Direct determination of the calcium profile structure for dipalmitoyllecithin multilayers using neutron diffraction

The distribution of calcium in lamellar phases of dipalmitoyllecthin (DPPC) multilayers was directly determined by neutron diffraction and stable isotope substitution of 44Ca for 40Ca. A significant resonance effect on the intensities of the lamellar diffraction pattern was observed for millimolar concentrations of these calcium isotopes. The calcium difference profile indicated that calcium was localized in the phospholipid headgroup region, being excluded from the hydrocarbon core as was water separately determined from the water profile structure obtained by H2O/D2O exchange. A reciprocal space analysis of the difference structure factors indicated that calcium binds preferentially to within 1-2 A of the phosphate moiety of the phospholipid head groups of the DPPC bilayer.