Interactions between proteins bound to biomembranes

We study a physical model for the interaction between general inclusions bound to fluid membranes that possess finite tension, as well as the usual bending rigidity. We are motivated by an interest in proteins bound to cell membranes that apply forces to these membranes, due to either entropic or direct chemical interactions. We find an exact analytic solution for the repulsive interaction between two similar circularly symmetric inclusions. This repulsion extends over length scales of order tens of nanometers, and contrasts with the membrane-mediated contact attraction for similar inclusions on tensionless membranes. For non circularly symmetric inclusions we study the small, algebraically long-ranged, attractive contribution to the force that arises. We discuss the relevance of our results to biological phenomena, such as the budding of caveolae from cell membranes and the striations that are observed on their coats.Comment: 22 pages, 2 figure

Dynamic phase separation of fluid membranes with rigid inclusions

Membrane shape fluctuations induce attractive interactions between rigid inclusions. Previous analytical studies showed that the fluctuation-induced pair interactions are rather small compared to thermal energies, but also that multi-body interactions cannot be neglected. In this article, it is shown numerically that shape fluctuations indeed lead to the dynamic separation of the membrane into phases with different inclusion concentrations. The tendency of lateral phase separation strongly increases with the inclusion size. Large inclusions aggregate at very small inclusion concentrations and for relatively small values of the inclusions' elastic modulus.Comment: 6 pages, 6 figure

Phospholipid membranes as substrates for polymer adsorption

A largely unsolved problem in soft materials is how surface reconstruction competes with the rate of adsorption. Here, supported phospholipid - bilayers of DMPC (1,2-dimyristoyi-sn-glycero-3-phosphocholine) were employed as substrates for the adsorption of a weak polyelectrolyte, polymethacrylic acid, whose time dependent ratio of charged to uncharged functional groups served to probe the local dielectric environment. Chains that encountered sparsely covered surfaces spread to maximize the number of segment-surface contacts at rates independent of the molar mass (which was varied by a factor of 30), but dependent on the phase of the lipid bilayer, gel or liquid crystal. Surface reconstruction rather than molar mass of the adsorbing molecules seemed to determine the rate of spreading. The significance of these findings is the stark contrast with well-known views of polymer adsorption onto surfaces having structures that are 'frozen' and unresponsive, and is relevant not just from biological and biophysical standpoints, but also in the formulation of many cosmetics and pharmaceutical products