Lipid raft-like domains in model and biological membranes have been heavily studied. Regulating the size of rafts in-vitro, however, remained a challenge. Although in biomembranes the size of rafts is ca. 10 – 30 nm, in-vitro saturated and unsaturated lipid mixtures phase separate into micro-domains. The origin of these differences is unclear and earlier studies used primarily imaging methods with limited resolution at the molecular level and so far were unable to resolve this puzzle.

We developed a unique approach for studying the phase behavior of lipid mixtures. Using the differences in the affinity of calcium ions to saturated and unsaturated dipolar membranes (Langmuir 2011a), solution X-ray scattering and the analysis software X+ developed in our laboratory (see X+ papers), we showed that calcium ions could noninvasively probe the phase behavior and structure of saturated, hybrid (containing a saturated and an unsaturated tail), and unsaturated zwitterionic lipid mixtures.

We confirmed that saturated and unsaturated lipid phase separate in-vitro into large domain. We then demonstrated that by controlling the concentration of a naturally occurring hybrid lipid, containing a saturated and an unsaturated tails, we could stabilize and regulate the size of the domains and even reach complete miscibility of the three lipids. We attribute this behavior to the reduction in the line-tension between the immiscible lipids, induced by the hybrid lipid (Langmuir 2011b).