Measurements of interactions between lipid membranes
Associate Professor of Physics, Indiana Univ Purdue Univ Indianapolis
Tuesday, November 27, 3:45 p.m. SB110
I will talk about measurements of interactions between membranes made of lipids, which are molecules found in biological cells. Some lipid molecules are electrically charged, but most common are electrically neutral. For both types of lipids, charge fluctuation forces (van der Waals) arise due to the fact that transient polarization in one membrane induce correlated polarization in the neighboring membrane and vice versa. These correlations cause an attractive interaction between neighboring membranes. However, attractive membranes do not collapse but reach a certain equilibrium distance which can be measured by x-ray scattering. Typical equilibrium distances at room temperature are on the order of 5-10 nm while the thickness of lipid membranes is about 4-5 nm. At equilibrium, the attractive van der Waals force between membranes must be exactly cancelled by repulsive forces. Apart from electrostatics, repulsive contributions have an entropic origin. One is due to the ordering of water molecules between lipid membranes and it is called a hydration force. The other repulsive effect is due to the fact that lipid membranes can thermally undulate and this contribution to the free energy of the system gives rise to an undulation force. I will present and compare measurements of membrane interactions for charged and neutral lipids and with and without the presence of mobile ions.