Polysaccharides structural interpretation

The Chapter by Wilkie (Aberdeen) on the hemicelluloses of the Gramineae serves to illustrate that not all areas of complex carbohydrates are yet amenable to neat and clear-cut structural interpretations as a result of modern technology. The article emphasizes the need for continued caution in attribution of precise structures to this enigmatic class of polysaccharides, and final answers can not yet be written on their structures and roles. 

X-Ray diffraction analysis of oriented polysaccharide fibers has had a long history. Marchessault and Sarko discussed this topic in Volume 22 of Advances, and a series of articles by Sundararajan and Marchessault in Volumes 33, 35, 36, and 40 surveyed ongoing developments. The comprehensive account presented here by Chandrasekaran (West Lafayette, Indiana) deals with some 50 polysaccharides, constituting a wide range of structural types, where accurate data and reliable interpretations are available. The regular helical structures of the polysaccharide chains, and associated cations and ordered water molecules, are presented in each instance as stereo drawings and discussed in relation to observed functional properties of the polymers. 

The gelation process is considered a very specific process. This specificity is interpreted on a structural basis. Therefore some relationships between the chemical constitution and gelation properties of pectins and some related polysaccharides are outlined. 

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... 

Investigations conducted at the Canadian Government Research Laboratories in Ottawa on the chemical structures of Neisseria meni-gitidis antigens have mainly been based on the interpretation of 13C-n.m.r. spectra. The polysaccharides of serogroups A, B, C, W135, X, Y,165 29e,166 and BO167 were characterized as complex polysaccharides,... 

In describing and interpreting some of the more important properties of plant galactomannans, comparisons will be made with structurally similar polysaccharides, including the closely related glucomannans and galactoglucomannans, and those based on (1 — 4)-/3-D-xylan main-chains (for example, the arabinoxylans) and (1 — 4)-/3-D-glucan main-chains [for example, the amyloids and sodium 0-(carboxymethyl)cellulose]. 

The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

The literature contains numerous observations on the properties of polysaccharides in cuprammonium solutions the work on cellulose is especially voluminous. Viscometric measurements in cuprammonium solution are regularly employed to determine the size of cellulosic molecules. However, before the spatial requirements for complexing with cuprammonium became known the properties of the complexes of polysaccharides could not be interpreted in terms of the structure of their monosaccharide units. With the present understanding of cupram-monium-glycol complexing, some of the earlier observations will be reexamined. 

Hess and coworkers53 have isolated mannose-containing polysaccharides from both ivory nuts and from pine wood pulp, and these mannans appeared to have identical properties. Both gave specific rotations of approximately —45° in N sodium hydroxide and +285° in cuprammonium (0.04 mole hexose anhydride, 0.10 mole copper, 10.0 moles ammonia per liter). The exact composition of these polysaccharides is not known Yundt54 has recently stated that mannan A from ivory nuts assayed only 50 percent mannose. The cuprammonium rotation data cannot be interpreted in terms of the fine structure until the composition of the mannans is known with certainty. 

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