Kinetics of reduction of cis-bis(dodecylamine)bis(1,10-phenanthroline)cobalt(III) perchlorate and cis-bis(dodecylamine)bis(2,2'-bipyridine)cobalt(III) perchlorate by Fe(II) in dipalmitoylphosphatidylcholine vesicles

The kinetics of reduction of the surfactant complex ions cis-[Co(phen)(2)(C12H25NH2)(2)](3+) and cis-[Co(bpy)(2)(C12H25NH2)(2)](3+) (phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine, C12H25NH2 = dodecylamine) in solutions of unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) were studied at different temperatures by a spectrophotometric method under pseudo-first-order conditions using an excess of the reductant. Experimentally the reaction was found to be second order and the electron transfer postulated as outer sphere. Below the phase temperature of DPPC the second-order rate constant for this electron-transfer reaction of both the complexes was found to decrease with increasing concentration of the liposome. Above the phase transition temperature the reaction rate was found to increase with increasing concentration of DPPC. Below the phase transfer temperature the results have been explained on the basis of electrostatic double layer repulsion which prevents the vesicles from accumulation of surfactant-cobalt(III) complexes in the interior of the vesicle membrane. Above the phase transfer temperature the release of the surfactant-cobalt(III) complex molecules from the interior core to the exterior surface of the vesicle through the strong hydrophobic effect increases the reaction rate. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between the surfactant complexes and vesicles surface. We could observe isokinetic relationships for both the complexes of the present study at the phase transfer temperature of DPPC.