THE LOW-FREQUENCY DIELECTRIC RESPONSE OF LIPOSOMES

The dielectric response of dipalmitoylphosphatidylcholine (DPPC) liposomes has been examined over a frequency range from 10(5) to 10(-2) Hz under a variety of conditions in order to investigate the low-frequency dispersion noted in previous studies. The data have been interpreted in terms of the model proposed by Hill and Pickup (J. Mater. Sci. 20, 4431 (1985)), which predicts that the response over this frequency range will be a function of the presence of an electrode barrier layer in series with a bulk layer. The barrier response was found to be related to the nature of the liposomal bilayer, as a linear relationship was observed between the low-frequency capacitance and the mole fraction of cholesterol in the phospholipid bilayer. The size of the vesicles, the use of different lipid isomers, and changing the head group of the phospholipid were all found to alter the bilayer response. It is suggested that the low-frequency response is associated with the presence of a layer of liposomes absorbed onto the electrode surface, hence the low-frequency technique has potential as a means of characterizing liposomal suspensions. (C) 1994 Academic Press, Inc.