An EXAFS study of the binding of Cd and Pb ions to lipid films

Extended X-ray Absorption Fine Structure (EXAFS) measurements performed on Langmuir-Blodgett (LB) films containing cadmium and lead ions reveal the different coordination structures of the two cations in lipid membranes. We describe the local atomic environment of cadmium and lead in LB films prepared with stearic and 1,2 distearoyl-Lα-phosphatidic acids. The measurements have been performed on films of two different thicknesses, one and seven molecular layers, and at two different values of relative humidity. The local atomic environment of Cd ions in stearate films is consistent with unidentate coordination in which a Cd ion binds two stearate molecules, while that of Pb ions is consistent with a bidentate coordination in which a Pb ion binds one stearate molecule. Furthermore, in lead stearate films, there is Pb-Pb coordination as already observed in Langmuir films. In films of Pb-phosphatidic acid, oxygen atoms of the organic phosphate and oxygen atoms of bound water form two distinct shells.