Effect of microfibril twisting in theoretical powder diffraction studies of cellulose Iß

Authors: HADDEN, JODI - University Of Georgia; French, Alfred - Al; WOODS, ROBERT - National University Of Ireland

Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2013
Publication Date: 9/25/2013
Citation: Hadden, J.A., French, A.D., Woods, R. 2013. Effect of microfibril twisting in theoretical powder diffraction studies of cellulose Iß. Cellulose. DOI: 10.1007/s10570-013-0051-z.

Interpretive Summary: Understanding how cotton responds to its environment requires an understanding of the fundamental structure of the cotton fiber, which is composed almost entirely of cellulose that occurs in small crystals. Two methods of studying the structure are X-ray diffraction and computerized molecular modeling. Many different computer models suggest that the cellulose crystals are twisted, but those models also have other small distortions that result from the modeling. Recently we have been calculating X-ray diffraction patterns from the models for comparisons with experiment, but it was impossible to separate the effects of the twist from those other distortions. There was concern that, if the real crystals were twisted, the analysis of those crystals might be invalid. This paper provides a way for untwisted models to also incorporate the other distortions introduced by the modeling so that the two calculated patterns, for twisted and untwisted models can be compared. It was concluded that the possibility of twisting at the low level indicated by the models or by some experiments would not adversely affect the proposed cellulose structure. This information is mostly of interest to biologists and chemists studying cellulose for various applications and uses.

Technical Abstract: Previous studies of calculated diffraction patterns for cellulose crystallites have suggested that the distortions arising once models have been subjected to MD simulation are likely the result of dimensional changes induced by the empirical force field, but have been unable to determine to what extent twisting itself is a factor. Here, simulations of a linearly constrained microfibril model, obtained by bonding chain terminals across the periodic boundary, as well as a finitely modeled (twisted) analog allowed for comparison of structures that were equivalently affected by the force field, and differed only by the presence or relative absence of twisting. Dimensional analysis confirmed that these structures were internally consistent, and theoretical powder diffraction patterns for the two were shown to be indistinguishable. This indicates that distortion from the crystallographic coordinates results entirely from dimensional changes induced by the force field as previously suggested and not from microfibril twisting. These findings further support the notion that a subtle degree of twist is not necessarily inconsistent with well-resolved crystallographic data. While powder patterns were shown to be unaffected by twisting, it may be that other experimental techniques are capable of detecting this subtle structural difference, and the method presented here for obtaining internally consistent twisted and linear microfibril structures for comparison provides a strategy for probing this possibility.