The structure and stability of phospholipid bilayers by atomic force microscopy

Area of Science:

• Lipid bilayer self-assembly and interfacial phenomena.
• Surface science and nanotechnology.
• Biophysical characterization of lipid membranes.

Background:

• Langmuir-Blodgett films provide model systems for cell membranes.
• Atomic force microscopy (AFM) is crucial for nanoscale surface analysis.
• Understanding lipid monolayer stability is key to biomimetic material design.

Purpose of the Study:

• To investigate the structure, stability, and defects of DSPC, DPPE, and DLPE lipid monolayers.
• To assess the resilience of these monolayers to mechanical stress and fluid exchange.
• To elucidate the pH-dependent behavior of fluid-phase DLPE monolayers.

Main Methods:

• Atomic force microscopy (AFM) for high-resolution surface imaging.
• Langmuir-Blodgett (LB) technique for controlled monolayer formation.
• Controlled fluid exchange and AFM tip manipulation to induce and observe defects.

Main Results:

• DPPE monolayers exhibited ordered parallel ridges (0.49 nm period); DSPC and DLPE showed no periodic order.
• Solid DSPC and DPPE monolayers were stable under AFM scanning.
• DLPE monolayer stability and defect behavior were highly pH-dependent, with increased instability at lower pH.
• Fluid exchange induced defects (gaps, holes), with stable defects at pH 11 but extensive, easily deformed defects at pH 9 and below.
• AFM tip stress induced ripple patterns (Shallamach waves) on deformable DLPE monolayers.

Conclusions:

• Lipid monolayer structure and stability vary significantly with lipid type and phase.
• DLPE monolayers exhibit pH-dependent mechanical resilience attributed to bending energy and spontaneous curvature.
• AFM is effective in characterizing lipid monolayer defects and mechanical responses, revealing insights into membrane behavior under stress.