Phase evolution in cholesterol/DPPC monolayers: atomic force microscopy and near field scanning optical microscopy...

Area of Science:

• Lipid bilayer biophysics
• Materials science
• Surface chemistry

Background:

• Understanding lipid monolayer phase behavior is crucial for cell membrane studies.
• Cholesterol is a key component influencing lipid membrane fluidity and domain formation.
• Atomic force microscopy (AFM) and near field scanning optical microscopy (NSOM) offer high-resolution imaging of molecular structures.

Purpose of the Study:

• To investigate the phase evolution in dipalmitoylphosphatidylcholine (DPPC)/cholesterol monolayers.
• To characterize the impact of varying cholesterol concentrations on monolayer structure.
• To evaluate the combined utility of AFM and NSOM for studying lipid domain formation.

Main Methods:

• AFM and NSOM were employed to image DPPC/cholesterol monolayers.
• Cholesterol concentrations were varied from 0% to 50%.
• BODIPY dye was used for fluorescence imaging in select samples.

Main Results:

• Monolayers transitioned from liquid expanded/condensed phases (<10% cholesterol) to liquid expanded and two cholesterol-containing phases (intermediate concentrations).
• A single homogeneous liquid ordered phase was observed at 33% cholesterol.
• AFM detected height differences, while fluorescence imaging (with BODIPY) highlighted fluid phases but could not distinguish intermediate cholesterol-rich/poor phases.

Conclusions:

• The combination of AFM and NSOM provides a comprehensive understanding of cholesterol/DPPC monolayer phase behavior.
• This dual-imaging approach surpasses conventional fluorescence microscopy for detecting submicrometer domains.
• The study elucidates the critical role of cholesterol in organizing DPPC monolayer phases.