Pulsed-field gradient NMR measurements on hydrogels from phosphocholine

Area of Science:

• Physical Chemistry
• Materials Science
• Biophysics

Background:

• Gels composed of diacylphosphatidylcholine in glycerol/butylene glycol mixtures are known to form networks of crystalline multilamellar vesicles (MLV).
• Understanding the molecular dynamics and diffusion within these gel networks is crucial for applications involving controlled release and material stability.

Purpose of the Study:

• To investigate the self-diffusion of water and butylene glycol molecules within diacylphosphatidylcholine gels using pulsed-field gradient NMR.
• To elucidate the structural and dynamic changes occurring during gelation and their impact on molecular transport across vesicle bilayers.

Main Methods:

• Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) measurements were employed to determine self-diffusion coefficients.
• The study analyzed molecular diffusion in both liquid crystalline and gel states above and below the phase transition temperature (T(m)).

Main Results:

• Self-diffusion coefficients revealed two distinct environments for water and butylene glycol molecules, even above T(m).
• A significant fraction of molecules exhibited hindered diffusion, suggesting encapsulation within closed domains (MLV bilayers), while others showed free diffusion.
• The fraction of entrapped molecules remained constant across the phase transition, indicating a stable network structure.
• Bilayer permeability drastically decreased below T(m), with water molecule residence time inside vesicles increasing from <50 ms to >1 s.

Conclusions:

• The gelation process in these systems is attributed to the transition of vesicle bilayers from a liquid crystalline to a crystalline state, without altering the overall network structure.
• Hindered diffusion is attributed to solvent molecules trapped between MLV bilayers, while free diffusion occurs outside the MLV.
• The significant decrease in permeability below T(m) highlights the impact of bilayer phase transition on molecular exchange, crucial for understanding gel properties and applications.