Advancing the pH-driven encapsulation technique of curcumin: Molecular interaction shifts due to structural and charge variations

Abstract

The pH-driven method, recognized for its material safety and avoidance of organic solvents, is considered a promising approach for the encapsulation of hydrophobic active substances. However, the interaction mode and mechanism between carriers and receptors in two stages (alkalization and neutralization) remain underexplored. In this study, the hydrophilic globular protein ovalbumin (OVA) was utilized as a carrier to load curcumin (Cur) by the pH-driven method, with an alkalization pH of 8–12. The results indicated that the solubility of Cur and the degree of protein unfolding increased with increasing alkalization pH, with a transformation in the secondary structure observed at pH 12. During the alkalization stage at pH 11 and 12, the loading efficiency of Cur exceeded 90%, with binding constants reaching as high as 5.79 × 105 M−1 and 1.44 × 105 M−1, respectively. The interactions transitioned through three phases in the alkalization process: electrostatic dominance to hydrophobic dominance, electrostatic assistance to hydrophobic dominance, and electrostatic repulsion hindrance. Notably, the generation of electrostatic repulsion and the potential loss of binding sites resulted in a decrease in the binding affinity between OVA and Cur at pH 12. In the neutralization stage, a stable structure dominated by hydrogen bonds and van der Waals forces was observed, with Cur encapsulated within the hydrophobic core of OVA. The mechanism proposed in this study elucidates the factors affecting the efficacy of the pH-driven method and helps to refine the application in hydrophobic active substance delivery systems.