Detection of membrane packing defects by time-resolved fluorescence depolarization

Area of Science:

• Membrane biophysics
• Lipid bilayer dynamics
• Cell membrane structure

Background:

• Packing defects in lipid bilayers significantly influence cell membrane biological activities.
• Lipid mixtures, such as dilinoleoylphosphatidylethanolamine/1-palmitoyl-2-oleoylphosphatidylcholine (PE/PC), can form distinct ordered phases (Lα, HII) and a mesoscopic packing defect state (D).
• Previous electron microscopy identified the D state by interlamellar attachments and HII phase precursors.

Purpose of the Study:

• To detect and characterize the onset of packing defects in binary PE/PC lipid mixtures.
• To investigate the sensitivity of rotational dynamics parameters in identifying membrane curvature-associating packing defects.
• To correlate packing defect characteristics with molecular dynamics and membrane curvature.

Main Methods:

• Time-resolved fluorescence depolarization of dipheny lhexatriene (DPH)-labeled PC (DPH-PC) in PE/PC mixtures.
• Application of a rotational diffusion model for rod-shaped fluorophores in curved matrices.
• Analysis of fluorescence depolarization decays to extract rotational dynamics parameters, order parameters, and diffusion constants.

Main Results:

• Abrupt increases in rotational dynamics parameters were observed around 60% PE concentration.
• These changes in parameters directly reflect the onset of packing defects in the PE/PC lipid matrix.
• Enhanced wobbling diffusional motion and localized curvatures characterize the packing defect state.

Conclusions:

• Rotational dynamics parameters are highly sensitive indicators for detecting curvature-associating packing defects in lipid membranes.
• The study successfully demonstrated the utility of fluorescence depolarization in characterizing membrane structural transitions.
• The findings provide insights into the molecular basis of packing defects and their role in membrane function.