Encapsulated Control: Shaping Insulin Fibrillation through Polymer Confinement

The understanding of insulin conformational changes trapped inside a polymeric capsule is obscure, especially at elevated temperatures above 40 °C. We studied the conformational changes of insulin in bulk solution and upon encapsulation into polymeric self-assemblies formed by poly(ethylene oxide)-<i>block</i>-poly(<i>N</i>,<i>N</i>,<i>N</i>-trimethylammonioethyl methacrylate) copolymer that is oppositely charged to the protein. We demonstrated that loading insulin into the nanoparticles does not affect its secondary structure but alters the pH sensitivity of insulin, making it more resistant to pH variation in the presence of the polymer. However, the temperature resistance of insulin is weakened in the environment of polyelectrolyte, which causes a lowering of the unfolding temperature, and the conformational changes begin already at 40 °C in the nanoparticle core. For the first time, we report that insulin fibrillation follows distinct pathways in free and encapsulated forms, a difference driven by insulin oligomeric state (hexamer in bulk and trimer within the polyelectrolyte/insulin complex).