Lipid molecular shape

FIG. 4 Schematic representation of various lipid phase structures influenced by lipid molecular shape. 

Interestingly, this elaborated sector approach to tubular materials can be executed using perhaps the simplest possible starting materials as molecular sectors. Variations within this basic framework alter the lipid molecular shape, which allows lipid assemblies to access a wide range of structural morphologies, ranging from micellar and hexagonal phases to the lamellar structure found in biomembranes. ... 

Several groups have studied the structure of chiral phases illustrated in Fig. IV-15 [167,168]. These shapes can be understood in terms of an anisotropic line tension arising from the molecular symmetry. The addition of small amounts of cholesterol reduces X and produces thinner domains. Several studies have sought an understanding of the influence of cholesterol on lipid domain shapes [168,196]. 

The inversed hexagonal structure, with water cylinders arranged in a matrix formed by the disordered hydrocarbon chains (Figure 1, left), is a common structure in aqueous systems of lipids of biological origin. There is usually no problem in determining the true alternative between the two hexagonal structures from the x-ray data, and the molecular dimensions can then be calculated. The occurrence of this structure in complex lipids results from the molecular shape two hydrocarbon chains are usually... 

Smisterova J, Wagenaar A, Stuart MCA et al (2001) Molecular shape of the cationic lipid controls the structure of cationic lipid/dioleylphosphatidylethanolamine-DNA complexes and the efficiency of gene delivery. J Biol Chem 276 47615-47622... 

Kumar, V.V. Complementary molecular shapes and additivity of the packing parameter of lipids. Proc Natl Acad Sci USA 88 (1991) 444-448. 

The three examples cited above illustrate the power of the simple block developed here. While complex physics governs the variation of the surfactant molecular shape with temperature and concentration, clear trends can be seen for the various classes of surfactants and lipids. 

To learn more about molecular shapes and behavior, visit the Chemistry Web site at chemistrymc.com Activity Research the structure of an amino acid, lipid, or other biological molecule. Make a model or poster to explain how the polarity of each bond and the overall molecule affect the shape, function, and reactivity of the molecule. 

Phospholipids are the major lipid building blocks most membranes and their molecules comprise of a hydrophobic (acyl chain) and a hydrophilic (polar) head group. The relative size of the hydrophobic tails and hydrophilic head of the molecule characterizes the molecular shape and determines the structure of the molecular assemblies in contact with w ater. Molecules with polar and non-polar regions (PC, PS, PI, Sphm) of equal size have a cylindrical shape and form lipid bilayers. Molecules that have a larger non-polar region are cone-shaped (PE, PA, Choi, Car), and form reversed micelles, in contact to water. When the polar region is larger (lysophospholipids) the molecule assembles an inverted cone and form micelles. Fig. (8). 

The kinetics of vesicle fusion, followed by monitoring the position of the minimum of the SPR reflectivity curves, depends on the composition and molecular shape ofthe vesicular lipids and on the nature ofthe substrate. As a rule, bilayer formation by vesicle unrolling onto a hydrophilic surface is faster than monolayer formation by vesicle fusion onto a hydrophobic surface. This is probably due to the fact that the processes involved in forming a planar bilayer starting from a vesicular bilayer are considerably less complex than those involved in forming a planar monolayer [8, 9]. 

The release profiles suggest that the lysoPPC and PA hydrolysis products, which are formed in a 1 1 molar ratio by SPLA2, are incorporated into the target membranes [51], leading to the increase in the permeability of the target liposomal membranes. These hydrolysis products, due to their non-bilayer forming molecular shapes, induce a curvature stress [52, 53] and/or form small-scale lipid domains [33, 36], which lead to membrane defects and consequently increased membrane... 

It is worth underlining at this point that the molecular shapes of saturated and cis unsaturated lipids are quite different, since the cis geometry confers a kink in the lipid hydrocarbon chain, with an angle of about 30° in the acyl chain. In contrast, the molecular shape of the saturated fatty acyl chain is straight, and it is interesting to note that CTI corresponds to a cancellation of the bending, because trans isomers, like saturated compounds, are also straight (Fig. 6.1). 

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