Vesicle surfaces, electron transfer

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)). 

Surfactant vesicles constitute a very flexible medium for the support of semiconductors. Semiconductor particles can be localized at the outer, the inner, or at both surfaces of single-bilayer vesicles (Fig. 102). Each of these arrangements has certain advantages. Semiconductor particles on outer vesicle surfaces are more accessible to reagents and can, therefore, undergo photosensitized electron transfer more rapidly. Smaller and more monodispersed CdS particles can be prepared and maintained for longer periods of time in the interior of vesicles than in any other arrangement... 

Figure 5. An idealized mechanism of photoinduced electron transfer from CdS conduction band to methylviologen (MV +)( resulting in formation of methylviologen radical cation (MV,+). The colloidal CdS particle as represented, was generated at the inside surface of the DHP vesicle. Its exact location is based on fluorescence quenching experiments (Figure 5). Inserts oscilloscope trace showing the formation of MV by the absorbance change at 396 nm, after a laser pulse at 355 nm.

If this scheme is valid, one can expect that, when both EDTA and MV2+ are located in the bulk solution and the membranes are used only as carriers of water-insoluble ZnTPP, electron transfer from EDTA to MV2+ sensitized by ZnTPP can occur on the outer surface of the vesicle with the participation of only one 3ZnTPP particle. In this case the initial rate of MV+ accumulation is expected to be directly proportional to the light intensity, and indeed such dependence was observed experimentally [57]. 

Ru(bpy)3+ complex placed into the inner cavity of the vesicle was used as such antenna . The lifetime of the triplet-excited state of this complex ( 0.6 ps) is sufficiently long, so that before its deactivation it can experience numerous collisions with the inner surface of the vesicle membrane and thus with the porphyrin molecules embedded into the membrane. Indeed, it was found that the introduction of Ru(bpy)2 + into the inner volume of the vesicle leads to the sixfold increase of the rate of the transmembrane PET [58, 61]. This effect results, first, from the spectral sensitization due to the light absorption by the ruthenium complex in the spectral region where porphyrin does not absorb, and, second, from the two-three fold increase of transfer from 3Ru(bpy)i+ to ZnTPPin. 

Charge Separation Potential. An electron can be transferred across the bllayer of surfactant vesicles from a donor to an acceptor. This transfer renders the donor side of the vesicle surface to be more positive than the acceptor side, thus it creates a potential, referred to as charge separation potential, i c.s ... 

Influence of Field Effect. Since electron transfer rates are directly related to the field, a judicious manipulation of the distance of a sensitizer and an electron acceptor (or donor) from a highly charged surface across the Stem layer (Figure 2, equation 7) is expected to result in altered efficiencies. This expectation has been realized In achieving effective charge separation under the Influence of a positive electric field, generated by DODAC vesicles (35). Rate constant for electron transfer from L-cystelne to the excited state of Ru(bpy) ... 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

Reactions that occur between components in the bulk solution and vesicle-bound components, i.e., reactions occurring across the membrane interface, can be treated mathematically as if they were bimolecular reactions in homogeneous solution. However, kinetic analyses of reactions on the surface of mesoscopic structures are complicated by the finiteness of the reaction space, which may obviate the use of ordinary equations of chemical kinetics that treat the reaction environment as an infinite surface populated with constant average densities of reactant molecules. As was noted above, the kinetics of electron-transfer reactions on the surface of spherical micelles and vesicles is expressed by a sum of exponentials that can be approximated by a single exponential function only at relatively long times [79a, 81], At short times, the kinetics of the oxidative quenching of excited molecules on these surfaces are approximated by the equation [102]... 

DBS) are employed. The rate of electron transfer and its dependence upon chromophore concentration suggest that the electron tunnels from a chromophore on the inner surface of the vesicle to one on the outer surface rather than being carried by flipping of the surfactant chromophore from one surface to the other. In the system involving amphiphilic zinc porphyrin... 

Figure 23 ZnSe particles on the surface of DHP vesicles lead to charge separation [(ZnSe(e +h )] upon irradiation [85]. ZnSe(e ) reacts with methylviologen (MV ) to give MV . Glucose (RCH2OH) depletes the holes (h ) by electron transfer and is oxidized, while the electrode reconverts MV" " to...

M EXPERIMENTAL FIGURE 8-19 Electron transfer from reduced cytochrome c (Cyt c " ) to O2 via the cytochrome c oxidase complex is coupled to proton transport. The oxidase complex is incorporated into liposomes with the binding site for cytochrome c positioned on the outer surface, (a) When O2 and reduced cytochrome c are added, electrons are transferred to O2 to form H2O and protons are transported from the inside to the outside of the vesicles. Valinomycin and are added to the medium to dissipate the voltage gradient generated by the translocation of H, which would otherwise reduce the number of protons moved across the membrane, (b) Monitoring of the medium pH reveals a sharp drop in pH following addition of O2. As the reduced cytochrome c becomes fully oxidized, protons leak back into the vesicles, and the pH of the medium returns to its initial value. Measurements show that two protons are transported per O atom reduced. Two electrons are needed to reduce one O atom, but cytochrome c transfers only one electron thus two molecules of Cyt c are oxidized for each O reduced. [Adapted from B. Reynafarje et al., 1986, J. Biol. Chem. 261 8254.1... 

Vesicles of dihexadecyl phosphate speed electron transfer from photoactivated Ru(bpy)3" to methyl viologen, and it was possible to distinguish between reaction on the inner and outer surface of the vesicle. However although the forward step is very rapid so is the back reaction. A modification of this system in which Ru(bpy) is on the inner surface of a dihexadecyl phosphate vesicle will generate hydrogen, provided that the vesicle contains PtO which catalyzes the decomposition of water [168],... 

The nucleophilic reactivity of cysteine has been exploited in Michael reactions with quinones. One example is a water-soluble naphthoquinone, which has been entrapped in chlorophyll-containing vesicles in order to study light-induced electron transfer through a membrane from glutathione to the quinone (Fore, 1983). Another example is an asymmetrical vesicle membrane made of a cysteine quinone carboxylate bolaamphiphile, where all the quinone is localized on the outer surface of the vesicle (see Scheme 7.2.6 Scheme 9.5.1). 

As a new type of electron relay, which is able to penetrate lipid membranes, we tested menaquinone (MQ, Fig. 7). Compounds of this type were not utilized earlier for artificial vesicle-based systems. However, these mimick the functioning of the Z-scheme of natural plant photosynthesis (see Figs 9 and 12). Indeed, the activity of MQ in the redox processes in a lipid bilayer membrane was revealed. However, the quantum yield of the transmembrane electron transfer from a CdS nanoparticle in the inner cavity to a CdS nanoparticle on the outer membrane surface with the participation of MQ appeared to be very low and did not exceed 0.2-0.4%. 

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