MECHANISMS OF VIOLOGEN-MEDIATED CHARGE SEPARATION ACROSS BILAYER-MEMBRANES DEDUCED FROM MEDIATOR PERMEABILITIES

The mobilities of several bipyridinium ions across the hydrocarbon bilayer membranes of dihexadecyl phosphate (DHP) small unilamellar vesicles have been determined by measuring the uptake of C-14-radiolabeled analogs. The N-alkyl-N'-methyl-4,4'-bipyridinium (viologen, CnMV2+) dications and the N-methyl-4,4'-bipyridinium cation were membrane-impermeable. However, the C(n)MV+ radical cations were permeable, provided that the n-alkyl chain length did not exceed 12 carbon atoms. Diffusion rates obeyed a simple first-order rate law; rate constants for n = 1 and n = 6 were nearly identical, and the constant for n = 12 was about 3-fold lower. From these values and the measured diffusion potential, the permeability coefficient of the N,N'-dimethyl-4,4'-bipyridinium radical cation was estimated to be 2 X 10(-8) cm s-1 at 23-degrees-C. When n = 16, radiolabel exchange was biphasic, with the major fraction of DHP-bound C16MV+ undergoing no transmembrane diffusion. This behavior confirms earlier suggestions, based upon the dynamics of viologen-mediated transmembrane redox reactions, that the short-chain viologens act as mobile charge relays, whereas the long-chain congeners transfer charge primarily by electron tunneling.