2026-04-14T03:11:37Zhttps://recercat.cat/oai/request

oai:recercat.cat:10256/278452025-12-05T14:57:34Zcom_2072_452955com_2072_2054col_2072_453073

00925njm 22002777a 4500 dc Huang, Haiyan author Kinziabulatova, Lilia author Manickoth, Anju author Zhang, Yiming author Barilla, Marisa A. author Blancafort San José, Lluís author Kohler, Bern author Lumb, Jean Philip author 2025-03-14 Eumelanin is a multifunctional biomaterial that colors the skin, hair and eyes of mammals. Despite years of effort, its molecular structure remains unknown, limiting our understanding of its biological function and the design of synthetic mimics. In an effort to address this challenge, we report an Iterative Chain Growth (ICG) of well-defined 5,6-dihydroxyindole (DHI) model compounds that provides direct, experimental evidence of a short effective conjugation length in the resulting oligomers. Our ICG highlights the C2-selective borylation of N-H indoles in complex settings, and the utility of Suzuki-Miyaura Coupling (SMC) to grow the chain. The resulting C2-C7′ linkage is installed selectively with good yields, affording products with up to 5-indole units. Access to these oligomers allows us to probe how DHI chain extension contributes to the emergence of sun screening in eumelanin. Our oligomers guarantee the absence of oxidized by-products that may otherwise complicate analysis, without substantially altering the photophysics of the indolic-backbone. Steady-state absorption and emission spectroscopy coupled with excited-state calculations reveal pronounced vibronic structure and excited state planarization, but only a moderate red shift with increasing chain length because of poor orbital coupling between adjoined π-systems. We conclude that eumelanin's characteristic ability to absorb visible light does not derive from long chains of fully reduced DHI sub-units. Our work takes an important step towards a more systematic exploration of eumelanin's structure through iterative synthesis, with the long-term goal of explaining the molecular origins of its properties Computational work at the University of Girona was supported by the Ministry of Science and Innovation of Spain (project Ref. PID2022-138062NB-I00) and was carried out with the use of supercomputing resources from Consorci de Serveis Universitaris de Catalunya (CSUC). A. M. acknowledges Secretaria d'Universitats i Recerca of Generalitat de Catalunya (Spain) and the European Social Fund for a doctoral fellowship (ref. 2021FI_B00690). Open Access funding provided thanks to the CRUE-CSIC agreement with RSC 9 http://hdl.handle.net/10256/27845 Materials biomèdics Biomedical materials Oligòmers Oligomers An iterative synthesis of poly-substituted indole oligomers reveals a short effective conjugation length in eumelanin model compounds