Compressing inverse lyotropic systems: Structural behavior and energetics of dioleoyl phosphatidyl ethanolamine

The pressure effects on the stability and energetics of lipid phases in the L-α-dioleoyl phosphatidyl ethanolamine (DOPE)–water system are presented. Using synchrotron diffraction experiments, performed at a wide range of concentrations, pressure-induced transitions from the inverse hexagonal ${(H}_{\mathrm{II}})$ to the lamellar $L_{\alpha }$ phase and from the $L_{\alpha }$ to the lamellar $L_{\beta }$ phase are demonstrated. Moreover, in the most dehydrated samples an intermediate phase is found between the $H_{\mathrm{II}}$ and the $L_{\alpha }$ phases, confirming that the lamellar-to-nonlamellar phase transition occurs through key intermediate structures. Simple molecular packing arguments lead to an interpretation of the phase behavior: in fact, pressure induces a progressive stiffening of the DOPE hydrocarbon chains and a reduction of the cross-sectional area. Because pressure is more effective in reducing the cross-sectional area near the terminal methyl groups than at the water-lipid interface, the curvature of that interface in the $H_{\mathrm{II}}$ phase is reduced during compression. The work of isothermal compression was then obtained and analyzed in terms of the elastic energetic contributions that should stabilize the DOPE phases during compression. As a result, we observe that the isothermal lateral compression modulus is almost independent of concentration, but it increases as a function of pressure, suggesting that the DOPE repulsion becomes very strong while the whole lipid shape becomes more cylindrical. On the other hand, the bending rigidity is observed to decrease with increasing pressure, while the spontaneous curvature becomes less negative. This suggests that the chain repulsion becomes relatively weaker, and thus less efficient in balancing the torque of head-group repulsion, as the order parameter increases.