Polysaccharidic molecule

Storage polysaccharides are an important carbohydrate form in plants and animals. It seems likely that organisms store carbohydrates in the form of polysaccharides rather than as monosaccharides to lower the osmotic pressure of the sugar reserves. Because osmotic pressures depend only on numbers of molecules, the osmotic pressure is greatly reduced by formation of a few polysaccharide molecules out of thousands (or even millions) of monosaccharide units. 

The high molecular weight of the polysaccharide molecules is the most important and limiting factor for any cross-linking reaction, such as the gelation process. The importance of a high molecular weight has been proved for practically every polysaccharide gel mixture and is therefore not discussed in detail (7, 9, 11, U, 19),... 

Probably the most promising polymeric drug carrier system involves polysaccharide molecules. These are natural polymers and are often biodegradable to products that are useful to the host or easily eliminated by the host. Dextrans have been the most extensively used polysaccharide for macromolecular prodrug preparations (79). These materials are biocompatible and the in vivo fate is directly related to their molecular weight. Moreover these macromolecules can be easily targetted to the hepatocytes with D-mannose or L-fucose (20). 

Both polysaccharide molecules are relatively stiff, stiffer even than simple cellulosics such as HEC, and have a molecular masses in excess of two million. Recent work by Rinaudo and coworkers (Personal communication) and Crecenzi and colleagues (Int. J. Biol. Macromol., submitted) has shown that succinoglycan molecules are also stiffer than those of xanthan. 

Polysaccharide solubility in aqueous solutions usually is dependent on polymer size and its allied three-dimensional structure. Even water-insoluble carbohydrates may be solubilized by controlled hydrolysis of o-glycosidic linkages to create smaller polysaccharide molecules. Thus, cellulose may be solubilized by heating in an alkaline solution until the polymers are broken up sufficiently to reduce their average molecular weight. Many such soluble forms of common polysaccharides are available commercially. 

The use of other functionalities (either indigenous or created) on polysaccharide molecules to effect a crosslinking reaction can be done in similar 2- or 3-step strategies. 

This simple strategy can be used to add amine residues to polysaccharide molecules after formation of aldehydes by periodate or enzymatic oxidation (Section 4.4, this chapter). Thus, glycoconjugates or carbohydrate polymers such as dextran may be derivatized to contain amines for further conjugation reactions. 

The carbonyl-reactive group on these crosslinkers is a hydrazide that can form hydrazone bonds with aldehyde residues. To utilize this functional group with carbohydrate-containing molecules, the sugars first must be mildly oxidized to contain aldehyde groups by treatment with sodium periodate. Oxidation with this compound will cleave adjacent carbon-carbon bonds which possess hydroxyl groups, as are abundant in polysaccharide molecules (Chapter 1, Sections 2 and 4.4). 

Greenwood, C. T., The Size and Shape of Some Polysaccharide Molecules, VII, 289-332... 

Figure 8 (Plate 7). Structure of the Escherichia coli FhuA protein serving as receptor for ferrichrome and the antibiotic albomycin. (a) side view (b) side aspect with partly removed barrel to allow the view on the cork domain (c) top view. A single lipo-polysaccharide molecule is tightly associated with the transmembrane region of FhuA (reproduced by permission of W. Welte and A. Brosig)...

Esterification is not difficult if xylan is first carefully precipitated from solution and dried to a fluffy, non-horny powder, or if the xylan can be highly swollen so as to make the polysaccharide molecules easily accessible to the esterifying reagents. Solechnik101 finds that swollen undried xylan, when treated with acetic anhydride containing sulfuric... 

Role of Neighboring Polysaccharide Molecules in Determining the Orientation of Tyrosine Residues During Coupling. These considerations suggest a third possible explanation for the exclusive formation of isodityrosine in the plant cell wall in vivo that the neighboring structural molecules of the wall constrain extensin to prevent dityrosine formation. This would mean that the biologically relevant substrate for peroxidase in the plant cell wall is not naked extensin but extensin complexed with another wall component, possibly an acidic polysaccharide to which the extensin would bind ionically. 

Marszalek PE, Li H, Oberhauser AE, Eemandez JM. Chair-boat transitions in single polysaccharide molecules observed with force-ramp atomic force microscopy. Proc Natl Acad Sci USA 2002 99 4278-4283. 

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