Dept. of Chemistry
Oregon Graduate Center
Electron transfer reactions in artificial vesicle solutions were studied. Binding of various redox active molecules to vesicles was demonstrated by gel exclusion chromatography, optical absorption spectroscopy, and ultrafiltration techniques. Photoexcitation of (5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphinato)zinc(II) ion (ZnTMPyP[superscript 4+]) or tris(2,2'-bipyridyl)ruthenium(II) ion (Ru(bpy)[subscript 3][superscript 2+]) adsorbed onto the external surface of dihexadecyl phosphate (DHP) vesicles containing internally bound methyl viologen dications(MV[superscript 2+]) leads to enhanced diffusion of viologens across the bilayer. The observed temperature dependence of both thermal (in the absence of sensitizer) and photoenhanced viologen diffusion suggests the photostimulated mechanism involves localized heating arising from nonradiative deactivation of the bound excited sensitizer. Oxidative quenching of unbound ZnTMPyP[superscript 4+] by alkylviologens (C[subscript n]MV[superscript 2+]) bound to phosphatidylcholine (PC) vesicles shows Stern-Volmer kinetics. Similarly, 5,10,15,20-tetrakis(4-sulfonatopyridyl)porphinato zinc(II)(ZnTPPS[superscript 4-]) ion undergoes bimolecular quenching with DHP-bound viologens and in steady-state photolysis gives rise to efficient net formation of reduced viologens. When ferricyanide ion is incorporated into the internal aqueous phase of DHP vesicles containing viologens bound to both surfaces, accumulation of reduced viologen is retarded, suggesting that the viologens are capable of transmembrane electron transfer. Reduction of (NH [subscript 3]) [superscript 5] Ru-4-(11'-dodecenyl) pyridine (III) ions bound to inner and outer surfaces of PC vesicles by external reductant is described. Addition of membrane impermeable reductants in stoichiometric excess resulted in biphasic ruthenium reduction. The fast component (<10[superscript -3] sec) is followed by a slow first-order component(t[subscript Â½] "equals approximately" 500 s at 23Â°C) which is independent of both the identity and concentration of the reductant. The slow component comprises ~30% of the total reaction, which corresponds to the inner surface to total vesicle surface area ratio. No transmembrane diffusion of the bound ruthenium ions on the time scale of the slow redox step could be found. These results suggest that the rate-limiting step in slow reduction is transmembrane electron exchange between ruthenium ions bound to opposite vesicle surfaces. The transfer rate and distance are treated by an electron tunneling model in which electrons hop to the intermediary trapping sites in the hydrocarbon phase of the bilayer. The temperature dependence of transmembrane redox is explained in terms of bilayer thinning at elevated temperatures, giving rise to shorter transferring distances.
Lee, Lester Y. C., "Transmembrane electron transfer in artificial bilayers" (1985). Scholar Archive. 82.