Origin of membrane dipole potential: contribution of the phospholipid fatty acid chains

The large intrinsic membrane dipole potential, phi(d), is important for protein insertion and functioning as well as for ion transport across natural and model membranes. However, the origin of phi(d) is controversial. From experiments carried out with lipid monolayers, a significant dependence on the fatty acid chain length is suggested, whereas in experiments with lipid bilayers, the contribution of additional -CH(2)-groups seems negligibly small compared with that of the phospholipid carbonyl groups and lipid-bound water molecules. To compare the impact of the -CH(2)-groups of dipalmitoylphosphatidylcholine (DPPC) near and far from the glycerol backbone, we have varied the structure of DPPC by incorporation of sulfur atoms in place of methylene groups in different positions of the fatty acid chain. The phi(d) of symmetric lipid bilayers containing one heteroatom was obtained from the charge relaxation of oppositely charged hydrophobic ions. We have found that the substitution for a S-atom of a -CH(2)-group decreases phi(d). The effect (deltaphi(d) = -22.6 mV) is most pronounced for S-atoms near the lipid head group while a S-atom substitution in the C(13)- or C(14)-position of the hydrocarbon chain does not effect the bilayer dipole potential. Most probably deltaphi(d) does not originate from an altered dipole potential of the acyl chain containing an heteroatom but is mediated by the disruption of chain packing, leading to a decreased density of lipid dipoles in the membrane.