Indole-5,6-quinones display hallmark properties of eumelanin

• Published in: Nature chemistry 2023-06, Vol.15 (6), p.787-793
• Main Authors: Wang, Xueqing, Kinziabulatova, Lilia, Bortoli, Marco, Manickoth, Anju, Barilla, Marisa A., Huang, Haiyan, Blancafort, Lluís, Kohler, Bern, Lumb, Jean-Philip
• Format: Article
• Language: English
• Subjects: 639/638; 639/638/403; 639/638/440/527; 639/638/563; Analytical Chemistry; Animals; Biochemistry; Biopolymers; Chemical bonds; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Decay; I.R. radiation; Indoles; Indoles - chemistry; Infrared absorption; Inorganic Chemistry; Intelligence; Mammals; Mechanical properties; Melanin; Melanins - chemistry; Near infrared radiation; Optical properties; Organic Chemistry; Oxidation; Paramagnetism; Phenols; Physical Chemistry; Pigmentation; Pigments; Polymerization; Quinones
• ISSN: 1755-4330; 1755-4349; 1755-4349
• DOI: 10.1038/s41557-023-01175-4

Melanins are ubiquitous biopolymers produced from phenols and catechols by oxidation. They provide photoprotection, pigmentation and redox activity to most life forms, and inspire synthetic materials with desirable optical, electronic and mechanical properties. The chemical structures of melanins remain elusive, however, creating uncertainty about their roles, and preventing the design of synthetic mimics with tailored properties. Indole-5,6-quinone (IQ) has been implicated as a biosynthetic intermediate and structural subunit of mammalian eumelanin pigments, but its instability has prevented its isolation and unambiguous characterization. Here we use steric shielding to stabilize IQ and show that ‘blocked’ derivatives exhibit eumelanin’s characteristic ultrafast nonradiative decay and its ability to absorb light from the ultraviolet to the near-infrared. These new compounds are also redox-active and a source of paramagnetism, emulating eumelanin’s unique electronic properties, which include persistent radicals. Blocked IQs are atomistically precise and tailorable molecules that can offer a bottom–up understanding of emergent properties in eumelanin and have the potential to advance the rational design of melanin-inspired materials. Indole-5,6-quinone (IQ) is a long-sought intermediate and structural subunit of eumelanin pigments whose instability has precluded isolation and characterization. It has now been shown that a sterically shielded derivative of IQ exhibits hallmark eumelanin properties, including near-infrared absorption, ultrafast nonradiative decay and a persistent semiquinone radical formed by comproportionation.