Compartmentalised enzyme-induced phase transformations in self-assembling lipid systems

HYPOTHESIS: Understanding the digestion of lipid-based pharmaceutical formulations and food systems is necessary for optimising drug and nutrient delivery and has been extensively studied in bulk emulsion systems using the pH-stat method [1]. However, this approach is not suitable for investigation of individual lipid droplets, in particular the interface where the lipase acts. Microfluidic approaches to study digestion at lipid-water interfaces using droplet trapping have been proposed, however the aqueous phase in that case washes over the interface presenting uncertainty over the stoichiometry of interactions [2]. The internal interface of a Janus-like droplet, containing distinct aqueous and lipid compartments, mimics the interface of a lipid droplet in aqueous solution with controlled stoichiometry [3]. Hence, it was hypothesised that the internal interface of Janus droplets can offer a precise way to study the enzymatic digestion of lipids formulations. EXPERIMENTS: Using microfluidic methods, Janus-like droplets were formed by coalescing emulsion droplets containing lipid formulation and pancreatic lipase. Polarised light microscopy (PLM) and in-situ small-angle X-ray scattering (SAXS) were used to investigate the droplets. FINDINGS: PLM revealed the growth of an aligned inverse hexagonal phase (H₂), and with SAXS showed that this phase transformation and alignment resulted from enzymatic digestion. A subsequent partial transformation from H₂ to inverse bicontinuous cubic phase occurred when simulated intestinal fluid was used instead of Tris buffer. Suggesting that phospholipids and bile salts could diffuse across the internal interface to locally affect their surroundings.