Identification of 4–O–5-Units in Softwood Lignins via Definitive Lignin Models and NMR

Lignins are complex and heterogeneous natural polymers in which the major units are characterized by certain prominent interunit linkages. Previous attempts to identify and quantify 4–O–5-linked units in softwood lignins by NMR were not successful. In this work, various lignin model compounds, including the tetramers formed by the 4–O–5-coupling of β–O–4-, β–β-, and β–5-model dimers, were synthesized. Such compounds are better able to model the corresponding structures in lignins than those used previously. 4–O–5-Linked structures could be clearly observed and identified in real softwood lignin samples by comparison of their 2D HSQC NMR spectra with those from the model compounds. When comparing NMR data of phenol-acetylated versus phenol-etherified model compounds with those of acetylated lignins, it was apparent that most of the 4–O–5-linked structures in softwood lignins are present as free-phenolic end units.