ROLE OF BILAYER TILT DIFFERENCE IN EQUILIBRIUM MEMBRANE SHAPES

Lipid bilayer membranes below their main transition have two tilt orde r parameters, corresponding to the two monolayers. These two tilts may be strongly coupled to membrane shape, but only weakly coupled to eac h other. We discuss some implications of this observation for rippled and saddle phases, bilayer tubules, and bicontinuous phases. Tilt diff erence introduces a length scale into the elastic theory of tilted flu id membranes. It can drive an instability of the flat phase; it also p rovides a simple mechanism for the spontaneous breaking of inversion s ymmetry seen in some recent experiments.