Electron bilayer membranes

An attractive way to overcome this problem is to use microheterogeneous photocatalytic systems based on lipid vesicles, i.e. microscopic spherical particles formed by closed lipid or surfactant bilayer membranes (Fig. 1) across which it is possible to perform vectorial photocatalytic electron transfer (PET). This leads to generation of energy-rich one-electron reductant A" and oxidant D, separated by the membrane and, thus, unable to recombine. As a result of such PET reactions, the energy of photons is converted to the chemical energy of spatially separated one electron reductant tmd oxidant. 

Epi-illumination Subcellular imaging structures Freeze fracture Preparation of cellular ultrastructures in frozen-hydrated and living state for electron microscopy macromolecular organization of bilayer membranes... 

Bilayer membranes and vesicles provide not only charged surfaces but also two phases, separating the reaction sites and products. It was first demonstrated that photoinduced electron transfer occurs from EDTA in the inner water phase of vesicles incorporated with surfactant Ru(bpy)2+ to MV2+ in the outer water phase22 (Eq. (14)). 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

In spite of the fact that the driving forces remain speculative, there is now little doubt that, at secretion, lipid droplets are enveloped in apical plasma membrane, with perhaps some contribution from secretory vesicle membrane. Many questions remain, however, regarding the nature and origin of the inner coat material which lies between the triacylglycerol core and the outer bilayer membrane. To what extent is this material derived from the amorphous surface material seen on lipovesicles within the cell (Dylewski et al. 1984 Deeney et al. 1985 Keenan and Dylewski 1985) and the electron-dense coat on the cytoplasmic face of the apical plasma membrane (Franke et al. 1981) Also to be considered is the clathrin-like coat observed on the outer surface of secretory vesicles (Franke et al. 1976 Mather and Keenan 1983), which may contribute a substantial quantity of material to the lipid droplets (Franke and Keenan 1979), if secretory vesicles do indeed contribute to the formation of the milk lipid globule membrane. 

Figure 10.8 Milk lipid globule membranes released by churning of washed globules and collected by ultracentrifugation retain densely staining coal material along one face of the bilayer membrane. As seen in this electron micrograph of glutaraldehyde and osmium tetroxide-fixed material, the...

Fig. 28. Schematic illustration of an artificial peptidic electron-transfer system designed to operate in a bilayer membrane. (Reproduced with the permission of Ref. 69)...

Fig. 22. Schematic representation of transmembrane electron transfer from a reducing agent (such as sodium dithiomte) to an oxidizing agent (such as potassium ferricyanide) by a zwitterionic caroviologen such as 99 incorporated in the bilayer membrane of a phospholipid vesicle [8.143].

Thus, the caroviologen approach does produce functional molecular wires that effect electron conduction in a supramolecular-scale system. Incorporation into black lipid bilayer membranes (BLM) should allow further investigations of the electron-transfer properties of these caroviologens and positive preliminary results have been obtained [8.145a]. A theoretical investigation of electron conduction in molecular wires has been made [8.145b]. 

The energy collected by the LH II antenna is transferred to another antenna complex known as LH I, which surrounds the RC. The photosynthetic reaction centers of bacteria consist mainly of a protein that is embedded in and spans a lipid bilayer membrane. In the reaction center, a series of electron transfer reactions are driven by the captured solar energy. These electron transfer reactions convert the captured solar energy to chemical energy in the... 

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