Flavoglycans: Evidence for Covalent Proanthocyanidin–Polysaccharide Linkages in Western Redcedar Bark

We investigated covalent linkages between proanthocyanidins (PAs) and cell wall polysaccharides in western redcedar bark (Thuja plicata Donn), a tannin-rich forestry co-product that has potential as a functional food ingredient. An alcohol-insoluble residue (AIR; 1.0% w/w relative to oven-dried bark) recovered from aqueous extracts contained both PAs (≥1.46% w/w of AIR) and polysaccharides (∼30% w/w of AIR) and resisted fractionation by inert solvents, consistent with recalcitrant PA–carbohydrate association. Acidic butanol hydrolysis coupled with LC–MS revealed diagnostic C-glycosylated anthocyanidins, including 6-C-pentosylcyanidin, 8-C-(deoxyhexosyl)cyanidin, and X-C-(deoxyhexosyl)-Y-C-pentosyldelphinidin. NMR analysis supported the presence of PA–polysaccharide conjugates, implicating hemicelluloses and pectins as the carbohydrate partners. These “flavoglycans” provide a chemical basis for incomplete tannin extractability from bark matrices and suggest that carbohydrate conjugation can modulate PA accessibility and interfacial interactions in aqueous systems relevant to bioactive recovery from bark matrices. Overall, this research provides converging lines of experimental evidence suggestive of C-glycosyl crosslinking between proanthocyanidins and carbohydrates in bark.