X-ray reflectivity study of a DPPC floating bilayer: Effect of Ca$^{2+}$ ions and temperature

We have used a floating bilayer of DPPC (1,2- dipalmitoyl-sn-glycero-3-phosphocholine) to examine the effects on Ca$^{++}$ ion concentration on membrane fluctuations. The density profile of the double bilayer system normal to the support was measured via x-ray specular reflectivity. We find an increase in membrane spacing with ion concentration which we attribute to Ca$^{++}$ ions preferentially binding to the head group of the bilayer. We also find an increase in interfacial roughness which we attribute to the Ca$^{++}$ ions causing a reduction in the layer's bending modulus. These effects are studied as a function of temperature up to the temperature at which the bilayer if found to unbind from the support.