Abstract
A modified TEMPO-catalyzed oxidation of the solvent-exposed glucosyl units of cellulose to uronic acids, followed by carboxyl reduction with NaBD4 to 6-deutero- and 6,6-dideuteroglucosyl units, provided a robust method for determining relative proportions of disordered amorphous, ordered surface chains, and anhydrous core-crystalline residues of cellulose microfibrils inaccessible to TEMPO. Both glucosyl residues of cellobiose units, digested from amorphous chains of cellulose with a combination of cellulase and cellobiohydrolase, were deuterated, whereas those from anhydrous chains were undeuterated. By contrast, solvent-exposed and anhydrous residues alternate in surface chains, so only one of the two residues of cellobiosyl units was labeled. Although current estimates indicate that each cellulose microfibril comprises only 18 to 24 (1 → 4)-β-d-glucan chains, we show here that microfibrils of walls of Arabidopsis leaves and maize coleoptiles, and those of secondary wall cellulose of cotton fibers and poplar wood, bundle into much larger macrofibrils, with 67 to 86% of the glucan chains in the anhydrous domain. These results indicate extensive bundling of microfibrils into macrofibrils occurs during both primary and secondary wall formation. We discuss how, beyond lignin, the degree of bundling into macrofibrils contributes an additional recalcitrance factor to lignocellulosic biomass for enzymatic or chemical catalytic conversion to biofuel substrates.
Highlights
Cellulose microfibrils of flowering plants are commonly described as para-crystalline arrays containing up to 36 (1®4)-b-d-glucan chains with diameters of ~3.2-3.5 nm (Ha et al 1998; Kennedy et al 2007a, b)
Current estimates indicate that each cellulose microfibril comprises only 18 to 24 (1, 4)- b eta-D-glucan chains, we show here that microfibrils of walls of Arabidopsis leaves and maize coleoptiles, and those of secondary wall cellulose of cotton fibers and poplar wood, bundle into much larger macrofibrils, with 67 to 86% of the glucan chains in the anhydrous domain
Some microfibril diameters as low as 2 nm have been inferred on the basis of the ratio of surface to core chains evaluated by solid-state NMR (Ha et al 1998), and more recent re-evaluation of solid-state NMR data indicate that diameters range from 2.3 to 3.0 nm, corresponding to a total of only 18 to 24 glucan chains in the crystalline core domain (Fernandes et al 2011; Newman et al 2013; Thomas et al 2013; Jarvis 2013)
Summary
Cellulose microfibrils of flowering plants are commonly described as para-crystalline arrays containing up to 36 (1®4)-b-d-glucan chains with diameters of ~3.2-3.5 nm (Ha et al 1998; Kennedy et al 2007a, b). Some microfibril diameters as low as 2 nm have been inferred on the basis of the ratio of surface to core chains evaluated by solid-state NMR (Ha et al 1998), and more recent re-evaluation of solid-state NMR data indicate that diameters range from 2.3 to 3.0 nm, corresponding to a total of only 18 to 24 glucan chains in the crystalline core domain (Fernandes et al 2011; Newman et al 2013; Thomas et al 2013; Jarvis 2013). Zhao et al (2007) show that amorphous cellulose in cotton must be removed by acid hydrolysis to reveal the microfibrillar structure, and imaging by atomic-force microscopy (AFM) suggests an ‘elemental fibril’ of 3nm x 5nm in maize stems (Ding and Himmel 2006) The consensus of these data suggests that the crystalline cores of an elemental microfibril are around 18 to 24 chains
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