Arabinose substitution effect on xylan rigidity and self-aggregation
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Substituted xylans play an important role in the structure and mechanics of the primary cell wall of plants. Arabinoxylans (AX) consist of a xylose backbone substituted with arabinose, while glucuronoarabinoxylans (GAX) also contain glucuronic acid substitutions and ferulic acid esters on some of the arabinoses. We provide a molecular-level description on the dependence of xylan conformational, self-aggregation properties and binding to cellulose on the degree of arabinose substitution. Molecular dynamics simulations reveal fully solubilized xylans with a low degree of arabinose substitution (lsAX) to be stiffer than their highly substituted (hsAX) counterparts. Small-angle neutron scattering experiments indicate that both wild-type hsAX and debranched lsAX form macromolecular networks that are penetrated by water. In those networks, lsAX are more folded and entangled than hsAX chains. Increased conformational entropy upon network formation for hsAX contributes to AX loss of solubility upon debranching. Furthermore, simulations show the intermolecular contacts to cellulose are not affected by arabinose substitution (within the margin of error). Ferulic acid is the GAX moiety found here to bind to cellulose most strongly, suggesting it may play an anchoring role to strengthen GAX-cellulose interactions. The above results suggest highly substituted GAX acts as a spacer, keeping cellulose microfibrils apart, whereas low substitution GAX is more localized in plant cell walls and promotes cellulose bundling.
KeywordsPlant cell wall Xylan Molecular simulation Neutron scattering
This research was supported by the Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0001090. This research used resources of two DOE Office of Science User Facilities: the National Energy Research Scientific Computing Center, a supported under Contract No. DE-AC02-05CH11231, and the High Flux Isotope Reactor at Oak Ridge National Laboratory. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U. S. Department of Energy under Contract DE-AC05-00OR22725.
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