Structural Properties of Inverted Hexagonal Phase: A Hybrid Computational and Experimental Approach

Inverted/reverse hexagonal (H_II) phases are of special interest in several fields of research, including nanomedicine. We used molecular dynamics (MD) simulation to study H_II systems composed of dioleoylphosphatidylethanolamine (DOPE) and palmitoyloleoylphosphatidylethanolamine (POPE) at several hydration levels and temperatures. The effect of the hydration level on several H_II structural parameters, including deuterium order parameters, was investigated. We further used MD simulations to estimate the maximum hydrations of DOPE and POPE H_II lattices at several given temperatures. Finally, the effect of acyl chain unsaturation degree on the H_II structure was studied via comparing the DOPE with POPE H_II systems. In addition to MD simulations, we used deuterium nuclear magnetic resonance (²H NMR) and small-angle X-ray scattering (SAXS) experiments to measure the DOPE acyl chain order parameters, lattice plane distances, and the water core radius in H_II phase DOPE samples at several temperatures in the presence of excess water. Structural parameters calculated from MD simulations are in excellent agreement with the experimental data. Dehydration decreases the radius of the water core. An increase in hydration level slightly increased the deuterium order parameter of lipids acyl chains, whereas an increase in temperature decreased it. Lipid cylinders undulated along the cylinder axis as a function of hydration level. The maximum hydration levels of PE H_II phases at different temperatures were successfully predicted by MD simulations based on a single experimental measurement for the lattice plane distance in the presence of excess water. An increase in temperature decreases the maximum hydration and consequently the radius of the water core and lattice plane distances. Finally, DOPE formed H_II structures with a higher curvature compared to POPE, as expected. We propose a general protocol for constructing computational H_II systems that correspond to the experimental systems. This protocol could be used to study H_II systems composed of molecules other than the PE systems used here and to improve and validate force field parameters by using the target data in the H_II phase.