Lipid structure, biomembranes

C. butyricum appears to regulate the stability of the bilayer arrangement of membranes by altering the ratio of ether versus acyl ethanolamine phospholipids in response to changes in the degree of lipid unsaturation of the membranes. Experiments with bacteria indicate that substitution of plasmenylethanolamine for phosphatidylethanolamine in biomembranes would have only small effects on lipid melting transitions, whereas the tendency to form non-lamellar lipid structures would be significantly increased. 

Electrochemical impedance spectroscopy provides a sensitive means for characterizing the structure and electrical properties of the surface-bound membranes. The results from impedance analysis are consistent with a single biomembrane-mimetic structure being assembled on metal and semiconductor electrode surfaces. The structures formed by detergent dialysis may consist of a hydrophobic alkyl layer as one leaflet of a bilayer and the lipid deposited by dialysis as the other. Proteins surrounded by a bound lipid layer may simultaneously incorporate into pores in the alkylsilane layer by hydrophobic interactions during deposition of the lipid layer. This model is further supported by the composition of the surface-bound membranes and by Fourier transform infrared analyses (9). 

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. 

Fig. 10. Membrane lipids. Structure of the main membrane sterols and their orientation and dimensions in a monolayer of the lipid bilayer of a biomembrane.

Life is driven by water. This is evident in the importance of the delicate balance between the hydrophilic and hydro-phobic forces responsible for protein structure and folding. It is nowhere as obvious as its role in the formation of biomembranes whose structures are principally determined by the amphipathic properties of the class of small biological molecules known as lipids. Membranes exist throughout organisms and many, perhaps most, important biological processes occur within them. In fact, the primordial formation of biomembranes might have been the first step in the emergence of life. 

Lohner, K (1996) Is the high propensity of ethanolamine plasmalogens to form non-lamellar lipid structures manifested in the properties of biomembranes Chem Phys Lipids, 61-184. 

Gennis RB (1989) Biomembranes molecular structure and function. Springer, New York Yeagle P (1992) The structure of biological membranes. CRC Press, Boca Raton McElhaney RN (1986) Differential scanning calorimetric studies of lipid-protein interactions in model membrane systems. Biochim Biophys Acta 864(3-4) 361-42l HianikT, Passechnik VI (1995) Bilayer lipid membranes structure and mechanical properties. Kluwer, Netherlands... 

Among the diverse components of biomembranes, lipids are essential in structural aspects. Lipids are defined operationally as derivatives of fatty acids and their metabolites. As lipids are usually amphiphilic molecules with hydrophobic hydrocarbon tails and hydrophilic head groups (as shown in Figure 1), the bilayer structures of biomembranes are held with hydrophobic forces to the tails and heads of lipids. Another major component of biomembranes is proteins. The weight proportion of proteins in biomembranes is often more than that of lipids. As proteins are more rigid than lipid assemblies, specific interactions by biomembranes are often related to proteins. Further, the sterols contained in biomembranes play unique roles in their apolar regions. As the functionality of biomembranes arises from the diversity of their composition, various kinds of molecules have been studied for biomembrane modeling. 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc 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 propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

In the 1970s the structure and dynamics of lipid bilayer membranes were extensively investigated by NMR. The principles of the NMR spectroscopy applied to the study of biomembranes are reviewed in Ref. 5, together with the fruitful achievements in the early stage. In the 1980s the NMR biomembrane research was carried out mainly by applying the solid-state NMR techniques [6-11]. Generally, the solid-state spectra are of low reso-... 

Although the drug delivery to the lipid bilayer membrane is just the first step for bioactivities and phopholipid vesicles are rather simple in view of the composite structure of biomembranes, the unambiguous specification of the preferential location of the drug is essential the successive processes of the action are expected to be induced via the delivery site in membranes. We expect more advances in the dynamic NMR study, so that we can get insight into the mechanism of DD in membranes. 

Although biomembranes contain a substantial amount of nonbilayer-forming lipids [23], the membrane is maintained as a bilayer. It has turned out that the amount and structure of these nonbilayer-forming lipids are precisely regulated [24,25], implying that the effects of the nonbilayer-forming lipids on the membrane are of biological importance [26]. 

Larsson, K., Cubic lipid-water phases Structures and biomembrane aspects. J. Phys. Chem. 1989,93,7304. 

Surfactant has a similar amphoteric structure as lipid, which makes it possible to form a stable membrane the same as a lipid membrane and can be used to embed proteins. A surfactant membrane has many characteristics similar to those of a biomembrane, so that it can retain the bioactivities of proteins well. The process of preparing a sur-factant/protein-modified electrode is simple and viable. There are usually two methods... 

Abstract To understand how membrane-active peptides (MAPs) function in vivo, it is essential to obtain structural information about them in their membrane-bound state. Most biophysical approaches rely on the use of bilayers prepared from synthetic phospholipids, i.e. artificial model membranes. A particularly successful structural method is solid-state NMR, which makes use of macroscopically oriented lipid bilayers to study selectively isotope-labelled peptides. Native biomembranes, however, have a far more complex lipid composition and a significant non-lipidic content (protein and carbohydrate). Model membranes, therefore, are not really adequate to address questions concerning for example the selectivity of these membranolytic peptides against prokaryotic vs eukaryotic cells, their varying activities against different bacterial strains, or other related biological issues. 

Here, we discuss a solid-state 19F-NMR approach that has been developed for structural studies of MAPs in lipid bilayers, and how this can be translated to measurements in native biomembranes. We review the essentials of the methodology and discuss key objectives in the practice of 19F-labelling of peptides. Furthermore, the preparation of macroscopically oriented biomembranes on solid supports is discussed in the context of other membrane models. Two native biomembrane systems are presented as examples human erythrocyte ghosts as representatives of eukaryotic cell membranes, and protoplasts from Micrococcus luteus as membranes... 

The structure and roles of membrane microdomains (rafts) in cell membranes are under intensive study but many aspects are still unresolved. Unlike in synthetic bilayers (Fig. 2-2), no way has been found to directly visualize rafts in biomembranes [22]. Many investigators operationally define raft components as those membrane lipids and proteins (a) that remain insoluble after extraction with cold 1% Triton X-100 detergent, (b) that are recovered as a low density band that can be isolated by flotation centrifugation and (c) whose presence in this fraction should be reduced by cholesterol depletion. 

In the fluid state, the lateral diffusion coefficient of lipids in the bilayer structure is 0( 10 1 ) m2 s-1 (the symbol O is used to indicate order of magnitude). Interestingly, it has been shown that the diffusion coefficients of phospholipids may differ greatly from the inner to the outer leaflet of the biomembrane layer [4,5]. Again, this is related to the differences in chemical... 

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