Effects of Passive Phospholipid Flip-Flop and Asymmetric External Fields on Bilayer Phase Equilibria

Abstract

Compositional asymmetry between the leaflets of bilayer membranes modifies their phase behavior and is thought to influence other important features such as mechanical properties and protein activity. We address here how phase behavior is affected by passive phospholipid flip-flop, such that the compositional asymmetry is not fixed. We predict transitions from ''pre-flip-flop'' behavior to a restricted set of phase equilibria that can persist in the presence of passive flip-flop. Surprisingly, such states are not necessarily symmetric. We further account for external symmetry breaking, such as a preferential substrate interaction, and show how this can stabilize strongly asymmetric equilibrium states. Our theory explains several experimental observations of flip-flop-mediated changes in phase behavior and shows how domain formation and compositional asymmetry can be controlled in concert, by manipulating passive flip-flop rates and applying external fields.

Experimentally determined leaflet–leaflet phase diagram of an asymmetric lipid bilayer

“…4 with Λ < 0.25 would result in Type IIA behavior and those with Λ > 0.25 would result in Type IA behavior. As mentioned above and noted previously by Williamson and Olmsted ( 15 ), this classification scheme maps directly to London’s coupling dominance model ( 34 ), with Type IA and Type IIA behavior corresponding to disordered-leaflet dominance and separated-leaflet dominance, respectively.…”

“…Leaflet–leaflet phase diagrams of asymmetric bilayers have been predicted using mean-field theories ( 12 – 15 ). We used a simplified theoretical approach employing a regular solution model with a single parameter each to describe in-plane and out-of-plane interactions between DPPC and DOPC, as described in Methods.…”

“…Translocase activity alters the distribution of lipids between leaflets while maintaining fixed overall bilayer composition. In the context of a leaflet–leaflet phase diagram, this corresponds to movement along trajectories perpendicular to the symmetric diagonal which—depending on the pre-flip-flop composition—can result in phase transitions ( 15 ). Phase diagrams like those shown in Fig.…”

“…In this regard, asymmetry is an additional thermodynamic degree of freedom that is relatively unexplored in the context of bilayer phase behavior; indeed, we are aware of no experimentally determined phase diagram for an asymmetric lipid bilayer. Despite a paucity of data, theorists have proposed leaflet–leaflet phase diagrams from mean-field models ( 12 – 15 ). In the simplest approach based on the Bragg-Williams approximation of regular solution theory, the free energy is governed by a competition between mixing entropy and the relatively unfavorable enthalpy of unlike lipid interactions; these are accounted for with a single parameter each for in-plane and out-of-plane interactions between unlike lipids.…”