Miscibility gap in fluid dimyristoylphosphatidylcholine:cholesterol as "seen" by x rays

Area of Science:

• Biophysics
• Materials Science
• Physical Chemistry

Background:

• Binary mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol exhibit a fluid miscibility gap under excess water.
• Understanding lipid-cholesterol interactions is crucial for membrane biophysics.

Purpose of the Study:

• To investigate the phase behavior of DMPC-cholesterol mixtures near and far from thermodynamic equilibrium.
• To characterize a previously unobserved phase transition in these mixtures.

Main Methods:

• Time-resolved small-angle X-ray diffraction was employed to study the mixture's structural changes.
• Experiments were conducted under varying temperature conditions to probe phase transitions.

Main Results:

• A reversible phase separation transition was observed upon heating, not currently depicted in existing phase diagrams.
• This transition exhibits a small temperature hysteresis (a few degrees).
• The transition is proposed to involve cholesterol migrating from the bilayer interface to the hydrophobic core.

Conclusions:

• The study identifies a novel phase transition in DMPC-cholesterol mixtures, expanding current phase diagrams.
• The findings suggest a temperature-dependent molecular rearrangement of cholesterol within lipid bilayers.
• This research provides new insights into lipid-cholesterol phase behavior and membrane dynamics.