Where are those lipid nano rings?

Area of Science:

• Soft matter physics
• Materials science
• Biophysics

Background:

• Toroidal micelles (donut-shaped structures) formed by block copolymers, around 100 nm in diameter, show promise for gene/drug delivery.
• These structures are thought to form from spherical or disc micelles triggered by external stimuli, leading to the concept of 'smart' materials.
• The same self-assembly principles apply to polymeric and lipid surfactants, suggesting lipid-based toroidal micelles could be possible.

Purpose of the Study:

• To investigate the formation of highly curved toroidal micelles (approximately 10 nm diameter) from 'smart' lipid surfactants.
• To explore the origin and stability of these smaller toroidal structures, which have not been experimentally observed.
• To support the hypothesis regarding the origin of polymeric toroidal micelle phases.

Main Methods:

• Utilized coarse-grained molecular dynamics (MD) simulations.
• Employed a state-of-the-art free energy calculation method (string method).
• Analyzed time-resolved X-ray spectra and DNA fragment interactions.

Main Results:

• Demonstrated that thermo-responsive lipid surfactants can form toroidal micelles via spontaneous perforation of disc-like micelles upon heat shock.
• Showed that these 'nano rings' are kinetically stable but have shorter lifetimes (milliseconds) than their larger polymeric counterparts.
• Identified characteristic X-ray spectral fingerprints and found that DNA fragment uptake can enhance stability.

Conclusions:

• The study supports the hypothesis on the origin of toroidal micelle phases in polymers.
• Highlights length-scale-dependent differences in the stability of kinetically controlled surfactant phases.
• Confirms the universality of surfactant behavior across different length scales, despite variations in phase stability.