Fluidity, biomembrane

The fluidity is one of the most vital properties of biological membranes. It relates to many functions involved in biological system, and effective biomembrane mimetic chemistry depends on the combination of both stability and mobility of the model membranes. However, in the polymerized vesicles the polymer chain interferes with the motion of the side groups and usually causes a decrease or even the loss of the fluid phases inside the polymerized vesicle (72,13). 

Penetration of the biomembrane is undoubtedly essential for most membrane activity. Araki and Rifkind (13) obtained esr spectra of stearic acid spin labelled erythrocyte membranes in the presence of diverse compounds including Triton XlOO, chlorpromazine and glutaraldehyde. The two surfactants chlorpromazine and Triton XlOO both increase the rate of haemolysis and are shown to increase membrane fluidity. Glutaraldehyde as expected decreases fluidity and decreases the rate of haemolysis. 

Various types of molecular interactions which occur in mixed mono-layers can be distinguished by simultaneous measurements of the surface pressure, potential, and fluidity of monolayers. Limitations of Goodrich s thermodynamic treatment of mixed monolayers are mentioned. Surface properties of cholesterol have been correlated with its function in biomembranes. 

All four systems illustrated in Fig. 4 exhibit properties differing from those of cell membranes. Methods a-c have no influence on the head groups and preserve physical properties, such as charge, charge density, etc. The fluidity of the hydrocarbon core, however, is drastically decreased by the polymerization process. In case d, fluidity is not affected, but there is no free choice of head groups. In comparison to biomembranes, all polymerized model membrane systems will show an increase in viscosity and a decrease in lateral mobility of the molecules. 

Most biomembranes are in the liquid crystalline phase under physiological conditions and their complex composition makes it more difficult to measure phase transitions. It should be mentioned that the effect of drug molecules on fluidity and order of a membrane can even be reversed below and above Tt [169] and NMR measurements of drug-membrane interactions are normally performed above Tt. 

Lipophilic compounds, such as the various terpenoids, tend to associate with other hydrophobic molecules in a cell these can be biomembranes or the hydrophobic core of many proteins and of the DNA double helix [10,18,24,25]. In proteins, such hydrophobic and van der Waals interactions can also lead to conformational changes, and thus protein inactivation. A major target for terpenoids, especially saponins, is the biomembrane. Saponins (and, among them, the steroid alkaloids) can change the fluidity of biomembranes, thus reducing their function as a permeation barrier. Saponins can even make cells leaky, and this immediately leads to cell death. This can easily be seen in erythrocytes when they are attacked by saponins these cells burst and release hemoglobin (hemolysis) [1,6,17]. Among alkaloids, steroidal alkaloids (from Solanaceae) and other terpenoids have these properties. 

Jain, M. K. (1983). Nonrandom lateral oiganization in bilayers and biomembranes. In C. A. Roland, ed. Membrane Fluidity in Biology Academic Press, New York, Vol. 1, pp. 1-37. 

In the popular fluid mosaic model for biomembranes, membrane proteins and other membrane-embedded molecules are in a two-dimensional fluid formed by the phospholipids. Such a fluid state allows free motion of constituents within the membrane bilayer and is extremely important for membrane function. The term "membrane fluidity" is a general concept, which refers to the ease of motion for molecules in the highly anisotropic membrane environment. We give a brief description of physical parameters associated with membrane fluidity, such as rotational and translational diffusion rates, order parameters etc., and review physical methods used for their determination. We also show limitations of the fluid mosaic model and discuss recent developments in membrane science that pertain to fluidity, such as evidence for compartmentalization of the biomembrane by the cell cytoskeleton. 

The Singer-Nicolson model of the membrane played a very important role in understanding membrane structure and function. However, many properties of biomembranes are not consistent with this model. In recent years, a growing consensus points at more complex membrane structure, which can be characterized as dynamically structured fluid mosaic. Compared with the original fluid mosaic model, the emphasis has shifted from fluidity to mosaicity. Experimental observations have led to the membrane microdomain concept that describes compartmen-talization/organization of membrane components into stable or transient domains. 

