Lignin Stereochemistry and its Biosynthetic Implications

Paper in proceeding, 2003

Study of the stereochemistry of lignins constitutes one approach to the elucidation of the biosynthesis of lignins. Results from stereochemical studies directed to an understanding of lignin biosynthesis are summarized. The distribution of the diastereomeric forms of different types of structural elements in lignins was determined. It can be expected that the distribution of diastereomers is unaffected during the isolation of lignin or that an equilibration occurs during this procedure. The compositions of equilibrium mixtures of representative lignin model compounds, obtained by acid-catalysed equilibration, were therefore determined. The results showed that the distribution of diastereomers in lignins differs from that of the equilibrium mixtures but agrees fairly well with that found in reaction mixtures obtained on oxidation of p-hydroxycinnamyl alcohols in vitro. These results reinforce the opinion that “random” polymerization of p-hydroxycinnamyl alcohols via radicals plays a role in the biosynthesis of lignins. Aiming at an elucidation of the “secondary structure” of lignins the conformation of a variety of crystalline lignin models was determined by X-ray crystallography. It was found that the bulky aromatic groups in many cases tend to be far apart from each other in the conformations adopted. This suggests that repulsion between aromatic groups plays a role. However, it is evident that other factors also influence the conformations of the model compounds examined.