Lipids and Biological Membranes

Forbes J, Bowers J, Shan X, Moran L, Oldfield E, Moscarello MA, Some new developments in solid-state nuclear magnetic resonance spectroscopic studies of lipids and biological membranes, including the effects of cholesterol in model and natural systems, /. Chem. Soc. Faraday Trans., 84 3821-3849, 1988. 

The rare gas xenon contains two NMR-sensitive isotopes in high natural abundance Xe has a spin of 1/2 and Xe is a quadrupolar nucleus with a spin of 3/2. The complementary NMR characteristics of these nuclei provide a unique opportunity for probing their environment. The method is widely applicable because xenon interacts with a useful range of condensed phases including pure liquids, protein solutions, and suspensions of lipid and biological membranes. It was found that the range of chemical shifts of Xe dissolved in common solvents is = 200 ppm, which is 30 times larger than that found for in methane dissolved... 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Seelig, J., and Seelig, A., 1981. Lipid conformadon in model membranes and biological membranes. Quarterly Review of Biophysics 13 19-61. 

Although for the moment this model is only partially supported by experimental data it offers the opportunity to design new experiments which will help to understand the mechanisms of pardaxin insertion and pore formation in lipid bilayers and biological membranes which at a molecular level are the events leading to shark repellency and toxicity of this marine toxin. 

It has been known for some years that gramicidin forms transmembrane ion channels in lipid bilayers and biological membranes and that these channels are assembled from two molecules of the polypeptide 213). The channels are permeable specifically to small monovalent cations [such as H+, Na+, K+, Rb+, Cs+, Tl+, NH4+, CHjNHj, but not (CH3)2NH2+J and small neutral molecules (such as water, but not urea). They do not allow passage of anions or multivalent cations 21 n. 

Wisniewska, A., J. Draus, and W. K. Subczynski. 2006. Is fluid mosaic model of biological membranes fully relevant Studies on lipid organization in model and biological membranes. Cell. Mol. Biol. Lett. 8 147-154. 

Silvius, J. R. and Nabi, I. R. (2006). Fluorescence-quenching and resonance energy transfer studies of lipid microdomains in model and biological membranes. Mol. Membr. Biol. 23, 5-16. 

Lipid-protein interactions are of major importance in the structural and dynamic properties of biological membranes. Fluorescent probes can provide much information on these interactions. For example, van Paridon et al.a) used a synthetic derivative of phosphatidylinositol (PI) with a ris-parinaric acid (see formula in Figure 8.4) covalently linked on the sn-2 position for probing phospholipid vesicles and biological membranes. The emission anisotropy decays of this 2-parinaroyl-phosphatidylinositol (PPI) probe incorporated into vesicles consisting of phosphatidylcholine (PC) (with a fraction of 5 mol % of PI) and into acetylcholine receptor rich membranes from Torpedo marmorata are shown in Figure B8.3.1. 

McMullen, T.P.W., Lewis, R.N.A.H., McElhaney, R.N. Cholesterol-phospholipid interactions, the liquid-ordered phase and lipid rafts in model and biological membranes. Curr. Opin. Colloid Interface Sci. 2004, 8, 459-68. 

A very brief description of biological membrane models, and model membranes, is given. Studies of lateral diffusion in model membranes (phospholipid bilayers) and biological membranes are described, emphasizing magnetic resonance methods. The relationship of the rates of lateral diffusion to lipid phase equilibria is discussed. Experiments are reported in which a membrane-dependent immunochemical reaction, complement fixation, is shown to depend on the rates of diffusion of membrane-bound molecules. It is pointed out that the lateral mobilities and distributions of membrane-bound molecules may be important for cell surface recognition. 

Vitamin E is a generic term that represents four tocopherols and four tocotrienols of varying biological potency. The term tocopherol correctly refers to the methyl-substituted derivatives of to-col and is not synonymous with the term vitamin E. The tocopherols and tocotrienols may be referred to collectively as tocochromanols. Many of the diverse deficiency syndromes observed in animals experimentally deprived of vitamin E can be explained by the vitamin s acting as an antioxidant in stabilizing unsaturated lipids in biological membranes. 

There are two main types of lipids in biological membranes Phospholipids and sterols. The predominant phospholipids in most membranes are phosphoglycerides, which are phosphate esters... 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

Stamatatos, L., R. Leventis, M.J. Zucker-mann, and J.R. Silvius. 1988. Interactions of cationic lipid vesicles with negatively charged phospholipid vesicles and biological membranes. Biochemistry 27 3917-3925. 

One long-term objective of this research is to utilize the finest attributes associated with the worlds of both biological and synthetic materials to create nanomechanical systems powered by biological motors. Important fields of application include miniaturized (nanofluidic) analytical systems,131 molecular sorting,132 controlled adaptation of materials on a molecular to mesoscopic scale,133 and engineering lipid and polymer membrane systems with cellular processes.134... 

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