X-ray diffraction from faulted cellulose I constructed with mixed Iα-Iβ stacking

Cellulose from higher plants is usually thought to be a composite of the I alpha and I beta allomorphs, with predominance of the latter. Instead of the pure allomorphs, this article proposes that I alpha and I beta stacking patterns coexist within each crystallite, forming a type of crystallographic defect known as stacking fault. Models of faulted crystallites are constructed with mixed I alpha-I beta stacking and their X-ray diffraction intensities are calculated using the Diffracted Intensities From Faulted Xtals (DIFFaX) computer program. Simulated powder diffractograms from faulted crystallites compare favorably with experimental data, modifying diffractogram regions that have been misfit by models based on the I beta crystal structure. Calculations also reveal that stacking faults generate a signature in the (hkl) dependence of diffraction line broadening, guiding further experimental verification and eventual quantification of stacking faults. Our findings bring an alternative view of native cellulose polymorphism and suggest that the proposed stacking faults are ubiquitous crystallographic defects in cellulose from higher plants.