Bilayer Membranes Containing the Ganglioside G_M1: Models for Electrostatic Potentials Adjacent to Biological Membranes

Although the Gouy-Chapman-Stern theory of the aqueous diffuse double layer describes well the electrostatic potential adjacent to negatively charged phosphohpid bilayer membranes, it does not describe adequately the f potential of biological membranes: the f potential of an erythrocyte is about half the value predicted from the theory by using the known density of negatively charged sialic acid residues. To investigate the factors responsible for this low electrophoretic mobility, we formed membranes from mixtures of the zwitterionic lipid phosphatidylcholine, PC, and the glycohpid galactosyl-N-acetylgalactosaminyl(Ar-acetylneuraminyl)-galactosylglucosylceramide, G_M1. This glycolipid differs from phospholipids in two respects. First, the negative charge on G_M1 is located about 1 nm from the surface, which tends to increase the electrophoretic mobility of vesicles. Second, the head group of G_M1 contains five sugar groups that exert a hydrodynamic drag, which tends to decrease the mobility of the vesicles. In a decimolar monovalent salt solution, where the Debye length is about 1 nm, the electrophoretic mobility of the PC-G_M1 vesicles is about half the mobility of PC-phosphatidylserine or PC-phosphatidylglycerol vesicles of equivalent composition. In addition, conductance measurements with planar bilayer membranes as well as ³¹P nuclear magnetic resonanee and fluorescence measurements with sonicated vesicles indicate that the potential at the surface of PC-G_M1 membranes is about half the value measured for PC-phosphatidylserine membranes in a 0.1 M monovalent salt solution.