MECHANISMS OF VECTORIAL TRANSMEMBRANE REDUCTION OF VIOLOGENS ACROSS PHOSPHATIDYLCHOLINE BILAYER-MEMBRANES

Zero-order kinetics were observed for one-electron transmembrane oxidation-reduction in vectorially organized phosphatidylcholine vesicles containing entrapped viologens and chemical reductants in the external aqueous phase. Oxygenation-reduction cycling established that the viologen was retained within the vesicle during this reaction. The rates were independent of the identities of the reactants and concentration of reductant as well as a wide variety of medium conditions. At high salt concentration, however, reduction of N,N'-diheptyl-4,4'-bipyridinium ion [(C7)2V2+], the most intensively studied viologen, exhibited anion-specific rate acceleration and autocatalysis, with the relative effectiveness of the anions following approximately the lyotropic series. Addition of valinomycin in K+-containing media or other lipophilic ions also accelerated the reaction rate. Sigmoidal kinetics were observed when valinomycin was present, and following reduction, the viologen was shown to have diffused out of the vesicles. The (C7)2V2+ dication was demonstrated to be membrane permeable by C-14-radioisotope-exchange techniques. These data were interpreted to indicate that net transmembrane oxidation-reduction occurred stepwise by (i) (C7)2V2+ ion diffusion across the bilayer, (ii) reduction in the external aqueous phase, and (iii) uptake of the (C7)2V+ product ion by the vesicles, with the overall reaction being rate limited by movement of charge-compensating aqueous ions. When other membrane-permeable ions were present, their translocation dissipated the membrane potential formed by outward diffusion of entrapped (C7)2V2+ ion, allowing accumulation of the (C7)2V+ radical cation at the external vesicle interface. This accumulation, in turn, gave rise to a (C7)2V+-mediated transmembrane redox pathway. The relative contribution of this pathway increased as the reaction proceeded, thereby accounting for the autocatalytic character of the reaction. Photostimulated transmembrane oxidation-reduction of entrapped (C7)2V2+ ion by EDTA ion was also observed with several photosensitizers that were confined to the external aqueous phase. These reactions could also be accommodated by the general reaction scheme presented but not by other previously advanced mechanisms.