Molecular modeling of lignin β-O-4 model compounds.: Comparative study of the computed and experimental conformational properties for a guaiacyl β-O-4 dimer

The conformational preferences of the threo and erythro diastereomeric forms of a guaiacyl beta-O-4 dimer have been investigated by molecular modeling using the CHARMM force field. Many low energy conformations have been identified for each diastereomer, showing that beta-O-4 dimers can adopt a large variety of shapes. A consistent structural model has emerged that indicates different conformational behavior for the threo and erythro forms, corresponding to a preferential extended overall shape for the threo form. All the low energy conformers are stabilized by intramolecular constant H-bonds. In particular, the highly directional H-bond between the alpha or gamma hydroxyl hydrogen and the aromatic methoxy oxygen governs the B aromatic ring orientation. However, it has appeared that the conformational. preferences are predominantly governed by local steric interactions rather than by differences in the H-bonding pattern. From the satisfactory agreement between computed data (geometries and the Boltzmann distribution of the low energy conformers) and the experimental data reported in the literature (X-ray crystal structures and (3)J(HalphaHbeta) NMR coupling constant), the CHARMM force field has been validated for the study of beta-O-4 structures. Clearly, the molecular modeling calculations have led to an improved rationalization of the conformational data collected by experimental techniques.