The Gaussian curvature elastic modulus of N-monomethylated dioleoylphosphatidylethanolamine: Relevance to membrane fusion and lipid phase behavior

The energy of intermediates in fusion of phospholipid bilayers is sensitive to κ_m, the saddle splay (Gaussian curvature) elastic modulus of the lipid monolayers. The value κ_m is also important in understanding the stability of inverted cubic (Q_∥) and rhombohedral (R) phases relative to the lamellar (L_α) and inverted-hexagonal (H_∥) phases in phospholipids. However, κ_m cannot be measured directly. It was previously measured by observing changes in Q_∥ phase lattice dimensions as a function of water content. Here we use observations of the phase behavior of N-mono-methylated dioleoylphosphatidylethanolamine (DOPE-Me) to determine κ_m. At the temperature of the L_α/ _∥ phase transition, T_Q, the partial energies of the two phases are equal, and we can express κ_m in terms of known lipid monolayer parameters: the spontaneous curvature of DOPE-Me, the monolayer bending modulus κ_m, and the distance of the monolayer neutral surface from the bilayer midplane, δ. The calculated ratio κ_m/κ_m is -0.83 ± 0.08 at T_Q ≈ 55°C. The uncertainty is due primarily to uncertainty in the value of δ for the L_α phase. This value of κ_m/ κ_m is in accord with theoretical expectations, including recent estimates of the value required to rationalize observations of rhombohedral (R) phase stability in phospholipids. The value κ_m substantially affects the free energy of formation of fusion intermediates: more energy (tens of k_BT) is required to form stalks and fusion pores (ILAs) than estimated solely on the basis of the bending elastic energy. In particular, ILAs are much higher in energy than previously estimated. This rationalizes the action of fusion-catalyzing proteins in stabilizing nascent fusion pores in biomembranes; a function inferred from recent experiments in viral systems. These results change predictions of earlier work on ILA and Q_II phase stability and L_α/Q_II phase transition mechanisms. To our knowledge, this is the first determination of the saddle splay (Gaussian) modulus in a lipid system consisting only of phospholipids.