Molecular structure and properties of lipids

See also Acetyl-CoA, Fats, Albumin, Fatty Acid Activation, Oxidation of Saturated Fatty Acids, Oxidation of Unsaturated Fatty Acids, Fatty Acid Biosynthesis Strategy, Palmitate Synthesis from Acetyl-CoA, Fatty Acid Desaturation, Essential Fatty Acids, Control of Fatty Acid Synthesis, Molecular Structures and Properties of Lipids (from Chapter 10)... 

See also Molecular Structures and Properties of Lipids, Phosphatidic Acid, Cardiolipin, Phosphatidylserine, Phosphatidylethanolamine, Phosphatidylglycerol, Phosphatidylcholine, CDP-Diacylglycerol, Phosphatidylglycerol-3-Phosphate, Phosphatidylinositol, Lung Surfactant, Sphingolipids, Glycosphingolipids,... 

See also Steroids, Molecular Structures and Properties of Lipids (from Chapter 10), Bile Acids, Steroid Hormone Synthesis... 

See also Molecular Structures and Properties of Lipids, Translocases... 

The molecular structure and conformation of an organic pollutant is a property which may affect adsorption onto a solid surface and/or partition into its organic lipid phase differently, thus hindering the expected correlation between KQM and K )W ... 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples are protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g., the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the structure and properties of biopolymer, biomembrane, and gel structures in aqueous media. 

Gennis, R. B., The structures and properties of membrane lipids. In Bio- -membranes Molecular Structure and Function. R. B. Geimis, Ed. Springer-Verlag New York, 1989. 

In summary, MD simulations of lipid assemblies are nowadays an established tool that can provide remarkable insights into the molecular structure and dynamics of model membranes. Even though some limitations persist, their ability to reproduce experimental data on membrane properties is truly amazing. Most importantly, the remaining drawbacks are under intense scrutiny, suggesting that future developments will lead to further improvements in their accuracy. In this context, the most challenging area appears to be that of protein-membrane interactions, a field that will probably remain the most significant application area for atomistic simulations of lipid assemblies. 

One of the key parameters for correlating molecular structure and chemical properties with bioavailability has been transcorneal flux or, alternatively, the corneal permeability coefficient. The epithelium has been modeled as a lipid barrier (possibly with a limited number of aqueous pores that, for this physical model, serve as the equivalent of the extracellular space in a more physiological description) and the stroma as an aqueous barrier (Fig. 11). The endothelium is very thin and porous compared with the epithelium [189] and often has been ignored in the analysis, although mathematically it can be included as part of the lipid barrier. Diffusion through bilayer membranes of various structures has been modeled for some time [202] and adapted to ophthalmic applications more recently [203,204]. For a series of molecules of similar size, it was shown that the permeability increases with octa-nol/water distribution (or partition) coefficient until a plateau is reached. Modeling of this type of data has led to the earlier statement that drugs need to be both... 

The present knowledge about molecular organization in lyotropic liquid crystalline phases is summarized. Particular attention is given to biologicaly relevant structures in lipid-water systems and to lipid-protein interactions. "New findings are presented on stable phases (gel type) that have ordered lipid layers and high water content. Furthermore, electrical properties of various lipid structures are discussed. A simple model of l/l noise in nerve membranes is presented as an example of interaction between structural and electrical properties of lipids and lipidr-protein complexes. 

Properties of Component Phases The composition and physicochemical properties of both the oil and aqueous phases influence the size of the droplets produced during homogenization (52). Variations in the type of oil or aqueous phase will alter the viscosity ratio, ri ,/ri(-, which determines the minimum size that can be produced under steady-state conditions. The interfacial tension of the oil-water interface depends on the chemical characteristics of the lipid phase, e.g., molecular structure or presence of surface-active impurities, such as free fatty acids, monoacylglycerols, or diacylglycerols. These surface-active hpid components tend to accumulate at the oil-water interface and lower the interfacial tension, thus lowering the amount of energy required to disrupt a droplet. 

From a biological standpoint an essential property of lipid molecules is their ability to form aqueous phases, possessing long-range order combined with disorder at molecular distances. A variety of different phases exist for a particular lipid (polymorphism), and a small change in solution conditions is sufficient to cause a transformation from one form or structure to another. 

Water behaves differently in different environments. Properties of water in heterogenous systems such as living cells or food remain a field of debate. Water molecules may interact with macromolecular components and supramolecular structures of biological systems through hydrogen bonds and electrostatic interactions. Solvation of biomolecules such as lipids, proteins, nucleic acids, or saccharides resulting from these interactions determines their molecular structure and function. 

In this chapter, the diversity in structure, chemical properties, and physical properties of lipids will be outlined. The various genetic approaches available to study lipid function in vivo will be summarized. Finally, how the physical and chemical properties of lipids relate to their multiple functions in living systems will be reviewed to provide a molecular basis for the diversity of lipid structures in natural membranes [1]. 

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