The Structures of Polysaccharides

Analytical work, alone, has educed much helpful information on the structure of polysaccharides.1104 Results in this field have come primarily... 

Polysaccharides that have been modified chemically, or altered physically, have been used as adsorbents for affinity chromatography. The modification of the structure of polysaccharides has been achieved by introducing cross-linkages between the chains of the polymer and bifunctional reagents. The alteration of the properties of polysaccharides by physical means can be effected by embedding the polysaccharide in a network of the support material. The molecular in-... 

The elucidation of the structure of polysaccharides and of the carbohydrate portion of glycoconjugates involves sugar analysis, linkage analysis, and the determination of anomeric configurations and the sequences of the sugar residues. When non-sugar substituents are present, these should also be identified and located. 

In an earlier article1 on the application of enzymic techniques to the analysis of the structure of polysaccharides, the -d- and /3-D-glucans were discussed, as well as more-general aspects of the preparation and use of catabolic enzymes in such analyses. The present article describes enzymic contributions to knowledge of the structures of other polysaccharides, but an account of subsequent research on a- and jS-D-glucans is also included. 

The properties and action patterns of glycosidases and polysaccharide depolymerases have been reviewed,2 5 as also have the enzymes involved in biosynthetic pathways.6,7 An understanding of biosynthesis can provide insights into the structures of polysaccharides. Reviews concerning particular polysaccharides have appeared, and references to these will be given in relevant sections. 

Organic-chemical studies cannot determine completely the structure of polysaccharides, as such methods can only account for about 95% of the linkages present. In many instances there is evidence that labile bonds exist, and the presence of these can only be determined from kinetic studies of the rates of hydrolysis of the polysaccharide under different conditions. The establishment of the presence or absence of such labile linkages should form an important preliminary in the study of the polysaccharides, especially as the breakdown occurring during isolation may be of this nature. 

Lipids and proteins that are part of cell membranes often are bonded to small polysaccharide groups that project from the surface of the cell. Slight differences in the structures of polysaccharides that are bonded to lipids in red blood cell membranes of humans are responsible for the A, B, and O blood types. Antibodies recognize these groups and cause cells to clump when their surface groups are not the same as those on the cells of the original individual. 

The problem of determining the structure of a polysaccharide is similar to that of determining the amino acid sequence of a protein, but it is complicated by the additional structural features unique to carbohydrates branching and linkage isomerism. The elements of carbohydrate chemistry discussed in following paragraphs are those relevant to the problems of determination of carbohydrate composition and the structure of polysaccharides. 

Naturally occurring polysaccharides are made up of a variety of monosaccharides connected by glyco-sidic bonds. Often, these polysaccharides are linked to proteins and lipids. The function of such polysaccharides is often critically dependent on the composition and sequence of their monomeric units. Determination of the structure of polysaccharides is a multistep process. In the early stages, it is crucial to ascertain both the relative amount and the chemical identity of the monomers. After a polysaccharide has been broken down to its monomeric units by hydrolytic or enzymatic digestion, a variety of analytical techniques can be employed to identify and quantify each monomer. Three of these techniques will be demonstrated in this laboratory exercise. 

NMR spectroscopy gives valuable information on the structures of polysaccharides, but its application requires preliminary isolation of polysaccharide samples from... 

I. J. Miller, Evaluation of the structures of polysaccharides from three taxa in the genus Hymenena and from Acrosorium decumbens (Rhodophyta, Delesseriaceae), Bot. Mar., 48 (2005) 148-156. 

When Hirst returned to Scotland in 1947, he knew that the Edinburgh laboratories already contained a very active research school in carbohydrate chemistry that had been built up by E. G. V. Percival, whose work was mainly concerned with the structure of plant mucilages and seaweed polysaccharides. These interests were complementary to Hirst s own recent studies in Manchester on the structure of polysaccharides, particularly the plant gums, and the next few years saw a renewal of the successful collaboration of the Birmingham period 1930-1933, until the untimely death of Percival in 1951. 

In a continuation of the series of bibliographic articles on the structures of polysaccharides as established by X-ray crystallographic methods, Sundararajan (Mississauga, Ontario) and Marchessault... 

It should be pointed out that, although infrared spectroscopy is a useful tool for the carbohydrate chemist, as with other physical methods, it does not supplant the classical chemical methods for determining the structure of polysaccharides. 

XI. The Formazan Reaction as a Means for Establishing the Structure of Polysaccharides. 162... 

Under the electron microscope, the uranyl complexes of the oxy-polysac-charide formazans show a distinct shading effect which imparts a markedly dark appearance to these polysaccharide materials. It is hoped that this will lead to the elaboration of a new method for the study of the structures of polysaccharides. 

The advent in recent years of (2-hydroxyethyl)apocupreine, the sulfonamides, and penicillin and other antibiotic substances (such as aureo-mycin, terramycin, and chloramphenicol), which provide effective treatment for pneumonia, has in no way diminished the interest shown earlier in the pneumococcal polysaccharides. In this article, we have endeavored to bring up to date the developments that have taken place in this field and, in particular, to emphasize the value of immunological techniques in determining the structure of polysaccharides. 

Reflect and Apply How would you expect the active site of a cellu-lase to differ from the active site of an enzyme that degrades starch Reflect and Apply Would you expect cross-linking to play a role in the structure of polysaccharides If so, how would the cross-links be formed ... 

Cross-linking can be expected to play a role in the structures of polysaccharides where mechanical strength is an issue. Examples include cellulose and chitin. These crosslinks can be readily formed by extensive hydrogen bonding. (See Figure 16.19.)... 

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