The unbinding transition of mixed fluid membranes

– A phenomenological model for the unbinding transition of multi-component fluid membranes is proposed, where the unbinding transition is described using a theory analogous to Flory-Huggins theory for polymers. The coupling between the lateral phase separation of inclusion molecules and the membrane-substrate distance explains the phase coexistence between two unbound phases as observed in recent experiments by Marx et al. [Phys. Rev. Lett. 88, 138102 (2002)]. Bellow a critical end-point temperature, we find that the unbinding transition becomes first-order for multi-component membranes. Introduction. – Adhesion of membranes and vesicles is responsible for cell-cell adhesion which plays an important role in all multicellular organisms. In general, bio-adhesion is governed by the interplay of a variety of generic and specific interactions [1]. The latter interaction acts between complementary pairs of proteins such as ligand and receptor, or antibody and antigen. Well-studied examples of such coupled systems are biotin-avidin complex [2], or selectins and their sugar ligands [3]. The problem of adhesion of multi-component membranes is intimately related to that of domain formation. Experimentally, adhesion-induced lateral phase separation has been observed for various systems [4–6], and it was reported that adhesion molecules aggregate spontaneously and form tight adhering domains. From the theoretical point of view, this problem has been considered in [7] using a phenomenological model, where the inter-membrane distance is coupled to the concentration of sticker molecules on the two adhering membranes. In a different approach, a lattice model for a multi-component membrane in contact with another substrate was proposed [8], and was extended using detailed Monte Carlo simulations [9, 10]. More recently, a work combining these two approaches has been published [11]. In a recent experiment by Marx et al. [12], the role of a long-range repulsion due to thermal fluctuations (Helfrich repulsion) of the adhering membranes has been addressed. By using Reflection Interference Contrast Microscopy [4], a multi-component bilayer membrane (∗) E-mail: [email protected] (∗∗) E-mail: [email protected]