Anomalous dielectric behavior of undulated lipid membranes. Theoretical model and dielectric spectroscopy measurements of the ripple phase of phosphatidylcholine

We measured the static and dynamic dielectric properties of phosphatidylcho line (PC) spherical membranes (liposomes) over a wide range of temperature and for different liposome radii, pH, and liposomes concentration. Within t he investigated temperature range, the physical state of the membrane goes from a gellike to a fluidlike phase, passing through a narrow (approximate to 10 degreesC) intermediate state, the Ripple phase, characterized by perm anent surface undulations of defined wavelength and amplitude. The dielectr ic properties of the ripple phase are anomalous, the mean static permittivi ty is higher than that of the gel and fluid phases (where no undulations ar e present), while the average relaxation frequencies is smaller. Furthermor e, the static dielectric permittivity of the fluid phase is much higher tha n that of the gel phase, while the relaxation frequencies behave just in th e opposite way. In order to rationalize this complex behavior we have devel oped a dynamic mean-field model aimed to calculate the electric polarizatio n and the dipolar relaxation for an array of strongly interacting dipoles a nchored to the membrane surface and rotating over either a planar (gel and fluid phases) or a corrugated surface (the ripple phase). The dipoles mimic the highly polar (approximate to 20 Debyes) head groups of PC molecules. T he model correctly predicts, albeit qualitatively, most of the observed die lectric anomalies essentially related to the changes of the dipole-dipole c orrelation induced by the membrane surface undulations. (C) 2001 American I nstitute of Physics.