ELECTROSTATIC CHARACTERIZATION OF LIPOSOMES

A variety of experimental techniques can be used to probe the electrostatic characteristics of lipid bilayers and liposomes. Many are based on the titration of the charge-dependent liposome properties. Determination of the lipid (headgroup) conformation, bilayer phase transition temperature, interbilayer force, mobility in external electric field, or transbilayer transport are but a few examples of this. Laser-Doppler velocimetry, based on dynamic light scattering by the lipid suspension, is probably the best currently available tool for the fast and/or routine determination of the electrostatic potential of liposomes; the proviso for this is that data analysis is done prudently. In addition to this potential-dependent adsorption or binding of ion or ionic probes to the liposome surface can be measured. Interesting organic ions can be made visible by suitable (fluorescent, nuclear, paramagnetic, etc.) labels. The conclusions drawn from any ion-liposome interaction study are only reliable, however, if Coulombic, as well as non-trivial electrostatic effects are both accounted for. When the magnitude of the latter is not known theoretically, the accuracy of the data analysis can be improved by subtracting a complementary set of data (measured with uncharged lipid vesicles or in a highly concentrated salt solution) from the corresponding results obtained with charged liposomes or at low salt concentration.