From angular to round: in depth interfacial analysis of binary phosphatidylethanolamine mixtures in the inverse...

Area of Science:

• Biophysics
• Materials Science
• Lipid Self-Assembly

Background:

• Lipidic inverse hexagonal HII phase exhibits packing stress due to geometric constraints.
• This stress arises from fitting cylindrical micelles into hexagonal unit cells, leading to lipid chain deformation and interface curvature.

Purpose of the Study:

• To investigate how doping with 1,2-palmitoyl-sn-phosphatidylethanolamine (DPPE) affects packing stress in 1-palmitoyl-2-oleoyl-sn-phosphatidylethanolamine (POPE) lipid bilayers.
• To analyze the impact of DPPE on the lipid/water interface shape and homogeneity.

Main Methods:

• Small-angle X-ray diffraction (SAXD) was used to obtain electron density maps.
• Analysis of electron density maps to determine the size, shape, and homogeneity of the lipid/water interface and methyl trough region.
• Application of a three-water layer model to differentiate headgroup, perturbed, and free water.

Main Results:

• Increasing DPPE concentration (0-15 mol%) led to a more circular lipid/water interface.
• The 9 mol% DPPE sample exhibited the most circular water core and the smallest water core area.
• This optimal circularity at 9 mol% DPPE resulted in the smallest number of lipids per circumference, effectively alleviating packing stress.

Conclusions:

• DPPE doping can mitigate packing stress in the hexagonal HII phase by increasing interface circularity.
• The hexagonal phase is most stable in compression zones (flat faces) and least stable in decompression zones (vertices).
• Understanding these structural and energetic aspects is crucial for designing lipid-based materials.