Resolving aromatic spectral congestion in whole-cell solid-state NMR: Signatures of condensed tannins and lignin in plant tissues

Solid-state NMR enables direct characterization of intact biological materials but is often hindered by spectral congestion arising from chemically similar components. A major challenge in plant tissues is distinguishing condensed tannins from lignin, two aromatic polymers that frequently coexist and generate overlapping 13 C resonances. Here, we establish experimentally validated spectral signatures for differentiating condensed tannins from lignin in whole-cell solid-state NMR spectra. By integrating simulated 13 C chemical shift maps with experimental 13 C 13 C and 13 C 1 H correlation spectra from 13 C-enriched spruce, eucalyptus, and pine tissues, we define regions of unavoidable spectral overlap and identify diagnostic tannin correlations. While tannin A-ring signals extensively overlap with guaiacyl and syringyl lignin aromatics, repetitive B- and C-ring correlations provide unique spectroscopic fingerprints that enable reliable tannin identification across tissues. Application of these signatures reveals pronounced species- and tissue-dependent variation in tannin abundance and identifies conditions under which tannins dominate aromatic regions, complicating lignin analysis. More broadly, this work provides a transferable framework for distinguishing chemically similar aromatic polymers in complex solid-state NMR spectra and improves structural interpretation of intact plant biomass.