Mechanisms of Electron Transfer Across Membranes

As it has been already indicated in Sect. 2, electron transport through the hydrocarbon core of the bilayer (see reactions (8), (14), (26)) is a key step of any transmembrane PET, and usually it controls the rate and efficiency of the PET process as a whole. Therefore, the data concerning the mechanism of this stage of PET and the factors which affect it are of crucial importance for the development of photochemical systems based on PET across the membranes. Unfortunately, for the majority of the systems listed in Table 1 the proposed mechanisms of electron transfer across the membrane seem to be rather tentative because of insufficient information about the localization of the redox-active components and their diffusion mobility inside the membranes. Only for few systems the studies were detailed enough to propose convincing mechanisms and to give a quantitative description of the kinetics of electron transfer across the membrane. 

As already mentioned in Sect. 2, there are two alternative pathways of transmembrane electron transfer (i) direct transfer via electron exchange reactions between the molecules located in the inner and outer monolayers of the membrane, such as reactions (8) and (14), and (ii) carrier-mediated transfer, provided by the diffusion of an electron carrier or a hole carrier from one side of the membrane to the other, such as reaction (26). 

In this Section we shall discuss experimental data concerning both these pathways. We shall start our discussion with the carrier-mediated mechanisms since it has been studied in more detail. 

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