The low natural abundance of 13C and 15N nuclei limits the achievable detail of structural elucidation of dissolved organic matter (DOM) using advanced NMR techniques. Isotopic labeling of natural organic matter such as DOM for detailed NMR investigations has not been attempted so far though the use of isotopic enrichment has been routine for protein and peptide structural determination. In this study, multidimensional 13C/1H/15N solid-state NMR spectroscopy was applied to 13C/15N-labeled high-molecular-weight (HMW) DOM isolated from Synechococcus cultures. The laboratory produced HMW DOM showed similar one dimensional NMR characteristics to HMW DOM isolated from natural waters. Isotopic labeling has greatly increased the sensitivity of advanced NMR techniques, and enabled the observation of C-C and C-N correlations. A series of advanced two-dimensional (2D) NMR experiments were applied to 13C- and 15N-enriched DOM, such as 13C{15N} rotational echo double resonance, 2D 15N-13C heteronuclear multiple quantum coherence, 15N-13C filtered 2D 13C-13C exchange NMR, and double quantum/exchange/single quantum NMR. These experiments uniquely identify two- to four-bond structures otherwise accessible only by three-dimensional NMR, and have demonstrated the presence of N-methyl groups, amadated sugar rings, and at least four types of N-acetyl groups, one of which is not linked to a carbon in a sugar ring.

New forms of dissolved organic nitrogen Identified by multibond 2D NMR spectroscopy

249th National Meeting (2015)

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