Conformational features of crystal‐surface cellulose from higher plants

Abstract

SummaryNative cellulose in higher plants forms crystalline ®brils a few nm across, with a substantial fraction of their glucan chains at the surface. The accepted crystal structures feature a¯at-ribbon 2 1 helical chain conformation with every glucose residue locked to the next by hydrogen bonds from O-3¢ to O-5 and from O-2 to O-6¢. Using solid-state NMR spectroscopy we show that the surface chains have a different C-6 conformation so that O-6 is not in the correct position for the hydrogen bond from O-2. We also present evidence consistent with a model in which alternate glucosyl residues are transiently or permanently twisted away from the¯at-ribbon conformation of the chain, weakening the O-3¢ ± 0-5 hydrogen bond. Previous molecular modelling and the modelling studies reported here indicate that this translational' chain conformation is energetically feasible and does not preclude binding of the surface chains to the interior chains, because the surface chains share the axial repeat distance of the 2 1 helix. Reduced intramolecular hydrogen bonding allows the surface chains to form more hydrogen bonds to external molecules in textiles, wood, paper and the living plant.

Nanostructure of cellulose microfibrils in spruce wood

“…Almost all the cellulose chains were at least moderately well oriented, as shown by both WAXS and polarized FTIR, but the disordered chains differed in conformation from the well ordered material as previously shown by NMR (5,13). Mostly the disordered chains retained the O3H-O5' hydrogen bond that ensures chain rigidity, as shown by the hydroxyl stretching region in FTIR, but they had a dynamic mixture of all three possible conformations at C-6.…”

“…S3). However for water-saturated spruce cellulose the observed column lengths were 2.9 nm normal to the (200) plane and a mean of 2.6 nm normal to the (110) and (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) planes. In the dry state the 200 Scherrer dimension increased to 3.3 nm, probably due to either coalescence of partially detached chains or some degree of crystallographic stacking of adjacent microfibrils over short lengths where the 200 planes were aligned.…”

“…Wood cellulose is composed of long microfibrils, each a few nanometers in thickness (3). These microfibrils contain both ordered and disordered components (4)(5)(6), but their structure is not known in detail.…”

“…Distinguishing these two forms is difficult in wood, but it has been suggested that cellulose resembling the Iβ form predominates in softwoods (8,12). Also present are partially ordered cellulose chains (7) and chains that differ conformationally from crystalline cellulose (5,6,13). Other polysaccharides, particularly glucomannans, may be closely enough associated with the microfibrils to be considered as a part of their structure (14,15).…”

Tools for Cellulose Analysis in Plant Cell Walls

“…Eventually, in the early 1980s, the spectroscopic technique of solid-state 13 C cross-polarization magic-angle spinning NMR spectroscopy was able to resolve this issue by showing that native cellulose I diffraction data from many natural sources were a composite of diffraction from the two crystalline allomorphs Ia (triclinic unit cell) and Ib (monoclinic unit cell; Atalla and VanderHart, 1984). 13 C-NMR spectroscopy would not only confirm that the crystalline structure of the cellulose microfibril in most plants was a composite of Ia and Ib crystalline forms (Viëtor et al, 2002) but, in combination with Fourier transform IR spectroscopy spectra, suggested that the more crystalline inner chains of the microfibril core are composed primarily of cellulose Ib, while both forms of cellulose compose the chains in the surrounding paracrystalline sheath (Sturcová et al, 2004).…”