Polysaccharides structure determination

A seminal review by Stacey in 1947 on Aspects of Immunochemistry, and a survey a year later on Chemistry of Immunopolysaccharides, pointed the way to much of his future research. Taken in toto, his chemical work laid a firm basis for correlating the chemical and serological approaches to structure determinations of antigenic polysaccharides. 

Most early publications on bacterial polysaccharides were concerned with impure products and poorly-described organisms. Many more recent papers are of limited value also, due to low yields, lack of characterization of products and arbitrary interpretations of data. Low yields of methylated polysaccharides may be due to degradation of the bacterial polysaccharide during methylation, or to degradation of the hydrolytic products of the methylated polysaccharide (to form methyl levulinate, etc.46). The great importance of (a) complete methylation of polysaccharide products prior to structural determination by hydrolysis and (6) quantitative identification of the hydrolytic products, has been emphasized previously. Other difficulties in end group analysis have been discussed recently.7... 

A number of recent papers have described methods which will undoubtedly become increasingly important as tools for the structural determination of bacterial and other polysaccharides. 

The integral intensities of signals of polysaccharides, obtained with a modem spectrometer under the usual operating conditions, are proportional or quasi-proportional, to the number of 13C nuclei present. This has been observed, in particular, in the case of linear hexo-pyranan structures containing one type,11,47,51,55,56 58-61 or two equal types, of linkage,62 or branched-chain polymers having two linkage types,53,58 where the resonances are readily resolved. In such cases, the T values would be low, 0.2 s or less, and the n.O.e. values would be approximately equal. However, few actual determinations of these values have been made, and extrapolation of such assumptions to more-complex polysaccharide structures is not recommended, as outlined in Section V,2,... 

Figure 4.21 An illustration of how the configuration of glycosidic bonds determine polysaccharide structure and function. The P-1-4 linkages in cellulose favourize straight chains, optimal for structural purposes, whereas the a-1-4 linkages are favourable to bent structures, better adapted to storage in a hydrated form.

Such chair-to-boat transitions have been suggested to play a role in determining polysaccharide structure (end-to-end lengths) in aqueous solution (51). 

X-ray fiber diffraction can be used to visualize highly hydrated polymer specimens at atomic resolution. An essential part of such an analysis is the inclusion of reliable stereochemical information to supplement the diffraction data. Structure determination involves modelling and refinement of putative structures, and adjudication amongst the optimized models. This technique has been successfully applied to a number of polysaccharides. The precision of resulting structures is often sufficient to identify the critical interactions within and between molecules, that are responsible for the unique properties of these materials. 

Establishing the complete structure of oligosaccharides and polysaccharides requires determination of branching positions, the sequence in each branch, the configuration of each monosaccharide unit, and the positions of the glycosidic links—a more complex problem than protein and nucleic acid analysis. 

How can the many complex polysaccharides found in nature be synthesized Are there genetically determined patterns How are these controlled The answer can be found in the specificities of the hundreds of known glycosyltransferases171 172 and in the patterns of expression of the genes for transferases and other proteins. As a consequence, a great variety of structurally varied polysaccharide structures arise, especially on cell surfaces. The structures are not random but depend upon the assortment of glycosyltransferases available at the particular stage of development in a tissue. 

Structural elucidation of natural macromolecules is an important step in understanding the relationships between the chemical properties of a biomolecule and its biological function. The techniques used in organic structure determination (NMR, IR, UV, and MS) are quite useful when applied to biomolecules, but the unique nature of natural molecules also requires the application of specialized chemical techniques. Proteins, polysaccharides, and nucleic acids are polymeric materials, each composed of hundreds or sometimes thousands of monomeric units (amino acids, monosaccharides, and nucleotides, respectively). But there is only a limited number of these types of units from which the biomolecules are synthesized. For example, only 20 different amino acids are found in proteins but these different amino acids may appear several times in the same protein molecule. Therefore, the structure of... 

One of the very important applications of the methyl ethers of sugars and polysaccharides has been in structure determination and for this purpose the complete substitution of all free hydroxyl groups of a given substance is required. Bourne and Peat61 have provided a full description of the principles, reagents, and conditions that are involved. 

Bhattacharjee, A.K., Jennings, H.J., Kenny, C.P., Martin, A. and Smith I.C.P. (1975) Structural determination of the sialic polysaccharide antigens of Neisseria meningitidis serogroup B and C with carbon 13 nuclear magnetic resonance. J. Biol. Chem. 250 1926-1932... 

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... 

This collection of papers was part of a unique symposium held during the 178th Meeting of the American Chemical Society. The symposium, Diffraction Methods for Structural Determination of Fibrous Polymers, had a pronounced international character, with scientists from 12 different countries. The speakers represented both the synthetic polymer and biopolymer fields, with contributions in each of the three classes of natural polymers nucleic acids, proteins, and polysaccharides. Most important, the symposium centered on methods and techniques for studying fibrous polymers, methods that are usually taken for granted despite their inadequacies. 

E. Romanowska, A. Romanowska, J. Dabrowski, and M. Hauck, Structure determination of the O-specific polysaccharides from Citrobacter 04- and 027-lipopolysaccharides by methyl-ation analysis and one- and two-dimensional H-NMR spectroscopy, FEBS Lett., 211 (1987) 175-178. 

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