Carbon monoxide (CO) is the second most abundant product of dissolved organic matter (DOM) photodegradation, but important questions remain about its formation mechanism. Inspired by previous results by Stubbins et al., we hypothesize that lignin’s methoxy aromatic groups (Ar-OCH3) can be the precursors of photochemically produced carbon monoxide. In this work, we first show that CO production correlates with substrate photodegradation only if lignin models possess at least one Ar-OCH3 group, regardless of the α-carbon oxidation state and the presence of additional substituents. We further demonstrate that Ar-OCH3 are efficiently cleaved during UVB irradiation, producing methanol and new hydroxy aromatic groups – analogous to O-demethylation catalyzed by ligninolytic enzymes. In the presence of hydroxyl radicals generated during irradiation of the model compounds, methanol may further react to form CO and other products such as formaldehyde, formic acid, and CO2. This study offers new mechanistic insights into the role of the methoxy groups in lignin as the precursor of C1 compounds, which are ubiquitous, abundant, and biolabile DOM photoproducts playing an essential role in C cycling.

Photodegradation of lignin model compounds releases methanol and carbon monoxide through formal O-demethylation of the aromatic core units

ACS Spring 2022

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