13C NMR data for the five dissolved organic matter

Dissolved organic matter (DOM) is one of the most complex, dynamic, and
abundant sources of organic carbon, but its chemical reactivity remains
uncertain. Greater insights into DOM structural features could facilitate
understanding its synthesis, turnover, and processing in the global carbon
cycle. Here we use complementary multiplicity-edited 13C NMR
spectra to quantify key substructures assembling the carbon skeletons of
DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We
find that one type of reaction mechanism, oxidative dearomatization (ODA),
widely used in organic synthesic chemistry to create natural product
scaffolds, is likely a key driver for generating structural
diversity during processing of DOM that are rich in suitable polyphenolic
precursor molecules. Our data suggest a high abundance of tetrahedral
quaternary carbons bound to one oxygen and three carbon atoms (OCqC3
units). These units are rare in common biomolecules but could be readily
produced by ODA of lignin and tannin derived polyphenols. Tautomerization
of (poly)phenols by ODA creates non-planar cyclohexadienones which are
subject to immediate and parallel cycloadditions. This combination amounts
to a proliferation of structural diversity of DOM compounds from early
stages of DOM processing, with an increase in oxygenated aliphatic
structures. Overall, we propose that ODA is a key reaction mechanism for
complexity acceleration in the processing of DOM molecules, creation of
novel oxygenated aliphatic molecules, and that it could be prevalent in
nature.