Transmembrane electron transfer in lecithin (phosphatidylcholine) vesicles was studied by pulse radiolysis Upon reduction, cetylmethylviologen (N-hexadecyl-N′-methyl-4,4′-bipyridinium CMV), cetylbipyridine (4-(N-hexadecylpyridinium-4-yl)pyridine, CB), and cetylnicotinamide (N-hexadecyl-3-(aminocarbonyl)pyridinium, CNA) transferred electrons from the bulk water phase to Fe(CN)63- in the internal water phase of the vesicles. The transmembrane electron transfer was found in all cases to proceed through diffusion of uncharged forms of the redox mediators (CMV0, CB0, and CNA0, respectively) but the kinetic behavior varied considerably. The mechanisms for CB and CNA were simple, the reaction following first-order kinetics, and the transmembrane diffusion was rate limiting (k = (1.5 ± 0.3) × 103 s-1 for CB and k = 3.2 ± 0.5 s-1 for CNA). The mechanism for CMV was more complicated, and the reaction followed second-order kinetics. The rate-determining step was proposed to be the disproportionation of two viologen radicals formed by the radiation pulse (2CMV+ ⇔ CMV0 + CMV2+), followed by rapid transmembrane diffusion of CMV0 and its subsequent reoxidation by Fe(CN)63-. In pulse radiolysis, and in phosphorescence quenching experiments with Pt2(P2O5)4H8 4-, CB0 and CB+ were used as models in order to obtain the rates of transmembrane diffusion of CMV0 and CMV+, respectively. Our results exclude the possibility of electron tunneling between viologens on opposite sides of the membrane, and they provide strong arguments against transmembrane diffusion of viologen radical (CMV+).
|Number of pages||9|
|Journal||Journal of physical chemistry|
|Publication status||Published - Dec 1 1993|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry