The formation of xylan hydrate crystals is affected by sidechain uronic acids but not by lignin

Xylans are the most abundant plant heteropolysaccharides and can account for over one-third of the content of cell walls in biomass. Xylans crystallize from dilute aqueous solution into crystal hydrates. Since lignin is closely associated with xylans in cell walls, we investigated the effect of residual lignin on crystal formation. We used β-(1 → 4) xylans from esparto grass, with and without 15% co-precipitated lignin. The xylan sample with lignin formed quasi-hexagonal platelets 605 nm in diameter, which were similar in morphology to those from the delignified xylan preparation. Crystallization, in this case, could serve as a mechanism for acquiring highly homogeneous and purified xylans. We then used Arabidopsis xylans that either contained or were deficient in glucuronic and 4-O-methylglucuronic acid sidechains—from wild-type and gux1/gux2 mutant plant lines, respectively—to test the effect of sidechains on xylan crystallization. We found that sidechains significantly impacted crystal dimensions, as xylans from wild-type Arabidopsis with sidechains formed smaller but highly crystalline nanoscale platelets with an average diameter of 412 nm. Xylans from the Arabidopsis mutant lacking sidechains formed spherulitic superstructures up to 5 microns in diameter. These spherulitic structures could be fragmented into crystallites measuring 46 nm by 27 nm. This study builds upon earlier xylan crystallization research, reporting, for the first time, on the preparation of a nanoparticle other than nanocellulose from a mutant plant line. Xylan hydrate crystals described herein expand the bioproduct repertoire for xylans and show that crystals with varying morphologies can be formed from a variety of xylan structures and purities.