Fatty acid unsaturation regulates astaxanthin encapsulation in bile salt micelles: Insights into self-assembly mechanisms and molecular interactions.

Astaxanthin (Asta), a hydrophobic carotenoid, exhibits poor bioavailability due to limited water solubility and intestinal absorption. Lipid-based delivery systems enhance its uptake via mixed micelles formed by fatty acids (FFAs) and bile salts (BS). Here, we systematically investigate how the degree of FFA unsaturation-using oleic (monounsaturated), linoleic (diunsaturated), and linolenic (triunsaturated) acids-modulates Asta encapsulation. Binary (FFA-BS) and ternary (FFA-BS-Asta) micelle models reveal that FFAs expand the hydrophobic core of pure BS micelles, improving stability without altering spherical morphology. Isothermal titration calorimetry (ITC) shows electrostatic interactions dominate micellization, with polyunsaturated FFAs exhibiting higher binding affinity to BS. Spectroscopic analyses, including circular dichroism (CD) and raman spectroscopy (Raman), confirm enhanced hydrogen bonding and hydrophobic interactions between Asta and unsaturated FFAs, promoting tighter molecular packing. Quartz crystal microbalance with dissipation (QCM-D) demonstrates increased interfacial thickness and stability for polyunsaturated systems. Molecular dynamics simulations visualize faster Asta encapsulation and higher loading capacity in linolenic acid micelles, correlated with enhanced hydrogen bond formation. These results establish that FFA unsaturation governs micellar architecture and interfacial interactions, providing a mechanistic basis for designing bile salt-lipid nanocarriers to improve lipophilic nutraceutical delivery.