Mapping phase diagrams of supported lipid bilayers by atomic force microscopy

Area of Science:

• Biophysics
• Materials Science
• Physical Chemistry

Background:

• Phospholipids are key components of biological membranes, and their phase behavior is crucial for membrane function.
• Understanding phospholipid mixtures is essential for developing advanced materials and drug delivery systems.
• Phase diagrams provide critical information about the structural organization and transitions of lipid systems.

Purpose of the Study:

• To construct a pseudophase diagram for a mixture of two specific phospholipids: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (PE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (PG).
• To investigate the influence of divalent cations (Ca2+) and substrate interactions (mica) on the phase behavior of the phospholipid mixture.
• To compare the efficacy of Differential Scanning Calorimetry (DSC) and Atomic Force Microscopy (AFM) in determining phospholipid phase transitions.

Main Methods:

• Differential Scanning Calorimetry (DSC) was employed to analyze liposomes, determining the onset (T_onset) and offset (T_offset) temperatures of the gel-to-liquid crystalline phase transition (L_β →L_α).
• Atomic Force Microscopy (AFM) was utilized to examine supported lipid bilayers, characterizing topographic images to identify phase transition temperatures.
• Pseudophase diagrams were constructed based on the transition temperatures obtained from both DSC and AFM analyses.

Main Results:

• The study successfully constructed pseudophase diagrams for the PE/PG phospholipid mixture under varying conditions.
• The presence of Ca²⁺ significantly altered the phase transition temperatures and the overall phase behavior of the phospholipid mixture.
• The substrate (mica) also influenced the observed phase transitions, indicating the importance of lipid-bilayer-substrate interactions.

Conclusions:

• Both DSC and AFM are valuable techniques for constructing phospholipid pseudophase diagrams, offering complementary information.
• Divalent cations like Ca²⁺ play a critical role in modulating the phase behavior of phospholipid mixtures.
• The interaction with the underlying substrate is a significant factor affecting the phase transitions of supported lipid bilayers.