Geometry of phase-separated domains in phospholipid bilayers by diffraction-contrast electron microscopy

Area of Science:

• Lipid bilayer structure and phase behavior
• Materials science of lipid mixtures
• Advanced electron microscopy techniques

Background:

• Lipid bilayers exhibit phase separation into distinct solidus (gel) and liquid crystalline phases.
• Understanding domain morphology is crucial for comprehending membrane properties and function.
• Previous studies relied on indirect methods to infer domain characteristics.

Purpose of the Study:

• To directly visualize and quantify the sizes and shapes of solidus phase domains in hydrated lipid bilayers.
• To investigate the influence of temperature on domain morphology in different lipid mixtures.
• To compare domain behavior in less miscible (DLPC/DPPC) and more miscible (PS/DPPC) lipid systems.

Main Methods:

• Low dose diffraction-contrast electron microscopy with an environmental chamber for controlled temperature and humidity.
• Electron optical filtering of diffraction patterns to enhance contrast between crystalline domains.
• Quantitative analysis of domain dimensions (width, area) from micrographs.

Main Results:

• Solidus phase domains were visualized as a patchwork with average widths of 0.2-0.5 micrometers.
• Domain percentages correlated with known phase diagrams; shapes resembled freeze-fracture observations.
• Temperature-induced changes in domain size and boundary were significant in DLPC/DPPC but not in PS/DPPC mixtures.

Conclusions:

• Direct visualization confirms distinct molecular packing in phase-separated lipid bilayers.
• Diffraction-contrast electron microscopy provides quantitative measurements of domain boundaries and defects.
• Lipid miscibility influences the temperature-dependent domain dynamics within bilayers.