Urea-Driven Epigallocatechin Gallate (EGCG) Permeation into the Ferritin Cage, an Innovative Method for Fabrication of Protein–Polyphenol Co-assemblies

The 8 nm diameter cavity endows the ferritin cage with a natural space to encapsulate food components. In this work, urea was explored as a novel medium to facilitate the formation of ferritin–polyphenol co-assemblies. Results indicated that urea (20 mM) could expand the 4-fold channel size of apo-red bean ferritin (apoRBF) with an increased initial iron release rate υ0 (0.22 ± 0.02 μM min–1) and decreased α-helix content (5.6%). Moreover, urea (20 mM) could facilitate the permeation of EGCG into the apoRBF without destroying the ferritin structure and thus form ferritin–EGCG co-assemblies (FECs) with an encapsulation ratio and loading capacity of 17.6 and 2.1% (w/w), respectively. TEM exhibited that FECs maintained a spherical morphology with a 12 nm diameter in size. Fluorescence analysis showed that urea intervention could improve the binding constant K [(1.22 ± 0.8) × 104 M–1] of EGCG to apoRBF. Furthermore, the EGCG thermal stability was significantly improved (20–60 °C) compared with free EGCG. Additionally, this urea-involved method was applicable for chlorogenic acid and anthocyanin encapsulation by the apoRBF cage. Thus, urea shows potential as a novel potential medium to encapsulate and stabilize bioactive polyphenols for food usage based on the ferritin protein cage structure.