The molecular basis of the adsorption of xylans on cellulose surface

In order to model the adsorption of xylan on cellulose, we have simulated, at the atomic level, the gas phase adsorption of small xylan fragments having 5 skeletal beta (1 -> 4) xylosyl residues (X-5), using molecular dynamics simulations. A first regime was considered, corresponding to a low surface coverage, with the adsorption of isolated X-5 in various initial orientations. In this regime, the simulation indicated that X-5 moved toward extended conformations, some of them being helical, with the possibility of either 2(1) or left-handed 3(1) helices. During the simulation, the X-5 fragments became preferentially oriented, parallel or anti parallel with respect to the cellulose chain axis. Substitution of the X-5 backbone by either GlcA and/or Araf side chains had no major influence on either the conformation or the efficiency of the interaction. However, the presence of side chains favored orientations of the X-5 backbone inclined with respect to the cellulose chain axis. In a second regime corresponding to monolayer coverage, the geometrical features of the adsorption of the xylan fragments on cellulose was roughly the same as that in the individual coverage situation. In this case, the monolayer became equilibrated at 0.14 g of xylan fragments for each g of cellulose, a figure that compared favourably with the values obtained in experimental adsorption of xylan on bacterial cellulose.