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 13C resonances. Here, we establish experimentally validated spectral signatures for differentiating condensed tannins from lignin in whole-cell solid-state NMR spectra. By integrating simulated 13C chemical shift maps with experimental 13C-13C and 13C-1H correlation spectra from 13C-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.