Demonstration of a common indole-based aromatic core in natural and synthetic eumelanins by solid-state NMR

Despite the essential functions of melanin pigments in diverse organisms and their roles in inspiring designed nanomaterials for electron transport and drug delivery, the structural frameworks of the natural materials and their biomimetic analogs remain poorly understood. To overcome the investigative challenges posed by these insoluble heterogeneous pigments, we have used L-tyrosine or dopamine enriched with stable C-13 and N-15 isotopes to label eumelanins metabolically in cell-free and Cryptococcus neoformans cell systems and to define their molecular structures and supramolecular architectures. Using high-field two-dimensional solid-state nuclear magnetic resonance (NMR), our study directly evaluates the assumption of structural commonality between synthetic melanin models and the corresponding natural pigments, demonstrating a common indole-based aromatic core in the products from contrasting synthetic protocols for the first time.