Oxidation of small phenolics compound by Mn(IV)

Authors: GRUENBERG, MADELINE - Wright State University; Halvorson, Jonathan; HAGERMAN, ANN - Miami University - Ohio; ENOMA, IKPONMWOSA - Wright State University; SCHMIDT, MICHAEL - Wright State University

Submitted to: Molecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2024
Publication Date: 9/12/2024
Citation: Gruenberg, M., Halvorson, J.J., Hagerman, A.E., Enoma, I., Schmidt, M.A. 2024. Oxidation of small phenolics compound by Mn(IV). Molecules. 29(18):4320. https://doi.org/10.3390/molecules29184320.
DOI: https://doi.org/10.3390/molecules29184320

Interpretive Summary: Plant secondary metabolites (PSM) are organic material that enter soils and contribute to forming soil organic matter (SOM). But, some PSM could react with metals that occur in the soil. This could degrade the PSMs and release CO2 to the atmosphere, possibly decreasing SOM. In this work we report how three phenolic PSM react with an environmentally relevant metal, Mn(IV). We demonstrate that gallic acid reacts with Mn(IV) less efficiently than pyrogallol. The products of the gallic acid:Mn(IV) reaction are more oxidized than the products of the pyrogallol reaction. Gallic acid is producing a less oxidized product and producing less CO2. Benzoic acid did not react with Mn(IV). This work provides a framework for understanding how different classes of PSM may be degraded abiotically by redox active metals in soils. This is important for understanding how PSM can affect soil processes, and inform better soil management.

Technical Abstract: Plant secondary metabolites including phenolics represent a large quantity of organic material that enters soil and contributes to the formation of soil organic matter (SOM). The process of phenolics forming SOM remains poorly understood. One possible mechanism is oxidation of the phenolic compound catalyzed by redox active metals such as Manganese (Mn) and Iron (Fe) in soils. In this work we report how three phenolic compounds react with a redox active environmentally relevant metal, Mn(IV). The reactions were monitored via Nuclear Magnetic Resonance (NMR), High Performance Liquid Chromatography (HPLC), and direct CO2 measurements. Using these techniques, we demonstrate that gallic acid reacts with Mn(IV) less efficiently than pyrogallol. The products of the gallic acid:Mn(IV) reaction are more oxidized than the products of the pyrogallol reaction. Gallic acid produces small molecules and releases CO2, while pyrogallol produces a less oxidized product, likely a quinone, and releases less CO2. Benzoic acid did not react with Mn(IV). This work provides a framework for how different classes of plant secondary metabolites may be degraded abiotically by redox active metals in soil.