Vesicle transmembrane reactions

Colloids of semiconductors are also quite interesting for the transmembrane PET, as they possess both the properties of photosensitizers and electron conductors. Fendler and co-workers [246-250] have shown that it is possible to fix the cadmium sulfide colloid particles onto the membranes of surfactant vesicles and have investigated the photochemical and photocatalytic reactions of the fixed CdS in the presence of various electron donors and acceptors. Note, that there is no vectorial transmembrane PET in these systems. The vesicle serves only as the carrier of CdS particles which are selectively fixed either on the inner or on the outer vesicle surface and are partly embedded into the membrane. However, the size of the CdS particle is 20-50 A, i.e. this particle can perhaps span across the notable part of the membrane wall. Therefore it seems attractive to use the photoconductivity of CdS for the transmembrane PET. Recently Tricot and Manassen [86] have reported the observation of PET across CdS-containing membranes (see System 32 of Table 1), but the mechanism of this process has not been elucidated. Note, that metal sulfide semiconductor photosensitizers can be deposited also onto planar BLMs [251],... 

Adhesion of different immune cells to one another or to epithelial cells has also been studied using planar bilayer models. For example, lymphocyte function-associated protein-1 (LFA-1) promotes cell adhesion in inflammation [i.e., a reaction that can be mimicked by binding to purified ICAM-1 in supported membranes (70)]. Similarly, purified LFA-3 reconstituted into supported bilayers mediates efficient CD2-dependent adhesion and differentiation of lymphoblasts (71). This work was followed by a study in which transmembrane domain-anchored and GPl-anchored isoforms of LFA-3 were compared (72). Because this research occurred before the introduction of polymer cushions and because the bilayers were formed by the simple vesicle fusion technique, the transmembrane domain isoform was immobile, whereas the GPl isoform was partially mobile. By comparing results with these two isoforms at different protein densities in the supported bilayer, the authors showed that diffusible proteins at a sufficient minimal density in the supported membrane were required to form strong cell adhesion contacts in this system. 

The fact that the pKa of the various oxidation states of quinones differ dramatically has been exploited in a recent design of artificial systems that mimic the light-driven transmembrane proton transport characteristic of natural photosynthesis [218]. Triad artificial reaction centers structurally related to 41 were vectorially inserted into the phospholipid bilayer of a liposome (vesicle) such that the majority of the quinone moieties are near the external surface of the membrane, and the majority of the carotenoids extend inward, toward the interior surface. The membrane... 

When the partition dynamics are rapid, the solute distribution in vesicles will obey the same laws as the distributions in micelles. However, when the transmembrane diffusion time of molecules entrapped within the aqueous vesicle core or incorporated into the hydrocarbon phases exceeds the characteristic time of their chemical transformation in chemical reactions, then their partitioning is set by their initial statistical distribution rather than their migration dynamics. In this case also, a Poisson law is appropriate to approximate their distribution among vesicles. This follows because the volume of the inner aqueous phase generally exceeds 10 A, and the maximiun number of molecules that can be entrapped inside the vesicle is correspondingly large. 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

The dilemma posed by these considerations was resolved for the DHP-organized system when it was noted that upon illumination the initially compartmented MV + partially leaked out of the vesicles [108]. Thus, the observed reaction actually occurred between reactants adsorbed on the same vesicle surface. The mechanism of this unprecedented light-induced scrambling of MV + is still not understood. Likewise, subsequent investigations of the PC-organized system provided evidence for transmembrane leakage of a small amount of the compartmented (C7)2V + ion, which then facilitated transmembrane electron transport [109]. Specifically, the reaction characteristics were duplicated when the amphiphilic Ru(bpy)3 + analog was bound only to the opposite side of the membrane as the oxidative quencher... 

The conventional approach of equilibrium thermodynamics is applicable only in the case of the so-called thermodynamic limit N/V = const, at F 00, where iV is a number of particles in the volume V, This approach ignores all the factors mentioned above and being applied to small systems may therefore lead to erroneous results. For example, considering within the formalism of the thermodynamic approach the transmembrane transfer of neutral P particles from closed vesicles loaded with the reaction mixture of P and Q particles (P -h Q PQ), we have obtained one paradoxical result for small enough vesicles the transfer of even one P particle along its concentration gradient can be thermodynamically unfavorable under certain conditions... 

Reaction Centers (RC s) from phototrophic bacteria catalyze light-driven transmembrane electron transfer as a first step in the (cyclic) electron transfer chain of such bacteria (for a review see Okamura et al., 1983 and Dutton et al., 1982). Many of the structural and functional features of RC s have already been elucidated the remaining questions mainly focus on (i) the effects of transmembrane gradients (of redox potential and electrochemical potential of protons) on the reactions catalyzed by the RC s and (ii) the interactions between RC s and physiological and artificial electron donors and acceptors. Many of the unsolved aspects can be optimally investigated under conditions, in which the RC s have been reconstituted into artificial membranes either in planar (Schonfeld et al., 1979) or vesicular form (Crofts et al., 1977 Pachence et al., 1979). Here I report on the structure of reconstituted RC vesicles and light-dependent unidirectional proton translocation catalyzed by these vesicles. 

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