CONFORMATIONAL-ANALYSIS OF ESTER AND ETHER LINKAGES IN LIGNIN-ARABINOXYLAN COMPLEXES

Conformational flexibility around the ester and ether linkages in 5-O-cinnamoyl-alpha-L-arabinofuranose (1) and 5-O-[(S)-1-phenethyl]-alpha-L-arabinofuranose (2), models for lignin-carbohydrate complexes, has been investigated by molecular orbital PCILO calculations. The structures of individual minima were refined by minimising the energy by adjusting geometrical parameters from the distinct low-energy regions in two-dimensional maps. The calculations show that the ester and ether linkages in lignin-carbohydrate complex models display a large amount of conformational freedom. This involves a rapid equilibrium between the various pseudorotamers of the furanose ring, rotamers of the exocyclic group, and rotation around the ester and ether linkages. The calculations reveal the strong influence of solvent on the conformer populations and that this effect is more pronounced in the ether linkage. The results for aqueous solution predict that (a) the North- and South-type pseudorotamers of the alpha-L-arabinofuranose ring are present in equilibrium in approximately equal amounts in the ester 1, whereas the North-type pseudorotamers prevail in the ether 2; (b) the gt conformation is more populated in both compounds; and (c) the antiperiplanar orientation is dominant in both the ester and ether. The present results give some insight into the conformational flexibility that both molecules display, shed light on the possible arrangement of hydrophilic and hydrophobic parts of lignin-carbohydrate complexes, and provide the basis for the generation of more accurate models for further studies of lignin-carbohydrate complexes.