Membrane, artificial fluidity

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. 

DiaryIpropanes. Excimer formation with diarylpropanes in solution talces place intramolecularly, making this process independent of concentration (44). Therefore, these molecules, especially 1Py(3)1Py (45), have been used extensively to probe the fluidity of micelles and artificial and biological membranes (17-19,46-50). Here, they are expected to be convenient indicators of the degree of mobility freedom of adsorbed molecules. 

Information about fluidity and viscosity of bilayers of artificial and natural membranes has been obtained from electron spin resonance studies in which the mobility of the spin-labelled species along the surface plane of the membrane is determined (17). However, the monolayer of either lipid, protein, or lipid-protein systems at the air-water interface, makes an ideal model because several parameters can be measured simultaneously. Surface tension, surface pressure, surface potential, surface viscosity, surface fluorescence and microviscosities, surface radioactivity, and spectroscopy may be determined on the same film. Moreover, the films can be picked up on grids from which they may be observed by electron microscopy, studied further for composition, and analyzed for structure by x-ray diffraction and spectroscopy. This approach can provide a clear understanding of the function and morphology of the lipid and lipid-protein surfaces of experimental membranes. However, the first objective is to obtain molecular correlations of surface tension, pressure, potential, and viscosity. 

The development of a portable and rugged sensing device requires that the selective recognition element be directly interfaced to the physical transducer. In the case of electrochemical transducers based on artificial BLMs, this entails stabilization of the assembly onto an electrode. The stabilization method must allow the membrane to retain characteristics of molecular mobility and fluidity which are essential for transduction and should provide sufficient ruggedness to permit use over an extended period of time (several months) without severe alteration of the response characteristics of the membrane. 

Andrich, M.P., and Vanderkooi, J.M., Temperature dependence of 1,6-diphenyl-1,3,5-hexa-triene fluorescence in phospholipid artificial membranes, Biochemistry, 15, 1257, 1976. Blitterswijk, W.J.V., Hoeven, R.P.V., and Dermeer, B.W.V., Lipid structural order parameters (reciprocal of fluidity) in biomembranes derived from steady-state fluorescence polarization measurements, Biochem. Biophys. Acta, 644, 323, 1981. 

As biological systems have always been an inspiration for scientists, intracellular compartments (such as lysosomes or mitochondria) also have their artificial equivalents in polymer vesicles, called polymersomes. Polymersomes are spherical compartments with a bi- or monolayer membrane, generated by self-assembly of di- or triamphiphilic block copolymers in diluted aqueous conditions. To favor the formation of structures such as polymersomes, it is necessary to have a hydrophilic fraction of the copolymer mass of 25-40%, and polymer concentration above the critical micellar concentration. Other parameters that affect the self-assembly process, and therefore the final architecture of the polymer supramolecular assemblies, are the molecular weight of the copolymer (Af ), block lengths, solubility, and glass transition temperature (Tg) [21,22], The relative mass or volume fraction of each block is a key parameter in the formation of a self-assembled structure with a certain membrane curvature, and ultimately, with a specific architecture. The of the copolymer (and thus the block lengths) dictates the membrane thickness and polymersome properties, such as membrane fluidity, stabihty, and permeabihty [21,74],... 

The lipid composition of membranes is a sensitive indicator of changes in environmental temperature. The fluidity of a membrane is critical to its functioning as a semi-permeable barrier, and is directly related to the fatty acid composition of the membrane. In artificial lipid membrane the liquid to crystalline transition occurs at lower temperatures for phospholipids containing higher proportions of shorter chain fatty acids or increased degree of unsaturation. Bacterial membranes with a greater proportion of unsaturated fatty acids are better able to function at low temperatures. 

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