Miscibility and phase separation in mixed erucylphosphocholine-DPPC monolayers

Area of Science:

• Biophysics
• Materials Science
• Pharmacology

Background:

• Binary Langmuir monolayers are crucial for understanding lipid interactions.
• Erucylphosphocholine (ErPC) is a novel phospholipid-like anti-cancer drug.
• DPPC (1,2-dipalmitoyl-sn-phosphatidylcholine) is a common phospholipid model.

Purpose of the Study:

• To investigate the miscibility and phase behavior of DPPC:ErPC binary Langmuir monolayers.
• To characterize the structural organization and molecular packing of these mixed monolayers.
• To understand the potential implications for drug delivery and formulation.

Main Methods:

• Classical Langmuir technique for surface pressure-area isotherms.
• Brewster angle microscopy (BAM) for visual characterization.
• Grazing Incidence X-ray Diffraction (GIXD) for structural analysis.
• Thermodynamic analysis of excess free enthalpy of mixing.

Main Results:

• Surface pressure-area isotherms revealed two distinct collapse points, suggesting immiscibility.
• Thermodynamic analysis and BAM confirmed miscibility at low surface pressures.
• GIXD and BAM demonstrated phase separation and 2D crystalline domain formation at high surface pressures (>50 mN/m).
• A molecular packing model proposed ordered domains at a 1:1 DPPC:ErPC ratio and random distribution otherwise.

Conclusions:

• DPPC and ErPC exhibit complex phase behavior in binary Langmuir monolayers.
• Phase separation occurs at higher surface pressures, leading to the formation of ordered lipid domains.
• The findings provide insights into the structural organization of phospholipid-based drug formulations.