Analytical techniques allowing for the rapid structural and quantitative assessment of chemical structures of condensed tannins mixtures, are of high priority if we are to improve the field of tannins chemistry as a green renewable resource and are relevant to nutritional and health research for both humans and animals. Due to the wide structural diversity and heterogeneity of condensed tannins novel approaches are needed for their analysis, including new techniques to corroborate and cross-correlate data with existing methods. The development of fast reliable and quantitative analytical techniques for the determination of the content of condensed tannins in complex mixtures, and their specific structure are required. In situ labeling of all labile H groups (aliphatic and phenolic hydroxyls and carboxylic acids) with a phosphorus-containing reagent (Cl-TMDP) followed by quantitative 31P-NMR acquisition constitutes a fast and reliable analytical tool for the analysis of tannins. The different 31P-labeled aliphatic, phenolic OH groups, and carboxylic acids show well dispersed and differentiated regions of absorbance, assigned on the basis of the corresponding signals offered by model compounds. The presence of a suitable internal standard allowes the facile quantification of these groups. presentn effort to fully explore and expose the potential of quantitative 31P-NMR for the structural characterization of condensed tannins and to further cross-correlate it with independent literature data we selected three commercial tannin samples and submitted them to structural evaluation by means of 31P-NMR. Furthermore, we expanded the frontiers of the use of NMR in the structural characterization of natural polyphenols by acquiring HSQC spectra of the enumerated samples and discussed them in the context of the quantitative 31P-NMR data. The concerted use of quantitative 31P NMR with two dimensional NMR has allowed for the validation of our conclusions that pertain to obtaining significant structural and purity information to otherwise complex and often elusive tannin samples. With the objective of explointg tannins antibiofilm potential, the aim of our work has focused on the tayloring of hydrophobic/hydrophilic properties of different tannins. A general chemical process for specific tannin functionalization has been developed and a library of specifically functionalized tannins carrying additional functional groups in specific loading factors has been created.

31P NMR: an invaluable tool for tannins analysis and valorization

ACS Spring 2022

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