RESUMO
The electrophoretic mobility of vesicles is measured for concentrations of poly(ethylene glycol) from 0 wt.% to 10 wt.%. Mixtures of phosphatidylcholines and phosphatidic acid are used. The zeta potential calculated from the electrophoretic mobility and the viscosity of the suspension becomes more negative for all vesicles studied. Binding of poly(ethylene glycol) to the phospholipid surface by addition of the poly(oxyethylene)-containing surfactant C12E8 has the opposite effect and a decrease of the zeta potential is observed. Independent measurements of the surface potential of the vesicles in the presence of PEG by use of a positively charged spin probe and ESR spectroscopy and a fluorescent pH indicator and fluorescence spectroscopy show that actually the surface potential is not changed. A theory of the electrophoretic behaviour of vesicles in the presence of PEG is given which explains the contradiction between the two methods. It is assumed that the polymer is excluded from the vesicle surface (depletion layer) and that the viscosity near the surface is lower than the viscosity in the bulk phase. The thickness of the depletion layer is calculated from the experiments. The decrease of the zeta potential in the presence of poly(oxyethylene) chains linked to the vesicle surface results from the friction increase.
Assuntos
Lipossomos/metabolismo , Polietilenoglicóis/metabolismo , Cálcio/farmacologia , Fenômenos Químicos , Físico-Química , Detergentes , Eletroquímica , Espectroscopia de Ressonância de Spin Eletrônica , Eletroforese , Concentração de Íons de Hidrogênio , Bicamadas Lipídicas/metabolismo , Potenciais da Membrana , Ácidos Fosfatídicos , Fosfatidilcolinas , Polietilenoglicóis/farmacologia , Espectrometria de Fluorescência , ViscosidadeRESUMO
Model membranes (egg-yolk PC liposomes) were exposed to the cationic form of amphiphilic drugs. Microelectrophoresis was used to measure the change of the electrokinetic potential as a function of the drug concentration. By use of the Gouy-Chapman theory the surface potential and surface charge density were calculated. A theoretical model postulating a simple partition equilibrium of the charged drug molecules between the membrane and the aqueous phase in the vicinity of the membrane failed to describe the experimental results. Modification of the partition law by introducing a mechanism of saturation at high drug concentrations, however, resulted in concordance of model and experiment. Some parameters of the model can be used as a means of evaluating the efficiency of neuroactive drugs.