Studies on outer-sphere electron transfer reactions of surfactant-cobalt(III) complexes with iron(II) in liposome (dipalmitoylphosphotidylcholine) vesicles

The effect of unilamellar vesicles of dipalmitoylphosphotidylcholine (DPPC), both below and above the phase transfer region, on the second-order rate constants for outer-sphere electron transfer between Fe2+ and the surfactant-cobalt(III) complexes, cis-[Co(en)(2)(C12H25NH2)(2)](3+) and cis-[Co(trien)(C12H25NH2)(2)](3+) (en = ethylenediamine, trien = triethylenetetramine, C12H25NH2 = dodecylamine) was studied by UV-Vis absorption spectroscopy. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC. It is concluded that below the phase transition temperature, there is an accumulation of surfactant-cobalt(III) complexes at the interior of the vesicle membrane through hydrophobic effects, and above the phase transition temperature the surfactant-cobalt(III) complex is released from the interior to the exterior surface of the vesicle. Through isokinetic plots, we have established that the mechanism of the reaction does not alter during the phase transition of DPPC.