Polysaccharides, crystal structure hydration

New Insights into the Crystal Structure Hydration of Polysaccharides... 

The donor and acceptor groups found in biological structures which form O-H- 0 bonds are given in Box 6.1. The C-OH -O hydrogen bonds are the primary intermolecular cohesive force between the carbohydrate molecules, water, and carboxylic acids. They occur extensively in the structures of cyclodextrins, polysaccharides, glycolipids, and glycoproteins. The OwH -O bonds are important in the ices, in hydrated crystal structures, and in the hydration shell of all biological molecules. 

The fact that hydrates are more common in the disaccharides provides an opportunity to study the way in which the inclusion of water molecules may influence the hydrogen-bonding patterns of oligo- and polysaccharides. The crystal structures of several compounds have been studied in both the anhydrous and hydrated forms. 

In the past SO years wood chemists have learned much about the composition and physical properties of hardwood xylans. Partially acetylated glucuronoxylans are model native hardwood xylans. They exhibit thermoplasticity, film forming properties, crystallization potential and are oriented in the secondary cell wall. Their crystal structure has been determined and the hydration of this crystalline polysaccharide has been defined. The structural regularity of these abundant polysaccharides can be interpreted using the principle of optical superposition. 

Mention has already been made of the numerous effects attendant upon chemical substitutions on the polysaccharide linear chain. Natural branches impart a dispersion stability to amylopectin that is not afforded amylose. One only has to compare cellulose ethers, deesterified chitin, and the lysis product of protopectin with the underivatized parent compound to appreciate the impact of chemical substituents on functionality. The loosening of compact, parallel structures with alkyl, hydroxyalkyl, and alkoxyl groups facilitates hydration and transforms insoluble, refractory polysaccharides to soluble, reactive polysaccharides. Not only do these substituents obstruct the crystallization tendency, they almost always confer secondary functionalities like q enhancement and foam, suspension, and freeze-thaw stabilization. 

In order to better understand the behaviour of this polysaccharide, the proposed crystalline structures should be examined. The "dry" and "hydrate" form of this polysaccharide have been studied by x-ray crystal 1 ography( ) and electron diffraction techniques. Table III gives a resume of the data available for this compound. In both polymorphs, the unit cell is orthorhombic. 

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