Phase transitions in phosphatidylcholine multibilayers

Area of Science:

• Biophysics
• NMR Spectroscopy
• Lipid Bilayer Dynamics

Background:

• Lipid bilayers are fundamental to cell membranes.
• Understanding lipid molecular motion is crucial for membrane function.
• Cholesterol's role in modulating lipid dynamics is of significant interest.

Purpose of the Study:

• To investigate the molecular dynamics of a specific phospholipid (1-myristoyl-2-[14,14,14-(2H3)]myristoyl-sn-glycero-3-phosphocholine) in a multilamellar dispersion.
• To elucidate the effect of cholesterol on lipid molecular motion using Deuterium NMR (2H NMR).
• To correlate NMR findings with known structural phases of lipid bilayers.

Main Methods:

• Recording 2H NMR spectra of a deuterated phospholipid with 1 mol% cholesterol.
• Analyzing spectral data to obtain motionally averaged quadrupole coupling constants and asymmetry parameters.
• Conducting experiments across a temperature range from -15°C to 36°C.

Main Results:

• Below -4°C, molecular rotation about the long axis is slow on the NMR timescale.
• Above the pretransition, fast, hindered rotational motion (180° flip) about the molecular long axis is observed.
• Between -4°C and the pretransition, spectra suggest either two populations of molecules or a single population undergoing flipping motion.
• In the absence of cholesterol, more complex spectral patterns correlate with observed ripple structures.

Conclusions:

• Cholesterol significantly influences lipid molecular motion, promoting hindered rotation and flipping above the pretransition.
• The observed spectral changes are consistent with distinct phases and structural transitions in lipid bilayers.
• NMR data provide insights into the molecular basis of lipid-cholesterol interactions and phase behavior.