Crystallography of polysaccharides: Current state and challenges

Polysaccharides are the most abundant class of biopolymers, holding an important place in biological systems and sustainable material development. Their spatial organization and intra- and intermolecular interactions are thus of great interest. However, conventional single crystal crystallography is not applicable since polysaccharides crystallize only into tiny crystals. Several crystallographic methods have been developed to extract atomic-resolution structural information from polysaccharide crystals. Small-probe single crystal diffractometry, high-resolution fiber diffraction and powder diffraction combined with molecular modeling brought new insights from various types of polysaccharide crystals, and led to many high-resolution crystal structures over the past two decades. Current challenges lie in the analysis of disorder and defects by further integrating molecular modeling methods for low-resolution diffraction data.

Digestion of single crystals of mannan I by an endo-mannanase from Trichoderma reesei

The enzymatic degradation of single crystals of mannan I with the catalytic core domain of a beta-mannanase (EC 3.2.1.78 or Man5A) from Trichoderma reesei was investigated by transmission electron microscopy and electron diffraction. The enzyme attack took place at the edge of the crystals and progressed towards their centres. Quite remarkably the crystalline integrity of the crystals was preserved almost to the end of the digestion process. This behaviour is consistent with an endo-mechanism, where the enzyme interacts with the accessible mannan chains located at the crystal periphery and cleaves one mannan molecule at a time. The endo mode of digestion of the crystals was confirmed by an analysis of the soluble degradation products.

X-ray diffraction of food polysaccharides

The morphologies of food polysaccharides described in this chapter illustrate the power of x-ray fiber diffraction in conjunction with computer modeling and sophisticated refinement techniques. On the other hand, the lack of information on structures such as xanthan reflects the inadequacy of the experimental techniques used to date. But the demands from academic and industrial sectors to investigate the molecular interactions in multicomponent systems, including protein-protein, protein-polysaccharide, polysaccharide-polysaccharide, and other complexes, are high and growing, because they have important food applications. These complexes are structurally more difficult than those solved in the past 40 years and it is improbable that any chosen system will be amenable for crystallographic investigation, crystals or fibers. Modern research facilities that include two-dimensional area detectors, millisecond exposures with synchrotron x-ray radiation, interactive computer graphics, sophisticated molecular dynamics calculations, unbelievably fast and inexpensive computers, and our own intellectual abilities are indispensable tools for the future of structural science in general and food polysaccharides in particular.

An evaluation of crystal structure of mannan I by X-ray powder diffraction and molecular mechanics studies

The present study reports the re-examination of the crystal structure of mannan I by the X-ray powder diffraction study combined with the miniature crystal model simulation. A primary objective of this study was to investigate an adequate chain staggering position along the fiber axis. Among the several crystal structure models proposed for mannan I, that derived from the electron diffraction study of the single crystals was adopted as a starting model. The X-ray crystallographic residuals were calculated for the single crystal structure at different chain staggering positions, while the ab projection was kept invariant. In a similar fashion, the miniature crystal models consisting of seven mannotetraoses were constructed, each having different chain staggering values and the three-dimensional structures of all the models were subsequently optimized by using the MM3(92) program without introducing any constraint. Both the X-ray residual and lattice energy plots with respect to the chain staggering position showed the common minima around the -0.25 x c chain staggering. The minimum models were subjected to a search for preferred orientations of O2 and O6 hydroxyl groups. It was found that the hydroxyl groups present inside the minicrystal tended to rotate into particular orientations during the structure optimizations to form the O5-H-O2 and O3-H-O6 intermolecular hydrogen bonds between the adjacent oligomers in an alternative direction.