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.

Methods
These files comprise 13C NMR section integrals for the five DOM, divided into buckets of 0.1 ppm width, and ranging from dC = 0 – 235 ppm. The total integral of these 13C NMR spectra is area normalized to Call = 100% for all five DOM, and to CH0123 according to its proportions. The 13C NMR section integral from dC = 47 - 51 ppm is set to zero for all five 13C NMR spectra of DOM, because of interference from the solvent 12CD3OD, which resonates at dC = 49.0 ppm.