Characterization and Fate of Hydrogen-Bonded Free-Radical Intermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 1,8-Naphthalenediol

Abstract

1,8-Naphthalenediol (dihydroxynaphthalene, 1,8-DHN) has been shown to be a potent hydrogen atom transfer (HAT) antioxidant compound because of the strong stabilization of the resulting free radical by intramolecular hydrogen bonding. However, the properties, reactivity, and fate of the 1,8-DHN phenoxyl radical have remained so far uncharted. Herein, we report an integrated experimental and computational characterization of the early intermediates and dimer products that arise by the oxidation of 1,8-DHN. Laser flash photolysis (LFP) studies of HAT from 1,8-DHN to the cumyloxyl and aminoxyl radicals showed the generation of a transient species absorbing at 350, 400, and >600 nm attributable to the 1,8-DHN phenoxyl radical. Peroxidase/H 2 O 2 oxidation of 1,8-DHN was found to proceed via an intense blue intermediate (λ max 654 nm) preceding precipitation of a black melanin-like polymer. By halting the reaction in the early stages, three main dimers featuring 2,2′-, 2,4′-, and 4,4′-bondings could be isolated and characterized in pure form. Density functional theory calculations supported the generation of the 1,8-DHN phenoxyl radical and its subsequent coupling via the 2- and 4-positions giving extended quinone dimers with intense transitions in the visible range, consistent with UV–vis and LFP data. Overall, these results allowed to elucidate the mechanism of oxidative polymerization of 1,8-DHN of possible relevance to melanogenesis in fungi and other processes of environmental and astrochemical relevance.

A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent π‐Electron Donor with Free‐Radical Properties that can be Tuned by Ionic Liquids

“…The UV-visible spectrum of the DMF suspension showed broadband absorption throughout the whole spectrum, typical of eumelanin polymers, with an intense absorption in the visible region, denoting a high degree of polymerization. Moreover, the presence of shoulders at 390, 460 and 580 nm suggested the presence of extended quinonoid moieties, as reported in a previous work, [20] indicating a high π-electron conjugation.…”

“…Chemical studies indicated that DHN tends to polymerize via 2,2′‐, 2,4′‐ and 4,4′‐bondings to give regular patterns of oligomers that may exhibit extended quinone‐methide moieties (Figure ) …”

“…[19] Chemical studies indicated that DHN tends to polymerize via 2,2'-, 2,4'-and 4,4'-bondings to give regular patterns of oligomers that may exhibit extended quinone-methide moieties ( Figure 1). [20] The symmetry and the meta-like peridioxygenation pattern of DHN, precluding labile o-quinone formation in the first step of the process, could drive the generation of relatively more regular melanin-type polymers and their efficient assembly into aggregates. This reactivity can account for the different properties exhibited by the DHN melanin compared to DHI-based eumelanins.…”

Astrochemistry and Astrobiology: Materials Science in Wonderland?

“…Recent studies have indicated that a representative oxyPAH, 1,8-dihdroxynaphthalene (1,8-DHN), undergoes polymerization to give a black, solid polymer mimicking fungal melanin (Figure 2) and displaying considerable H-atom donor behavior, coupled with strong chemical robustness toward hydrogen peroxide degradation and bleaching [16,17,18]. Thus, oxidative polymerization of hydroxylated PAHs may lead to allomelanin-like materials with suitable properties for biointerfaces, organic electronics, and theranostics [19].…”

High‐Energy Proton‐Beam‐Induced Polymerization/Oxygenation of Hydroxynaphthalenes on Meteorites and Nitrogen Transfer from Urea: Modeling Insoluble Organic Matter?

“…All three energetically favorable coupling isomers were obtained, namely para-para a, ortho-ortho b,a nd para-ortho 10 c combinations, respectively. [29] The relative amountso fd imers were found to be dependent on the nature of the meteorite;c ompounds 10 b and 10 c were generally the most abundant derivatives ( Table 1, entries 22 and 23, 45 and 46, and 59 and 60, respectively).…”