Mechanisms of oxidation—reduction across vesicle bilayer membranes: An overview

Abstract

Chemical and photochemical reduction of amphiphilic alkylviologen ions, C n MV 2+ ( n = 1, 6, 8, 10, 12, 14, 16), bound to dihexadecylphosphate (DHP) vesicle membranes has been studied. Reduction of viologen is concomitant with bleaching of photoexcited (5,10,15,20-tetrakis-(4-sulfonatophenyl)porphinato)zinc(II) ion ( 3ZnTPPS 4−), occurring via two parallel first order pathways. The relative contributions from these processes to the overall rate of viologen radical formation depends on the C n MV 2+:DHP ratio. Comparison of 3ZnTPPS 4− lifetime quenching rates and the rate of C n MV· + formation indicates that a very efficient charge separation process results in the formation of a longlived viologen radical intermediate. Reduction of short chain alkylviologens ( n < 12) by sodium dithionite obeys the rate law d[C n MV· +]/d t = k 1 [C n MV z+] [Na 2S 2O 4] 1 2 A second parallel pathway develops for n $ ̆ 12 having the same form of the rate expression as the first process, but with a different rate constant. The relative contributions from the two components are insensitive to the extent of C nMV 2+ loading, indicating the presence of two different forms of viologen. Similar kinetic observations were made using another chemical reductant, pentacyanocobaltate(II) ion, except that reduction of viologens with n < 12 becomes limited by the formation of the redox-active hexacyano species. These results are discussed within the context of current views of transmembrane electron transfer across bilayer membranes, and conceptual problems existing within the field are reviewed.