Self-Assembly of Protamine Biomacromolecule on Halloysite Nanotubes for Immobilization of Superoxide Dismutase Enzyme

An antioxidant material composed of halloysite nanotubes (HNTs), protamine sulfate polyelectrolyte (PSP), and superoxide dismutase (SOD) enzyme was prepared by self-assembly of the PSP and SOD biomacromolecules on the nanoparticulate support. The structural, colloidal and biocatalytic features were assessed. Adsorption of PSP on the oppositely charged HNT surface at appropriate loadings gave rise to charge neutralization and overcharging, which resulted in unstable and stable dispersions, respectively. The formation of a saturated PSP layer on the HNT led to the development of positive surface charge and to remarkable resistance against salt-induced aggregation making the obtained HNT-PSP hybrid suitable for immobilization of negatively charged SOD. No enzyme leakage was observed from the HNT-PSP-SOD composite indicating sufficient structural stability of this material due to electrostatic, hydrophobic, and hydrogen bonding interactions taking place between the particles and the biomacromolecules. Enzymatic assays revealed that SOD kept its functional integrity upon immobilization and showed high activity in superoxide radical dismutation. In this way, stable antioxidant bionanocomposite dispersions were obtained, which can be used as antioxidants in heterogeneous samples.