One-electron reduction of dihexadecyl phosphate vesicle-bound viologens by dithionite ion

Abstract

N-Methyl-N'-alkyl-4,4'-bipyridinium (C/sub n/MV/sup 2 +/) ions bound to dihexadecyl phosphate vesicles are rapidly reduced to radical cations by S/sub 2/O/sub 4//sup 2 -/ in weakly alkaline solutions. For the short-chain viologens investigated (n = 1, 6, 8), the reaction obeys the rate law d(C/sub n/MV/sup +/)/dt = k(C/sub n/MV/sup 2 +/)(S/sub 2/O/sub 4//sup 2 -/)/sup 1/2/. The measured rate constants were identical for preparations in which viologen was added to preformed vesicles and in which vesicles were formed in the presence of viologen, although in the latter systems 10-30% of the viologen was inaccessible to the reductant. For longer chain C/sub n/MV/sup 2 +/ (n = 12, 14, 16, 18, 20), viologen radical formation was biphasic when binding was constrained to the external aqueous-vesicle interface, but it was triphasic when viologen was bound at both interfaces. Where determined, each step was first order in C/sub n/MV/sup 2 +/ and half order in S/sub 2/O/sub 4//sup 2 -/; viologen radical yields were identical in vesicles prepared by either method. The results are interpreted to indicate the simultaneous existence of multiple binding domains for long-chain viologens within the vesicles. Possible structures and relationships to transmembrane redox pathways are briefly discussed.