THE LOW-FREQUENCY DIELECTRIC RESPONSE OF LIPOSOMES

The dielectric response of dipalmitoylphosphatidylcholine (DPPC) lipos omes 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-frequenc y 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 ra nge will be a function of the presence of an electrode barrier layer i n series with a bulk layer. The barrier response was found to be relat ed to the nature of the liposomal bilayer, as a linear relationship wa s 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 th e phospholipid were all found to alter the bilayer response. It is sug gested that the low-frequency response is associated with the presence of a layer of liposomes absorbed onto the electrode surface, hence th e low-frequency technique has potential as a means of characterizing l iposomal suspensions. (C) 1994 Academic Press, Inc.