Computational Studies of the Three-Dimensional Structure of Guaiacyl ß-O-4 Lignin Models

Abstract

In this report, MM3 is validated for preliminary conformational screening of guaiacyl β-O-4 structures. Using the repeating unit of a guaiacyl β-O-4 oligomer, 2-(2-methoxyphenox),1,3-propanediol. the results of MM 3 and ab initio (HF/6-31G * ) optimizations show very good agreement in predicting the geometry of all major intra-molecular hydrogen bonding patterns involving both hydroxyl groups. The total energy of stabilization of a hydroxyl-aromatic ether oxygen hydrogen bond is,however, underestimated by MM3 and other molecular mechanics approaches. Limited ab initio conformational searching of this same structure suggests that the simultaneous hydroxyl-methoxyl/Cβ-ether oxygen intra-molecular hydrogen bond is, in the absence of external constraints, the primary stabilizing force in the guaiacyl β-O-4 linkage of lignin. Extending the ab initio optimizations to the full guaiacyl β-O-4 dimer, the preferred erythro conformers of 1-(3-methoxy-4-hydroxyphenyl)-2-(2-methoxyphenoxyl-1,3- propanediol are presented. These conformers are characterized by an extended ring orientations and a hydroxyl/methoxyl hydrogen bond which stabilizes the B aromatic ring in an oblique orientation. The relative energy differences between the low energy structures indicate a significant restriction in the distribution of the major conformers. This distribution and the conformations of the glohal minima agree with the NMR J HαHβ measured in CDCλ 3 The asymmetric methoxyl substituent, characteristic of the guaiacyl aromatic ring, indirectly determines the preferred A ring variation of these structures through aromatic hydrogen/hydroxyl oxygen interaction. These preferences indicate that the specific hydroxyl orientation of one β-O-4 linkage can influence the conformation of a succeeding guaiacyl linkage.