Simulations of Inverted Hexagonal Lipid Structures

Phospholipid molecules are composed of multipolar headgroup and two electrically neutral hydrocarbon tails. The inverted hexagonal phase (H-II) belongs to the biologically most significant non-lamellar phospholipid phases in biomembranes. Hence the geometric properties and conditions of transition to the H-II phase are nowadays widely studied. In our derivation of the free energy of lipid monolayers we assume that phospholipid molecules are in general anisotropic with respect to the axis perpendicular to the membrane plane. In our model the expression for the phospholipid monolayer free energy consists of two energy contributions: the bending energy which involves also deviatoric term, and the interstitial energy which describes the deformation energy due to stretching of the phospholipid molecule chains. On the basis of the derived expression for the phospholipid monolayer free energy we theoretically predict optimal geometry and physical conditions for the stability of the inverted hexagonal phase.