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Molecular Orbital Investigations on Lignin Model Compounds. XX. Conformational Analysis of Benzyl Ether and Benzyl Ester Linkages in Lignin-Carbohydrate Complexes

The quantum chemical PCILO method has been used to perform a detailed Conformational analysis of benzyl ether and benzyl ester linkages of lignin-carbohydrate complexes (LCC). The ether and ester bonds were modeled by simple dimeric units 4-0-methyl-6-0-(l-phenylethyl)-ß-D-glucopyranose (I) and 4-0-methyl-6-0-(l-phenylethyl)-ß-D-glucuronate (II), respectively. Calculations including optimalization of four torsion angles which determine the conformation of these molecules (rotation around C-C-O-C-C bonds of connecting chain) showed that both dimers exist in stable conformations with all-trans arrangement of C-C-O-C-C fragment. Further, the calculations of two-dimensional Conformational maps (rotation around C-O-C bonds) and potential energy curves have been performed. The energy minima obtained in this way were in the general agreement with the results of the PCILO geometry optimalization.