Amphiphiles synthetic phospholipids

In this chapter, we have surveyed a wide range of chiral molecules that self-assemble into helical structures. The molecules include aldonamides, cere-brosides, amino acid amphiphiles, peptides, phospholipids, gemini surfactants, and biological and synthetic biles. In all of these systems, researchers observe helical ribbons and tubules, often with helical markings. In certain cases, researchers also observe twisted ribbons, which are variations on helical ribbons with Gaussian rather than cylindrical curvature. These structures have a large-scale helicity which manifests the chirality of the constituent molecules. 

Brandenburg, K., Hawkins, L., Garidel, P., Andra, J., Muller, M., Heine, H., Koch, M.H.J., Seydel, U. Structural polymorphism and endotoxic activity of synthetic phospholipid-like amphiphiles. Biochemistry 43 (2004) 4039-4046. 

Phospholipids or similar water-insoluble amphiphilic natural substances aggregate in water to form bilayer liquid crystals which rearrange when exposed to ultrasonic waves to give spherical vesicles. Natural product vesicles are also called liposomes. Liposomes, as well as synthetic bilayer vesicles, can entrap substances in the inner aqueous phase, retain them for extended periods, and release them by physical process. 

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). 

Phospholipids are found in all living cells and typically constitute about half of the mass of animal cell plasma membranes (Cevc, 1992). The reason forthe variety of membrane lipids might simply be that these amphiphilic structures have in common the ability to arrange as bilayers in an aqueous environment (Paltauf and Hermetter, 1990). Thus, the use of endogenous phospholipids to form vesicles as drug carriers may have much less adverse effects in patients compared to synthetic drui carrier molecules. 

A large number of macromolecules possess a pronounced amphiphilicity in every repeat unit. Typical examples are synthetic polymers like poly(l-vinylimidazole), poly(JV-isopropylacrylamide), poly(2-ethyl acrylic acid), poly(styrene sulfonate), poly(4-vinylpyridine), methylcellulose, etc. Some of them are shown in Fig. 23. In each repeat unit of such polymers there are hydrophilic (polar) and hydrophobic (nonpolar) atomic groups, which have different affinity to water or other polar solvents. Also, many of the important biopolymers (proteins, polysaccharides, phospholipids) are typical amphiphiles. Moreover, among the synthetic polymers, polyamphiphiles are very close to biological macromolecules in nature and behavior. In principle, they may provide useful analogs of proteins and are important for modeling some fundamental properties and sophisticated functions of biopolymers such as protein folding and enzymatic activity. 

Stabilization of a lipid membrane onto a solid support by covalent attachment also provides the physical stability necessary for the development of practical sensors. An oriented membrane can be prepared by allowing self-assembly of individual amphiphilic molecules onto a solid surface through either the reaction of terminal silane moieties with a hydroxylated surface to form a silyl ether [33,34], or by the reaction of sulfur-terminated compounds (alkylthiols or disulfides) with gold surfaces [35,36]. A variety of species, both with and without polar head groups, have been deposited onto surfaces such as glass, quartz, silicon, and gold [37-39]. These include phospholipids, fatty acids, and fatty amines which were synthetically altered so as to contain either a silyl chloride or a thiol moiety at the terminus of the acyl chain [40]. Both monolayers... 

Finally we note that, apart from double-tailed surfactant molecules, vesicular aggregates can also be formed from a number of other building blocks, including nonionic amphiphiles to form niosomes, single-tailed surfactants and complex (co)polymers, polypeptides, and dendrimers. In this chapter we restrict our discussion to encapsulation processes by vesicles formed from synthetic surfactants and phospholipids. 

The rates of these hydrolysis reactions (Nu = H2O) can be compared with nucleophilic substitution by bromide ions (Nu = Br ). A kinetic study has been made of these reactions in the presence of vesicles formed from synthetic amphiphiles, phospholipids, and mixtures of both types of amphiphiles. °° Particular attention was paid to the effect of addition of n-dodecyl-j8-glucoside (C Glu) as a mimic for glycohpids. Kinetic data were... 

Due to interactions between water molecules and the hydrophobic phosphate groups of the phospholipids, the lipid bilayer closes in on itself. This process of liposome formation is spontaneous because the amphiphilic phospholipids self-associate into bilayers. Inihally liposomes were made of phospholipids from the egg yolk but now with advances in materials science, a variety of synthetic materials are being used to produce liposomes. 

Preparation and characterization of liposomes formed with natural phospholipids were well established. However, in using liposomes for simulation of enzymatic functions, especially in acid-base catalysis, difficulties would be encountered due to their chemicai and morphological instabilities. Thus, bilayer membranes composed of synthetic amphiphiles are more favorable candidates for enzyme mimics. For example, artificial vitamin Bg-dependent enzymes were constructed from catalytic bilayer membranes in combination with a bilayer-forming peptide lipid (10), a hydrophobic vitamin derivative (11), and metal ions (Fig. 5). The catalyst acts as an artificial aminotransferase, showing marked substrate specificity, high enantioselectivity, and turnover behavior for the transamination of a-amino acid with a-keto acids. In addition, the reaction fields provided by the catalytic bilayer membranes are suitable to establish multienzyme systems through functional ahgnments of artificial enzymes and natural ones in a sequential manner. 

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