Oxidation of Small Phenolic Compounds by Mn(IV)

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.

Keywords: CO2; HPLC; NMR; manganese; oxidation; phenolic acids; soil organic matter.

Figure 1

Chemical structures of the phenol compounds.

Figure 2

NMR spectra of unreacted (black) and reacted (blue) samples for gallic acid (A), pyrogallol (B), and benzoic acid (C). Unreacted samples were 1 mM. Reacted samples were a 1:1 molar ratio of phenol to Mn(IV) with a final concentration of 1 mM for each.

Figure 3

HPLC chromatograms for gallic acid (A), pyrogallol (B), and benzoic acid (C). Top panels represent the samples reacted with Mn(IV); bottom panels are unreacted samples. The detection wavelength 220 nm (left axis) is displayed in blue, and 400 nm (right axis) is displayed in red. Insert panels are full spectra of the peaks that appeared in the reacted samples. Unreacted samples were 1 mM. Reacted samples were in a 1:1 molar ratio with Mn(IV) with a final concentration of 1 mM for both.

Figure 4

CO₂ produced when Mn(IV) was reacted with gallic acid (red), pyrogallol (blue), and benzoic acid (black). Gallic acid and benzoic acid were reacted with Mn(IV) in a 1:1 molar ratio and a final concentration of 1 mM for both the phenol and Mn(IV). Pyrogallol was 1:1 with Mn(IV), and a final concentration of 5 mM for both. Final volume for all reactions was 50 mL. Values are means, error bars indicate the standard error of the mean, n = 3.