Surface potentials and electric dipole moments of ganglioside and phospholipid monolayers: contribution of the polar headgroup at the water/lipid interface

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Monolayers of different gangliosides (G M1, G D1a, G T1b, G Mix), ceramide (Cer), sulfatide (Sulf), phospholipids (DOPC, DPPE, DPPS, DOPA), a quaternary ammonium salt (DOMA) and fatty acids (C 16, C 18, C 20) were investigated at the air/water interface on pure water as well as on buffered subphases. Monolayers at the air/water interface consist of two interfaces: the water/lipid and the lipid/air interfaces. The normal components of the effective total dipole moments (Δμ), the effective local dipole moments (Δμ α) and effective local surface potentials ( ΔV α ) of polar headgroups at the water/lipid interface have been calculated from surface potential ( ΔV) and mean molecular area ( A) measurements of close-packed monolayers. The contribution of the lipid/air interface was previously determined experimentally by partial dipole compensation approach (Vogel, V. and Möbius, D. (1988) Thin Solid Films 159, 73–81). The surface potentials ( ΔV) of ganglioside monolayers are quite similar (e.g., G MI = −17 mV, G Tib = −39 mV; at surface pressure π = 30 mN/m triethanolamine (TEA)/HCl buffer, pH 7.4, as subphase); this indicates that variations in molecular structure of gangliosides like the influence of the number of negative charges per ganglioside which lead to appreciate changes in the average molecular packing do not cause large changes in surface potential. The local surface potentials ( ΔV α ) reach to minus several hundred millivolts for nearly all compounds, but clear differences are shown between negatively charged phospholipids (e.g., DPPS = −296 mV at π = 30 mN/m) and glycolipids (e.g., G M1 = −413 mV), and within glycolipids at different surface pressures (e.g., G D1n: − 342 mV at π = 20 mN/m versus −453 mV at π = 30mN/m nly in gangliosides (except for G D1b) the total dipole moments (Δμ) are negative (−0.029 up to − 0.078 D) and directed to the water. Unlike DOPC (+0.069 D) and DOMA (+0.421 D), the local dipole moments (Δμ α of all hydrated polar headgroups are negative (e.g., DPPS = −0.331 D; G M1 = −0.729 D) and directed from the monolayer (−) to the water (+). Under well-defined conditions investigated such data are helpful for a better understanding of the large functional role of gangliosides especially in determining the surface potential of biological membranes.