Transmembrane electron transfer mediated by a viologen: A mechanism involving diffusion of doubly reduced viologen formed by disproportionation of viologen radical

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 CMV²⁺ 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 ([CMV⁺]_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⁺ = CMV⁰ + CMV²⁺). CMV⁰ diffuses rapidly through the membrane and is re-oxidized in the vesicle interior. A brief study of Ru(bpy)₃²⁺-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 [CMV⁺]_out.