Peroxidase-Like Reactivity at Iron-Chelation Sites in a Mesoporous Synthetic Melanin

High catalytic activity and substrate specificity make enzymes a rich source of inspiration for catalyst development. Co-opting the advantages of natural materials while tuning them to a modified form and purpose, however, is not a straightforward synthetic task. Polymerization of L-3,4-dihydroxyphenylalanine (L-DOPA) results in amorphous polymer nanoparticles that are similar in many ways to natural eumelanin. Herein, the authors introduce mesoporosity and iron ion chelation to synthesize a variant of the L-DOPA polymer with high peroxidase-like activity. Our results indicate catalytic reaction with peroxide under mildly acidic conditions (pH 5.4 and 6) with a greater maximum reaction velocity (V_max) than horseradish peroxidase (HRP) at optimal pH 3.5–4.5. Comparison between Fe(III) and Fe(II) loading indicates that either can be used as a starting point to trigger reactivity, though Fe(II) loading leads to materials with twice the V_max of the Fe(III)-loaded sample. The lack of catalyst degradation despite the redox changes and presence of radical species is consistent with the robust nature and redox versatility of polydopamine-based materials and demonstrates strong potential as a versatile redox-catalysis platform.