The stereochemistry and conformation of lignin as judged by X-ray crystallographic investigations of lignin model compounds: Arylglycerol beta-guaiacyl ethers

Abstract

The conformations of a variety of crystalline lignin models representing different diastereomeric forms of structural elements in lignin of the arylglycerol b-guaiacyl ether type have been studied using X-ray crystallography. Based on X-ray crystallographic data, sequences of units attached to each other by b-guaiacyl ether linkages were constructed. The appearance of the resulting oligomers shows that stereoisomerism can be expected to influence the shape of the lignin molecules to a great extent. The constructed oligomers provide an idea about the contribution to the complexity of lignins by stereochemistry alone. It is proposed that structural differences, together with conformational variations, are the main reasons for the broadness of the signals in NMR spectra of lignins.

Structural Motifs of Wheat Straw Lignin Differ in Susceptibility to Degradation by the White-Rot Fungus Ceriporiopsis subvermispora

“…The observations that threo -diastereomers of β- O -4′ aryl ethers were preferentially degraded and incorporation of p -coumarate ( p- CA) at the γ-OH position increased resistance suggested that local steric hindrance and conformation might be major determinants for susceptibility against fungal degradation. Langer et al showed that threo -forms of β- O -4′ aryl ethers existed in a more “extended” or “open” form and, consequently, might be more accessible to or promote interaction with the diffusible oxidants . To the best of our knowledge, conformational studies on structural motifs exclusively occurring in grass lignin have not been performed.…”

“…Langer et al showed that threo-forms of β-O-4′ aryl ethers existed in a more "extended" or "open" form and, consequently, might be more accessible to or promote interaction with the diffusible oxidants. 84 To the best of our knowledge, conformational studies on structural motifs exclusively occurring in grass lignin have not been performed. However, steric hindrance by γ-acylating p-coumaryl groups can be easily conceived.…”

Radical Nature of C-Lignin

“…42,43 To this effect, works of Langer and Lundquist have predicted substantial effects of lignin stereochemistry on structure. 44 We therefore extend the optimized, minimum-energy dimer structures in this study out to longer polymers holding the same internal bond angles, evaluating the extent to which the C-lignin polymer structure can differ on account of stereochemistry.…”

“…44,57−60 Langer and Lundquist have suggested that the stereochemical diversity of lignin may be among the primary factors that result in complicated NMR signals, beyond the obvious heterogeneity of linkages. 44,57,61 This corresponds to the intrinsic challenge to control the chemical decomposition reactions required for efficient and economical lignin processing.…”

“…Among many possible influences of structure, changes to the solvent and ion accessibility within lignin polymers would play a crucial role in the capability to either chemically or biologically degrade it into smaller high-value products. Moreover, the ability of subtle modulations in molecular level structure to impart significant material property alterations is well-known to be of great importance, despite the mystery that still enshrouds the precise interrelation of these principles. , To this effect, works of Langer and Lundquist have predicted substantial effects of lignin stereochemistry on structure . We therefore extend the optimized, minimum-energy dimer structures in this study out to longer polymers holding the same internal bond angles, evaluating the extent to which the C-lignin polymer structure can differ on account of stereochemistry.…”