Effect of NaCl and CaCl2 on the lateral diffusion of zwitterionic and anionic lipids in bilayers

The influence of addition of NaCl or CaCl2 (0.3 and 0.1 M, respectively) on the lateral diffusion coefficient (DL) of dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylglycerol (DOPG) was measured by the pulsed field gradient NMR technique. DL of DOPC was unaffected, whereas the DOPG diffusion decreased with salt concentration. 23Na NMR quadrupole splittings of DOPG between 20 and 60 °C and added NaCl between 0 and 15 wt% decreased only slightly with salt content, but increased with increasing temperature. Similar results were obtained for palmitoyloleoylphosphatidylglycerol, in which the palmitoyl chain order parameter increased slightly with salt. A model with free and “bound” ions was used to interpret the splitting data.With increasing salt content a decrease in the water permeability for DOPG was observed, but not for DOPC, as measured by water diffusion perpendicular to the oriented lipid bilayers.It was concluded that calcium and sodium ions interacted with the DOPG head-groups resulting in a decrease in the “free area” per lipid molecule due to a screening of the charged lipid head-groups. Thus, there was a closer packing of DOPG, leading to a decrease in DL and water permeability. DOPC did not show any changes in the bilayer properties upon the addition of ions.