| The global energy minima for the threo and erythro {dollar}beta{dollar}-O-4 model dimers formed from p-coumaryl, coniferyl, and sinapyl alcohol have been established at the ab initio (HF/6-31G*) level of computation after preliminary conformational searching using MM3. A plane of pseudosymmetry is present in the threo and erythro conformers within each monolignol type. In the p-coumaryl {dollar}beta{dollar}-O-4 linkage, intra-molecular hydrogen bonds from the {dollar}alpha{dollar}- and {dollar}gamma{dollar}-hydroxyl groups to the ether oxygen affect the rotational characteristics, and thus the preferred orientation, of the aromatic ring. In the guaiacyl and syringyl conformers, a stabilizing, intra-molecular hydrogen bond from one or both hydroxyl groups to the ring methoxyl oxygens is present. The distribution of low energy guaiacyl conformers and the conformation of the global minima agree with the experimental NMR J{dollar}rmsb{lcub}Halpha Hbeta{rcub}{dollar} determined in deuterated chloroform. The specific hydroxyl orientation of one {dollar}beta{dollar}-O-4 linkage can influence the conformation of a succeeding guaiacyl linkage. Rotation of the C{dollar}sb{lcub}alpha{rcub}{dollar}-C{dollar}sb{lcub}beta{rcub}{dollar} bond is implicated in an elastic response to external tensile forces. Estimates of the C{dollar}sb{lcub}alpha{rcub}{dollar}-C{dollar}sb{lcub}beta{rcub}{dollar} rotation barrier at the ab initio level indicate that this response is independent of monolignol type. In contrast, the barrier to aromatic ring rotation in response to external compressive forces increases from p-coumaryl to guaiacyl to syringyl type monolignols. One of the functions of the methoxyl group is apparently to modulate the elasticity of the lignin polymer. |
