Zwitterionic and Charged Lipids Form Remarkably Different Structures on Nanoscale Oil Droplets in Aqueous Solution

Area of Science:

• Physical Chemistry
• Surface Science
• Biophysics

Background:

• Lipid monolayers at interfaces are crucial for biological systems like cell membranes and lipid droplets.
• Understanding the molecular structure of these monolayers informs their function.

Purpose of the Study:

• To determine the molecular structure of zwitterionic and charged lipid monolayers on oil droplets.
• To compare the structural differences between zwitterionic and charged lipid monolayers.

Main Methods:

• Combined second harmonic (SH) and sum frequency (SF) spectroscopy.
• Analysis of interfacial vibrational signatures from acyl chains, headgroups, and hydrating water.

Main Results:

• Zwitterionic phospholipids form densely packed monolayers with ordered acyl tails.
• Charged phospholipids form low-density monolayers with disordered acyl tails and perpendicular headgroup orientation.
• Significant structural differences observed compared to studies on planar interfaces.

Conclusions:

• Phospholipid headgroups play a critical role in determining monolayer properties on oil droplets.
• Nanoscale effects alter charge condensation behavior, leading to different monolayer structures than expected from planar interfaces.
• Findings highlight the importance of headgroup interactions in lipid droplet and membrane properties.