Calcium-mediated rigidity in PIP2 lipid domains

In lipid mixtures containing the highly negatively charged lipid PIP2 (a crucial component in cell membrane mechanics) multivalent ions such as calcium can drive the formation of PIP2-rich domains by mediating attractions between the lipids. Although the existence of ion-mediated attractions is well known in macromolecular systems, their form is poorly understood because they result from strong correlations between the charged molecules and ions. Within a numerical model of a lipid monolayer, we analyze the mechanics of PIP2-rich domains. We show that they are liquid-like at moderate values of the PIP2-charge but rigid at higher PIP2-charge. We use a recently introduced method to extract the effective pair interaction between the charged lipids in the many-body system, in which the calcium ions and remaining lipids are integrated out.