Path dependence of three-phase or two-phase end points in fluid binary lipid mixtures.

The phase behavior of anionic/zwitterionic mixtures can be controlled by tuning the charge state of the anionic lipid. In the case of dioleoylphosphatidic acid (DOPA)/dioleoylphosphatidylcholine (DOPC) mixtures, demixing occurs either when DOPA is protonated or when DOPA(2-):Ca(2+) complexes form. Herein it will be shown that the final end point, a three-phase or two-phase system, depends on the order in which the charge state is manipulated. The facile accessibility of different end points is a clear demonstration of the inherent flexibility of biological systems.

Formation of complex three-dimensional structures in supported lipid bilayers.

Cell membranes are continually undergoing a wide range of shape transformations. Here, we demonstrate the formation of several structures in supported bilayers, including tubules, caps, and giant multivesicular structures. The key elements required for these transformations are osmotic pressure imbalances, insertion of lipids with positive curvature, and lipids whose curvature is dependent on the screening environment. With these elements, a wide variety of transformations can be achieved in the absence of protein.

Controlling the charge and organization of anionic lipid bilayers: effect of monovalent and divalent ions.

It is shown that the organization of lipid bilayers containing phosphatidic acid (PA) and phosphatidlycholine (PC) can be controlled by altering the monovalent and divalent ion concentrations. At high pH and/or calcium concentration, 1:1 Ca(2+)-PA(2-) complexes form; these complexes demix, and PA-rich and PC-rich regions are observable with epifluorescence microscopy. The results are compared with predictions from electrostatic theory. It is noted that the complex formation correlates in a roughly linear fashion with the monovalent/divalent ion ratio, a parameter that cells adjust.

Using ionic strength to control liquid-ordered/liquid-disordered separation.

In this Letter, we will show that liquid-ordered/liquid-disordered separation can be controlled with ionic strength. Using this observation, a robust method was developed for creating visible, by fluorescence microscopy, liquid-ordered domains in supported lipid bilayers. The details of the method will be discussed.