Transmembrane Electron Transfer Mediated by a Viologen: A Mechanism Involving Diffusion of Doubly Reduced Viologen Formed by Disproportionation of Viologen Radical

Article abstract of DOI:10.1021/j100191a052

A stopped-flow study of electron transfer (ET) through vesicle membranes composed of egg lecithin (phosphatidyl choline) was made.Cetylmethylviologen (N-methyl-N'-hexadecyl-4,4'-bipyridine, CMV) acted as transmembrane redox mediator by transferring electrons from dithionite in the bulk aqueous phase to CMV(2+) or ferricyanide in the vesicle interior.Transmembrane ET was observed when viologen was bound only to the outer interface and the overall reaction was second order in viologen radical on the outside (_out).Both facts exclude electron tunneling/self-exchange as mechanism.A mechanism is proposed where the rate-determining step is the disproportionation of two viologen radicals initially formed at the outer interface of the vesicle (2CMV(+) = CMV(0) + CMV(2+)).CMV(0) diffuses rapidly through the membrane and is re-oxidized in the vesicle interior.A brief study of Ru(bpy)3(2+)-sensitized, photoinduced transmembrane ET was made, giving further evidence for transmembrane viologen migration.Preliminary results from pulse-radiolytically induced transmembrane ET support the observation that the reaction is second order in _out.