Translational diffusion in phospholipid bilayer membranes

The translational diffusion in artificial phospholipid bilayers has attracted much interest in the past. This process is widely accepted to be of importance for the diffusion of biological membrane components. The lipid bilayer is known as the permeability barrier in biological membranes. So far the diffusion of the membrane components has been determined indirectly by sophisticated methods, e.g., by the help of fluorescent probes. We present here experimental data on the orientational relaxation of the lipid headgroup in alternating electric fields. The dipole relaxation time measured by high-frequency spectroscopy is shown to be related to the self-diffusion coefficient. We have determined the electric dipole relaxation of two phospholipids and of their mixture. The dielectric relaxation time and thus the diffusion coefficient was measured as a function of temperature in both the liquid and the gel crystalline phase. The mixture even allows one to study the relaxation and the diffusion behavior in a state where both gel and liquid phases coexist. The experimental study has revealed the potential of high-frequency dielectric spectroscopy to determine the diffusion coefficient and its thermal activation energy in phospholipid membranes where the dipolar relaxation of the headgroup is associated with a site change.