Calcium-induced phase separation phenomena in multicomponent unsaturated lipid mixtures

Area of Science:

• Biochemistry
• Physical Chemistry
• Materials Science

Background:

• Lipid phase behavior is crucial for biological membranes.
• Calcium ions are known to influence lipid organization.
• Cholesterol's role in lipid mixtures is complex and not fully understood.

Purpose of the Study:

• Investigate calcium's ability to induce phase separation in multicomponent lipid mixtures.
• Determine the influence of cholesterol on calcium-induced lipid phase transitions.
• Explore the interaction of cholesterol with different phospholipid types.

Main Methods:

• Utilized 31P Nuclear Magnetic Resonance (NMR) spectroscopy.
• Employed 3H NMR spectroscopy.
• Conducted small-angle X-ray diffraction (SAXD) studies.

Main Results:

• Calcium failed to induce lateral phase separation in unsaturated phosphatidylglycerol (PG)/phosphatidylethanolamine (PE) and phosphatidic acid (PA)/phosphatidylcholine (PC) mixtures, leading to a hexagonal (HII) phase.
• Cholesterol facilitated calcium-induced lamellar to hexagonal (HII) transitions or mitigated phase separation in existing HII phases.
• Cholesterol showed no preferential interaction with phosphatidylcholine (PC), phosphatidylethanolamine (PE), or phosphatidylserine (PS) in the liquid-crystal state.
• Various sterols, including ergosterol and stigmasterol, could induce hexagonal (HII) phase formation in PC/PE mixtures.

Conclusions:

• Calcium alone is insufficient to induce lateral phase separation in certain unsaturated lipid mixtures.
• Cholesterol plays a significant role in modulating calcium-induced lipid phase behavior and HII phase formation.
• Sterols broadly influence lipid phase transitions, suggesting a common mechanism in membrane stabilization or destabilization.