Zinc ions are highly abundant in biological systems and interact with various, enzymes, proteins and biomembranes. Our in-house state-of-the-art time-resolved solution SAXS and WAXS setup was used to study the interactions of divalent ions with charged membranes. We showed that unlike calcium ions that strongly couple and crystallize charged membranes very rapidly, zinc ions exhibit a fast time scale (seconds) for the strong coupling of the bilayers and a much slower one (hours) for the 2D lateral crystallization of the bilayers. This difference is attributed to the smaller zinc ion size (compared to calcium ions), which requires higher energy to shed its hydration shells. The rate of crystallization depends on the structure of the lipid tails and is slower for the unsaturated lipid, DOPS, than the saturated lipid, DLPS. We attribute this to the stronger steric repulsion between unsaturated DOPS tails, which have kinks, and to the weaker cohesive electrostatic energy, induced by the zinc ions, owing to the larger area per headgroup of DOPS. The Avrami model for a 2D growth mechanism with an instantaneous nucleation describes well the crystallization process. The crystallization involves various structural changes in the bilayer structure and lipid conformations within each bilayer. We determined those structural changes as a function of time (Soft Matter 2011).
