Modelling the feasibility of intramolecular dehydrodiferulate formation ingrass walls

Molecular modelling is a useful tool for predicting and visualising molecul ar interactions, eg providing a means of predicting the feasibility of cova lent bond formation between macromolecules. Molecular modelling was used to evaluate the feasibility of intramolecular diferulate formation. Two ferul ates were positioned at various locations along the backbone of an arabinox ylan (16 xylose residues) and the optimised structure generated using MM2 p arameters. For ferulates separated by several xylose residues, diferulates could only form if the xylan backbone relaxed allowing chain folding to bri ng the two ferulates within spatial proximity for bonding. In positions tha t would allow overlap of ferulates, one or both of the ferulates would have to rotate along the xylan backbone for radical coupling. In both cases hig h-energy barriers prevented the complete rotation to allow bond formation. It therefore seems unlikely that intramolecular dehydrodiferulates form rea dily within grass cell walls. When two xylose residues separate two arabino se moieties containing ferulate units it is feasible for rotation to a posi tion allowing the formation of the 5-5 linked diferulate with no relaxation of the backbone. This is the only diferulate that can form without bond st rain when the ferulates are positioned three xylose residues apart on the s ame xylan backbone. This suggests restricted positioning of ferulates for 5 -5 coupling. (C) 1999 Society of Chemical Industry.