Bilayer membranes containing the ganglioside GM1: models for electrostatic potentials adjacent to biological membranes

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Abstract
Although the Gouy-Chapman-Stern theory of the aqueous diffuse double layer describes well the electrostatic potential adjacent to negatively charged phospholipid bilayer membranes, it does not describe adequately the .zeta. potential of biological membranes: the .zeta. potential of an erythrocyte is about 1/2 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, membranes from mixtures of the zwitterionic lipid phosphatidylcholine, PC, and the glycolipid galactosyl-N-acetylgalactosaminyl(N-acetylneuraminyl)-galactosylglucosylceramide, GM1 were formed. This glycolipid differs from phospholipids in 2 respects. First, the negative charge on GM1 is located about 1 nm from the surface, which tends to increase the electrophoretic mobility of vesicles. Second, the head group of GM1 contains 5 sugar groups that exert a hydrodynamic drag, which tends to decrease the mobility of the vesicles. In a decimolar monovalent salt solution, where the Debeye length is about 1 nm, the electrophoretic mobility of the PC-GM1 vesicles is about 1/2 the mobility of PC-phosphatidylserine or PC-phosphatidylglycerol vesicles of equivalent composition. Conductance measurements with planar bilayer membranes as well as 31P NMR and fluorescence measurements with sonicated vesicles indicate that the potential at the surface of PC-GM1 membranes is about 1/2 the value measured for PC-phosphatidylserine membranes in a 0.1 M monovalent salt solution.

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