Quantitative 31P NMR Analysis of Lignins and Tannins

The development of sustainable biorefinery products is confronted, among others, with the challenge of lignin and tannin valorization. These abundant, renewable aromatic biopolymers have not been widely exploited due to their inherent structural complexity and high degrees of variability and species diversity. The lack of a defined primary structure for these polyphenols is further compounded with complex chemical alterations induced during processing, eventually imparting a large variety of structural features of extreme significance for any further utilization efforts. Consequently, a protocol for the rapid, simple, and unequivocal identification and quantification of the various functional groups present in natural polyphenols, is a fundamental prerequisite for understanding and accordingly tailor their reactivity and eventual utility. Quantitative P-31 NMR offers the opportunity to rapidly and reliably identify unsubstituted, o-mono substituted, and o-disubstituted phenols, aliphatic OHs, and carboxylic acid moieties in lignins and tannins with broad application potential. The methodology consists of an in situ quantitative lignin or tannin labeling procedure using a suitable P-31 containing probe, followed by the acquisition of a quantitative P-31 NMR spectrum in the presence of an internal standard. The high natural abundance of the P-31 nucleus allows for small amounts of the sample (similar to 30 mg) and short NMR acquisition times (similar to 30-120 min) with well-resolved P-31 signals that are highly dependent on the surrounding chemical environment of the labeled OH groups.

Automated summary

The development of sustainable biorefinery products faces challenges in lignin and tannin valorization due to their complexity and variability, requiring a protocol for rapid and unequivocal identification and quantification of functional groups in polyphenols for further utilization. Quantitative P-31 NMR provides an efficient method for identifying different functional groups in lignins and tannins with high application potential, offering clear signals highly dependent on the surrounding chemical environment.