Molecular modeling of syringyl and p-hydroxyphenyl beta-O-4 dimers. Comparative study of the computed and experimental conformational properties of lignin beta-O-4 model compounds.

As a new approach for the study of the ultrastructure of lignin, the conformational preferences of lignin beta-O-4 model compounds have been investigated by molecular modeling. The computed results have been compared with the experimental data (X-ray crystal structures and (3)J(H)(alpha)(H)(beta) NMR coupling constant values) reported in the literature. This comparison has led to an improved understanding of the influence of the structure, stereochemistry, and intramolecular H-bonding upon the conformational properties of the beta-O-4 dimers. A large number of low-energy conformations have been predicted for the structures. It has also appeared that the conformational features are predominantly governed by local steric interactions rather than by differences in the H-bonding interactions. The threo and erythro forms differ significantly in their conformational features, with a preferential extended overall shape for the threo form in which the bulky aromatic groups are distant from each other.