Kinetic evidence for the incorporation of the [(pentamethylcyclopentadienyl) (2,2'-bipyridyl)(aquo)rhodium(III)] complex into DPPC vesicles

The binding of the [(pentamethylcyclopentadienyl) (2,2'-bipyridyl)(aquo)rhodium(III)] complex [Cp*RhIII(bpy)H2O]2+, to L-alfa-dipalmitoylphosphatidyl choline (DPPC) vesicles has been estimated by studying the kinetics of the electron transfer reaction between the rhodium(III) complex and formiate ions. Kinetic measurements carried out under anaerobic conditions in absence and presence of DPPC show that the total reaction is composed of two steps. The rate of the first reaction increases with the phospholipid concentration, while that of the second process is independent of the concentration of DPPC. This is consistent with a reaction, where the two reacting species are partitioned between the aqueous and the vesicular pseudo-phases. The determined binding constants of [Cp*RhIII(bpy)H2O]2+ and formiate to the DPPC bilayers suggest that the palisade layer and the phospholipid head-group region of the DPPC vesicles are the respective solubilization sites.