Assignment of 2D TOCSY Spectra of Lignins: the Role of Lignin Model Compounds

Abstract

The validity of using model compound data to facilitate the interpretation of solution-state two-dimensional TOCSY spectra of soluble wood lignins is demonstrated. The correspondence between model data and lignin correlations was such that it was possible to match the structure and stereochemistry of model compounds to structures present in the lignins by mapping model compound 1H chemical shift data to the cross peaks observed in the TOCSY spectra. For systematic comparisons of model data and lignin correlations, an XY scatter plot of model compound side-chain data was generated and then overlayed on the lignin TOCSY spectra. In addition to providing an accurate way of comparing model compound data with lignin correlations, this technique permitted the elucidation of some previously unassigned correlations. In situations where not all possible combinations of model compound side-chain stereochemistry and aromatic ring substitution were available, it was possible to generate internally consistent side-chain data from substituent effects. This allowed the prediction of the mode of attachment of certain inter-unit structures in the lignins.