Methyl viologen-mediated oxidation-reduction across dihexadecylphosphate vesicles involves transmembrane diffusion

Numerous reports have appeared describing oxidation-reduction across bilayer membranes. Mechanisms proposed for specific systems include the following: (i) electron tunneling across the hydrocarbon barrier between interfacially bound redox partners, (ii) molecular diffusion of bound redox components across the barrier, and (iii) formation of barrier-penetrating aggregates, or electron-conducting channels, across the bilayer. Nonetheless, the actual reaction mechanisms remain obscure due to the general unavailability of transverse diffusion rates, possible loss of compartmentation of reactants, particularly in photochemical systems, and the ambiguities inherent in deducing reaction mechanisms from rate data, which form the primary evidence in most systems studied. The reactions of dihexadecylphosphate (DHP) vesicle-bound methyl viologen (MV/sup 2 +/) describes in this report are unique in allowing deduction of molecular details of a transmembrane redox event from the product composition and microphase distribution. Specifically, they have found that MV/sup 2 +/ bound at the outer vesicle interface mediates reduction of inner-localized MV/sup 2 +/ by dithionite ion in bulk solution in a manner that requires comigration of MV/sup +/ with the electron transferred across the membrane barrier.