X-ray diffraction and molecular modeling studies of cellulose and its interaction with xyloglucan
Abstract
Polysaccharides have widespread commercial uses and play important roles in biological systems. The structures and interactions of two polysaccharides, cellulose and xyloglucan, are investigated using molecular modeling and x-ray fiber diffraction analysis. Cellulose is a high molecular weight polymer of glucose and is an essential component of plant structure and function. Xyloglucans have a cellulosic backbone that is heavily substituted by short oligosaccharide sidechains, and play important roles in the plant cell wall. Molecular modeling is used to investigate the ways in which plant cell wall xylogucans might bind to the surface of cellulose microfibrils. Two kinds of model are obtained, one of which involves hydrogen bonding between the xylosyl residues and cellulose surfaces, and the other involving both backbone-cellulose and xylose-cellulose interactions. The xyloglucan sidechains may therefore mediate, as well as modulate, the binding. The positions of reflections on the diffraction pattern from a polycrystalline fiber depend on the crystal system and the orientation of the fiber axis relative to the unit cell axes. General expressions describing the reflection positions for a triclinic system and any orientation of the fiber axis are derived. Calculations using these expressions illustrate characteristics, and aid in the interpretation, of fiber diffraction patterns. Native cellulose exists in nature as a mixture of two crystalline allomorphs referred to as cellulose I$\alpha$ and cellulose I$\beta$. Previous structural studies of native cellulose I, using x-ray fiber diffraction, were based on only a single phase (I$\beta$). The structures of cellulose I$\alpha$ and I$\beta$ are determined by joint refinement against existing x-ray diffraction data from Valonia cellulose.
Degree
Ph.D.
Advisors
Millane, Purdue University.
Subject Area
Biophysics|Polymers
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