X-ray diffraction structures of some phosphatidylethanolamine lamellar and inverted hexagonal phases

Area of Science:

• Biophysics
• Materials Science

Background:

• Diacyl phosphatidylethanolamines (PEs) are crucial membrane lipids.
• Understanding their phase behavior (lamellar L(alpha) and inverted hexagonal H(II)) is key to membrane function.
• Previous studies showed less variation in PE phase dimensions.

Purpose of the Study:

• To determine low-resolution structures of hydrated PE dispersions using X-ray diffraction.
• To analyze the internal dimensions of L(alpha) and H(II) phases as a function of temperature.
• To investigate how PE hydrocarbon chain structure affects phase behavior.

Main Methods:

• X-ray diffraction was employed on aqueous dispersions of seven different diacyl phosphatidylethanolamines (PEs).
• Low-resolution structures of both liquid-crystalline lamellar (L(alpha)) and inverted hexagonal (H(II)) phases were solved.
• Key dimensions including d-spacing, water layer thickness, and headgroup area were measured.

Main Results:

• Significant variations in L(alpha) (51.2–56.4 Å) and H(II) (74.9–82.7 Å) phase d-spacings were observed, dependent on PE chain structure.
• D-spacings decreased with increasing temperature in both phases, with a slower rate in the L(alpha) phase.
• Results contrast with earlier findings of near-constant d-spacings for these PEs.

Conclusions:

• PE hydrocarbon chain structure strongly influences the dimensions of both L(alpha) and H(II) phases.
• Temperature affects d-spacing in both phases, but the rate of change differs.
• A detailed molecular model of the L(alpha)/H(II) phase transition is presented, offering new insights into lipid self-assembly.