Effect of ions on the organization of phosphatidylcholine/phosphatidic acid bilayers, Biophys

ABSTRACT Lipid bilayers are two-dimensional fluids. Here, the effect of monovalent ion concentration on the mixing, and conse-quently the organization, of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-sn-glycero-3-phosphate (DOPA) bi-layers has been examined. Epifluorescence microscopy was used to visualize the organization. Fluorescence recovery after photobleaching and attenuated total reflection-Fourier transform infrared spectroscopy were used to assess the fluidity of the lipids. At high ionic strength the DOPC and DOPA lipids appear uniformly mixed. Upon lowering the ionic strength, rapid separation is observed. The DOPA-rich regions appear fractal-like and exhibit hysteresis in their properties. The lipids freely exchange be-tween the two regions. These experiments clearly demonstrate the significant effect that electrostatics can have on membrane organization