Zn2+-induced electronic polarization and local reactivity in melanin-like dopamine oligomers: a DFT study.

CONTEXT: Melanin-like dopamine oligomers are promising biomaterials due to their biocompatibility and metal-chelating ability. In this work, Zn2+-chelated melanin-related structures were investigated to clarify the effects of metal coordination on their electronic structure. The results indicate that Zn2+ coordination induces significant electronic polarization, modifies local reactivity patterns, and activates the coordination sites. The theoretical results are also consistent with available spectroscopic observations, providing molecular-level insight into the antibacterial behavior of these complexes and supporting the design of related bioactive materials. METHODS: Density functional theory (DFT) calculations were performed for 5,6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), and DHI-based oligomeric structures in different deprotonation states. Geometry optimizations and electronic-structure analyses were carried out using the B3LYP functional and 6-31G(d,p) basis set, including Grimme correction (GD3) and solvent effects through the polarizable continuum model (PCM) for water, as implemented in the Gaussian 16 software package. Local reactivity was investigated through condensed-to-atom Fukui indices (CAFIs) and molecular electrostatic potential (MEP) analyses. Zn2+ coordination and complex stability were further evaluated using calculations performed with the ORCA computational package and complementary automated interaction site screening (aISS) analyses followed by geometry optimizations with the GFN2-xTB approach implemented in the xTB package.