McElhaney RN. The relationshhip between membrane fluidity and phase state and the ability of bacteria and mycoplasmas to grow and survive at various temperatures. Biomembranes 1984 12 249-276. 

Hopanoids (the most common organic natural product on earth) must have been involved in the evolution of the biomembrane itself. All known membranes contain terpene derivatives, such as cholesterol or carotenoids, which belong to, or can be derived from, hopanoids. However, we still do not know their biological function. Their most commonly proposed mechanism is to regulate membrane fluidity. Another obvious effect is their influence on the lipid bilayer (or monolayer in the case of archaebacteria) curvature. The different types of hopanoids occurring will certainly favour the relative stability of either the planar or of the intrinsically curved membrane conformation. The ether lipids of archaebacteria, which are hopanoid derivatives, forming curved bilayers as discussed above, therefore provide evidence for cubosomes as the first organised form of life. 

Membrane fluidity is determined by following anisotropic rotation of fluorescent or spin probes. Liquid-crystalline (or fluid) to gel thermotropic phase transition of lipids (Figure 1) (cf. Section 3.1.1 l)in liposomes or intact biomembranes can be followed by Fourier transform infrared (FTIR) spectroscopy or differential scanning calorimetry (DSC). 

Tlie polar head group of the molecule regulates the overall charge and hydrophilicity of the membrane. The major polar groups in biomembranes may be choline, ethanolamine, serine, inositol, galactose or in some cases n-acetylneuraminic acid. The non-polar acyl-chains influence greatly the fluidity and the packing of the membrane. Saturated fatty acid residues while decrease the fluidity and produce more compact membranes, unsatiirated acyi-chains increase the fluidity and minimize the lipid orientation of the membrane. 

One of the objectives in performing hydrogenation reactions in situ was to modulate the fluidity of biomembranes and to examine the role of membrane lipid fluidity in biochemical and physiological functions. Secondly, because membrane lipids with six or more unsaturated double bonds were found to be significant components of some membranes, such as the retinal rod membranes of the eye, the hydrogenation of these lipids was thought to be a useful tool to identify their role in these membranes. 

Natural biomembranes generally have a fluldllke consistency. In general, membrane fluidity is decreased by sphingolipids and cholesterol and Increased by phospho-glycerldes. The lipid composition of a membrane also influences its thickness and curvature (see Figure 5-8). 

Notwithstanding this complexity, the need for three-dimensional, structural information at the atomic level of resolution is central and indispensable to biomembrane science. X-ray, and to a lesser extent neutron-diffraction, as the most important sources for such information have, therefore, been widely used in this field (for reviews, see Refs. 1-4). The success of this approach, however, has generally been less spectacular than for instance in the cases of protein or nucleic acid structure. The reasons for this lie in the very nature of biological membranes with few, notable exceptions (such as the purple membrane of halobacterium halobium, which can be viewed essentially as a two-dimensional crystal of bacteriorhodopsin with only little lipid. Refs. 5, 6,25) biological membranes are characterized by highly complex and variable molecular compositions, and by the structural dynamics, fluidity , which is in many cases essential for enzymatic, or other, functions of membranes. As a reflection of this most natural membranes do not crystallize, and a full, three-dimensional atomic structure analysis seems out of reach. 

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. 

The changes in the fluidity of the membranes affect the spatial arrangement of the membrane proteins. Mobility of the membrane protein and its organization are probably influenced by the motional characteristics of the lipids in the boundary region of the proteins (1). The physical state of lipids in biomembranes affects the protein dynamics (2,3). Low levels of cholesterol cause a decrease in the fluidity of the thylakoid membranes and brings about a significant inhibition of the interaction between electron transport components (4). 

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