Diffraction from nonperiodic models of cellulose crystals

Abstract

Powder and fiber diffraction patterns were calculated for model cellulose crystallites with chains 20 glucose units long. Model sizes ranged from four chains to 169 chains, based on cellulose Ib coordinates. They were subjected to various combinations of energy minimization and molecular dynamics (MD) in water. Disorder induced by MD and one or two layers of water had small effects on the relative intensities, except that together they reduced the low-angle scattering that was otherwise severe enough to shift the 1 " 1 0 peak. Other shifts in the calculated peaks occurred because the empirical force field used for MD and minimization caused the models to have small discrepancies with the experimental intermolecular distances. Twisting and other disorder induced by minimization or MD increased the breadth of peaks by about 0.2-0.3°2-h. Patterns were compared with experimental results. In particular, the calculated fiber patterns revealed a potential for a larger number of experimental diffraction spots to be found for cellulose from some higher plants when crystallites are well-oriented. Either that, or further understanding of those structures is needed. One major use for patterns calculated from models is testing of various proposals for microfibril organization.

Evolution and impact of cellulose architecture during enzymatic hydrolysis by fungal cellulases

“…The relative intensities of the Bragg peaks are similar to those recorded by X-ray diffraction for the powder samples. This observation shows that cellulose nanocrystals are not affected by water, somehow in agreement with the finding reported recently showing a minimal effect from two solvation shells on the diffraction patterns of modelled cellulose crystals [58].…”

Effect of microfibril twisting on theoretical powder diffraction patterns of cellulose Iβ

“…While the patterns for the twisted and linear structures indeed display distortions relative to that of the crystallographic structure, as previously discussed by Nishiyama et al (2012), they are effectively indistinguishable from each other. The PWHM for the 200 peak (averaged over patterns for 100 simulation frames) was equivalent for the two MD models, but was broader (by less than 10%) than that of the crystal coordinates, with the value for 2-θ shifted by less than 2% (Table 1).…”

“…The PWHM for the 200 peak (averaged over patterns for 100 simulation frames) was equivalent for the two MD models, but was broader (by less than 10%) than that of the crystal coordinates, with the value for 2-θ shifted by less than 2% (Table 1). Because the twisted and linear structures give rise to equivalent theoretical diffraction patterns, the distortions relative to the pattern for the crystallographic structure must arise from dimensional changes induced by the force field during simulation, as suggested by Nishiyama et al (2012), and not from microfibril twisting. This is expected to hold for all microfibrils, provided that the model is large enough that twisting does not excessively disorder the unit cell of the crystalline assembly.…”

Wide-Angle X-Ray Scattering and Solid-State Nuclear Magnetic Resonance Data Combined to Test Models for Cellulose Microfibrils in Mung Bean Cell Walls

“…WAXS diffractogram of mung bean cell walls (dots) after subtraction of the diffractogram of adhesive tape, for radiation of wavelength 0.06199 nm, and diffractograms calculated for a 36-chain microfibril model without disorder (blue) and with type 1 disorder (orange; A) and two models with type 2 disorder as described in the text (purple and green; B). Nishiyama et al (2012) calculated diffraction patterns for twisted microfibrils, such as those illustrated by Fernandes et al (2011), and showed that the peaks were broader than those calculated for untwisted microfibrils. The broadening was attributed, at least in part, to changes in lattice parameters arising from energy minimization for the twisted model.…”