Iron catalysed formation of methyl radicals as a common source of environmentally important volatile carbon compounds

Organic and inorganic volatile compounds containing one or two carbon atoms (C1, C2), such as carbon dioxide, methane, methanol, formaldehyde, carbon monoxide, chloromethane, formic acid, acetic acid, ethane and ethene are ubiquitous in the environment and play an important role in atmospheric physics and chemistry as they act as greenhouse gases, destroy stratospheric and tropospheric ozone and control the atmospheric oxidation capacity. Furthermore, these compounds play an important role in global carbon cycling. Up to now, most C1 and C2 compounds in the environment were associated to complex metabolic and enzymatic pathways in organisms or combustion processes of biomass. So far, it was not recognized that many C1 and C2 compounds in the geobiosphere might also have a common origin in methyl groups from methyl-substituted substrates that are cleaved by the iron-catalysed formation of methyl radicals.

We performed a set of laboratory experiments containing methyl-substituted substances, an iron species (e.g. hematite, ferrihydrite or bispidine-iron complexes for the better understanding of the mechanism), H₂O₂ for the activation of the iron species and ascorbic acid as a radical scavenger. The experiments were conducted under ambient conditions (atmospheric pressure and 22°C) and variable parameters such as pH value, substrate concentration and O₂ saturation.

We show that a range of organic and inorganic C1 and C2 compounds can be produced by environmentally important methyl-substituted substances such as dimethyl sulfoxide (DMSO), methionine, choline, trimethylamine, synapyl alcohol (lignin component) and galacturonic acid methyl ester (pectin component). Applying isotopically labelled (²H/¹³C) methyl groups from DMSO and methionine we unambiguously demonstrate that labelled methane, ethane, methanol, formaldehyde and acetic acid are produced from methyl-substituted substances.

Based on our preliminary results we hypothesise that formation of methyl radicals by abiotic and possibly also by biochemical processes is ubiquitous in the environment with various heteroatom-methylated substrates. We propose that by generating methyl radicals formation of the entire set of C1 compounds with carbon oxidation states of -IV to +IV but also formation of C2 compounds is possible. The relative amounts of the formed individual C1 species might depend on the redox milieu and biogeochemical conditions such as the availability of methyl radical donors, iron species, pH, O₂ concentration and possibly a range of other parameters.  To thoroughly understand, the chemistry behind these processes and to verify mechanistic scenarios, we also performed computational modeling based on density functional theory and ab-initio quantum-chemical studies.

The investigated methyl moieties are ubiquitous in the terrestrial and marine biosphere. Thus, for future studies we will put our assembled knowledge into practice and study these reactions in water and soil samples collected from the field.