DIELECTRIC-SPECTROSCOPY AS A SENSOR OF MEMBRANE HEADGROUP MOBILITY AND HYDRATION

Dielectric spectroscopy is based on the response of the permanent dipo les to a driving electric field. The phospholipid membrane systems of dimyristoylphosphatidylcholine and dioteoylphosphatidylcholine can be prepared as samples of multilamellar liposomes with a well known amoun t of interlamellar water. For optimal resolution in dielectric spectro scopy one has to design the experimental set-up so that the direction of the permanent headgroup dipole moment is mostly parallel to the fie ld vector of the external radio frequency (rf) electric field in this layered system. A newly developed coaxial probe technique makes it pos sible to sweep the measuring frequency between 1 and 1000 MHz in the t emperature range 286-323 K. The response yields both the dispersion (e psilon') and the absorption part (epsilon'') of the complex dielectric permittivity, which are attributed to the rotational diffusions of th e zwitterionic phosphatidylcholine headgroup and the hydration water, respectively. Although the contributions of the headgroup and the hydr ation dipole moments to the dielectric relaxation are found to be situ ated close together, we succeeded in separating them. In the language of the Debye description, we propose to assign the lower frequency por tion of the signal response to the relaxation contributed by the headg roups. The respective relaxation frequency is a discrete Value in the range of 15-100 MHz and it shows normal temperature dependence. The co ntribution of the hydration water molecules exhibits a similar behavio r in the range of 100-500 MHz but with the attributed relaxation frequ ency as the center of an asymmetric distribution of frequencies in ana logy to simulation models known from the literature. Activation energi es are derived for each of these relaxation processes from the Arrheni us plots of the temperature-dependent relaxation frequencies.