Miscibility tricritical points in asymmetric lipid bilayers*

Cellular membranes are lipid bilayers which are composed of two asymmetric leaflets and play an important role in regulation. In particular, the coupling and inductions between leaflets can help to colocalize proteins in the inner and outer leaflets, and thereby contribute to signaling processes. In experiments with model membranes, it was shown that in asymmetric bilayers domain formation in one leaflet may suppress or induce domain formation in the other leaflet, ultimately leading to three-phase coexistence in the membrane. This finding of three-phase coexistence hints to the fact that asymmetric membranes exhibit a tricritical point with special thermodynamic properties. To investigate the conditions under which such a tricritical point occurs, we consider an Ising-type lattice model with two coupled layers representing the two leaflets of cell membranes. Combining Monte-Carlo simulations with a Landau free energy approach, we find a line of tricritical points requiring both a coupling between the leaflets and leaflet asymmetry. Moving forward, we hope to suggest specific experiments to test our model predictions.