Structures lipid assemblies

These are also just as important as the biological syntheses of proteins if a holistic view of biological macromolecule and lipid assembly structures and functions is to be obtained. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

The most fascinating characteristic some amphiphile molecules exhibit is that, when mixed with water, they form self-assembly structures. This was already discussed in Chapter 2 on micelle formation. Since most of the biological lipids also exhibit self-assembly structure formation, this subject has been given much attention in the literature (Birdi, 1999). Lipid monolayer studies thus provide a very useful method to obtain information about SAM formation, both concerning technical systems and cell bilayer structures. 

A certain type of lipid (or lipid-like) molecules are found that when dispersed in water tend to make self-assembly structures (Figure 4.13). Detergents were shown to aggregate to spherical or large cylindrical-shaped micelles. It is known that if egg phosphatidylethanolamine (egg lecithin) is dispersed in water at 25°C, it forms a self-assembly structure, which is called liposome or vesicle. 

Macro domains of micrometer size have been reported when using an ordinary microscope. The typical procedure is to use 2% of a fluorescent lipid analog, which renders visible a domain pattern. This, of course, assumes that the fluorescence moiety has no effect on the assembly structure. There is (generally) no information about the thickness of the domains. The shapes of domains are varied, and very complex (circular or near-circular domains, parallel stripes, or more irregular, wormlike, or similar kinds of structures). 

In the past decades, it has become more and more obvious that students and scientists of chemistry and engineering should have some understanding of surface and colloid chemistry. The textbooks on physical chemistry tend to introduce this subject insufficiently. Modern nanotechnology is another area where the role of surface and chemistry is found of much importance. Medical diagnostics applications are also extensive, where both microscale and surface reactions are determined by different aspects of surface and colloid chemical principles. Drug delivery is much based on lipid vesicles (self-assembly structure) that are stabilized by various surface forces. 

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

The synthesis of active compounds of the lipid A class has received considerable attention recently. This attention is reflected in the four chapters in the last section of this volume. Each chapter presents a distinctive approach to the assembly of the fundamental lipid A structure, comprising a glucosaminyl disaccharide substituted with N- and O-fatty acyl and Ophos-... 

Kleinschmidt JH, Tamm LK (2002) Structural transitions in short-chain lipid assemblies studied by P-31-NMR spectroscopy. Biophys J 83 994—1003... 

Safinya CR, Koltover I (1999) Self assembled structures of lipid-DNA nonviral gene delivery systems from synchrotron X-ray diffraction. In Huang L, Hung M-C, Wagner E (eds) Nonviral vectors for gene therapy. Academic, San Diego... 

Hierarchic design of assembled structures should be important for preparation of functional mesoscopic structures. Typical examples can be seen in biological systems where tissues of organisms in living systems consist of assemblies of cells with cell membranes composed of self-assembled lipids, proteins, saccharides, etc. Therefore, tissues and organisms can be regarded as (at least) two-level assemblies lipid to... 

Another use of the DNA-lipid complex was reported by Wong and coworkers [33]. The DNA molecule structural features can be imprinted into CdS nanostructures by using self-assembled DNA-membrane templates. The initial application of the DNA-lipid multilamellar structure was the use as gene carriers in gene therapy, and many reports [34-39] were published (Fig. 6b). To prepare the DNA templates, anionic DNA was reacted with a mixture of... 

Line shape analysis of the static 31P NMR spectra and its corresponding CSA values have been successfully used to study the perturbation effect induced by proteins. 31P data for PLs bilayers interacting with antimicrobial peptide (AMP) magainin-2, aurein-3,3, incorporated into structures of supramolecular lipid assemblies such as toroidal pores and thinned bilayers have been reported.90 Various types of PL systems (l-palmitoyl-d3i-2-oleoyl-s -glycero-3-phosphotidylcholine... 

The physical properties, especially the bending flexibility of the lipid assemblies, facilitate the formation of highly complex structures in which topology, vesicle volumes, and tube lengths as well as radii can be constructed with high precision and modified at will (Fig. 23.3). 

Among the various lipids that form cubic phases, monoglycerides have been studied most. Phosphatidylcholines exhibit cubic phases only at very low water content, whereas phosphatidylethanolamines form cubic phases in similar regions of the lipid-water system as monoglycerides e.g. GMO). Lysophospholipids (single-chain lipids) exhibit cubic phases of quite different kinds, liiey will not be discussed here as they are not considered to be relevant in the formation of self-assembled structures of biological... 

Different other attractive way to modify the active electrode of amperometric sensors, besides deposition of polymers or electropolymerization, is formation of self-assembled structures, e.g., self-assembled monolayers (SAM) on solid supports 88 89 or bilayer lipid membranes (BLM) on various types of support.90 Both types of theses structures can either induce selectivity of sensor to particular analytes and... 

In addition to lipid-like molecules, polymers can also form self-assembling particulates [8]. As expected, the amphiphilic polymers do form polymeric micelles with polar part orienting towards the surface of the particle and the hydrophobic part orienting towards the core of the particle. Also, in this case, different complicated self-assembled structures can be tailored by using block copolymers with regular blocks of monomer units but, only the simpler ones have been investigated so far in the context of drug delivery. 

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