DIELECTRIC ANALYSIS OF PHOSPHORYLCHOLINE HEAD GROUP MOBILITY IN EGG LECITHIN LIPOSOMES

Purpose. A knowledge of the interfacial properties of lecithin underpi ns our understanding of many of the physicochemical characteristics of drug delivery systems such as liposomes and lecithin stabilized micro emulsions. In order to further this understanding, a high frequency di electric study of the interfacial properties of egg lecithin liposomes was undertaken. Methods. The effect of temperature, lecithin concentr ation and probe sonication on the interfacial dielectric properties of liposomal suspensions was investigated by high frequency dielectric r elaxation spectroscopy between 0.2-6 GHz.Results. The frequency depend ent permittivity of each suspension exhibited a dielectric dispersion centred around 100 MHz, corresponding to the relaxation of zwitterioni c head groups. The activation energy for head group reorientation was estimated as Delta H = 6.3 kT mol(-1). There was an increase in extent of inter-head group interactions on increasing the liposome volume fr action, whereas the effect of probe sonication showed that: (i) head g roups in both the outer and inner lamellae contribute to the dielectri c response; (ii) the head groups may be less restricted in liposomes o f high surface curvature with few lamellae; (iii) the high frequency p ermittivity of the suspension increased on sonication, as a result of a reduction in the amount of (depolarized) interlamellar water followi ng a reduction in the number of lamellae per liposome. Conclusions, Di electric analysis of the zwitterionic head groups of lecithin therefor e provides a means for investigating the surface of lecithin liposomes , and may be used to investigate the effect of drugs and other solutes on membranes.