Zwitterionic dipoles as a dielectric probe for investigating head group mobility in phospholipid membranes. 
For phospholipid membranes with zwitterionic head groups, the dipole can be considered as a specific label for tracing the changes in the dynamic behaviour of this region of the bilayer in its various phases.  Measurements of the dielectric properties of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayers in the frequency range 1--50 MHz show a dispersion which is attributed to the motion of the phosphocholine dipoles in the plane of the bilayers.  When the temperature is varied, both the permittivity and loss factor increase sharply at the pretransition (35 degrees C) and the main transition (42 degress C).  The relaxation time and amplitude were also determined for this dispersion and these further reflect the structural changes occurring with temperature.  The relaxation times varied between 4 ns at 30 degrees C and 2.3 ns at 50 degrees C. Due to steric hindrances a restriction in the angle of head group rotation occurs at lower temperatures but is greatly reduced above the main transition.