature37 38. Especially the spreading behaviour of diacetylenic phospholipids at the gas-water interface was recently described by Hupfer 120). 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

The hydrated nature of amino acid residues lining the porin channels presents an energetically unfavourable barrier to the passage of hydrophobic molecules. In rough strains, the reduction in the amount of polysaccharide on the cell surface allows hydrophobic molecules to approach more closely the surface of the outer membrane and cross the outer membrane lipid bilayer by passive diffusion. This process is greatly facilitated in deep rough and heptose-less strains which have phospholipid molecules on the outer face of their outer membranes as well as on the inner face. The exposed areas of phospholipids favour the absorption and penetration of the hydrophobic agents. 

When natural phospholipids are the surfactant, the formed vesicles are termed liposomes. They are made of fragmented phospholipid bilayers in aqueous solution, and closed liposome structures encapsulate some aqueous solution within. Lipids are natural surfactants having two hydrocarbon tails per molecule and they behave similarly to synthetic surfac-... 

Lipid transfer proteins have proved to be a useful tool for studying artificial and natural membranes (for a recent review see Bloj and Zilver-smit, 1981a). With the ability of phospholipid transfer proteins to replace selectively the phospholipid molecules on the exposed surfaces of membranes, information about the asymmetric distribution of phospholipids across a bilayer and the rate of transbilayer movement of phospholipid... 

Supported bilayer lipid membranes (s-BLMs) have been prepared from mixtures of natural phospholipids and synthetic amphiphatic molecules. Three classes of compounds have been immobilized into these s-BLMs as follows ... 

Protrusion Model for the Hydration Force. Recently Is-raelachvili and Wennerstrom (IW) proposed that the origin of the hydration force is due to the head-group protrusion of the phospholipid molecules into the solvent region (27), and, therefore, the force is more akin to a steric force acting between polymer-covered surfaces. Because such an explanation of the nature of the hydration force does not involve electrostatic concepts, we will not present the IW theory here. A detailed description of a protrusion model is available in a recent review by Israelachvili and Wennerstrom (28), and a critique of IW theory of protrusions can be found in Parsegian and Rand (29). 

Lipids. After 15 years of systematic measurement, interactions between amphiphilic assemblies, particularly between phospholipid bilayer membranes, are as well elaborated as for any class of materials (for a recent review, see reference 6). By their very nature, amphiphilic molecules self-assemble in water into structures that separate polar group-water compartments from hydrophobic compartments. Aqueous spaces in these assemblies swell and shrink under the combined influence of all the forces that stabilize the molecular assemblies as well as those that determine their association. Because the subject has been so thoroughly reviewed, we need mention only general points here. 

The BLM vesicles are usually prepared from amphiphiles with two long alkyl chains. This leads to 32 or so CH goups per molecule, which is enough to produce the desired insolubility. Typical examples are natural phospholipids (see Table 2.2.3) and dimethyldioctadecylammonium bromide (DODAB). The inner... 

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