Polysaccharides, crystal structure

Since its introduction several years ago, the virtual bond, constrained optimization method has proved very useful in studies of polysaccharide crystal structure. Notable among the successes that can be ascribed to it are the structural determinations of the double-helical amylose (.11), the cellulose polymorphs of different chain polarities (.12, 13), and of a number of other polysaccharides and their derivatives. As described in a review of amylose structures elsewhere in this volume, the use of this refinement method has produced structural detail that has previously been unavailable (ll). These results have provided much-needed... 

In order to discuss the hydrogen bonding in polysaccharide crystal structures, two interrelated problems have to be resolved. One is to determine the intrachain hydrogen bonding, which may be inter- or intramonomer residue. This bonding must be consistent with fitting the polymer repeat unit to the length of the fiber axis, which is usually a well-defined quantity. When the fiber axis coincides with the axis of the polymer, the conformation of the polymer repeat unit must fit the symmetry of the fiber axis. 

Figure 3. Flowchart of operations involved in evaluation of x-ray fiber diagram for polysaccharide crystal structure analysis left, experimental procedure and evaluation of fiber diagram right, computational refinement...

Two helices are packed antiparallel in the orthorhombic unit cell. Association of the helices occurs through a series of periodic carboxylate potassium water - carboxylate interactions. An axial projection of the unit-cell contents (Fig. 23b) shows that the helices and guest molecules are closely packed. This is the first crystal structure of a polysaccharide in which all the guest molecules in the unit cell, consistent with the measured fiber density, have been experimentally located from difference electron-density maps. The final / -value is 0.26 for 54 reflections, of which 43 are observed, and it is based on normal scattering factors.15... 

The side chains in the latter are flexible disaccharides on account of poor-quality diffraction patterns, their tentative molecular structures are known only from computer modeling.1" On the other hand, well-defined crystal structures are available for gellan and welan, and they can be correlated with the physical properties of the polysaccharides the details are presented here. Their conformation angles are listed in Table VI. 

How modeling has been useful in the crystal structure analysis of polysaccharides—and how it could lead to a better understanding of other condensed j)hase states—can be illustrated with structural worK done on cellulose. It is one of the world s most important and widely used raw materials whose structure, properties, derivatives, and transformations remain under continuous study. Some of the results, problems, and indications of future directions resulting from the study of its crystalline structure—and the attendant roles for molecular modeling—are briefly described in the following. 

This article, which is the fourth in the current series,1 presents a bibliographic account of the crystal structures of polysaccharides that have been published during 1977-1979. Several comprehensive reviews on the structures of glycosaminoglycans,2-5 bacterial capsular polysaccharides,3 4,6 starch,7-9 and cell-wall polysaccharides,10 and on... 

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