Theoretical studies of the phase behavior of DPPC bilayers in the presence of macroions

A molecular theory is developed to assess the effect that the adsorption of a macroion onto a DPPC phospholipid bilayer has on its phase behavior. The proposed theoretical approach considers the molecular details of the phospholipid molecules, including their charge density, size and molecular conformations. It was found that the favorable electrostatic interactions between the negatively charged macroion and the zwitterionic phosphocholine head-groups lead to the stabilization of the DPPC bilayer gel phase. Consequently, the main chain transition temperature from the gel phase to the liquid-crystalline phase is raised by tens of degrees upon adsorption of the negatively charged macroion. The shift in the main transition temperature increases with the surface charge density of the adsorbed macroion. These results are in line with experimental observations and show how changes in the phospholipid bilayer environment can result in profound effects on the structural and thermodynamic behavior of the lipid films.