Mannans conformation

Multi-layer sorption, on the other hand, would have to involve an interaction between mannan chains themselves. Mannan sorption seems, however, to be built up of two stages firstly, the initial very rapid sorption, followed by a much slower sorption phase, which may be the multilayer sorption (20). A change in the mannan conformation after sorption may perhaps enable or facilitate mannan - mannan interactions and allow multilayer sorption. Studies at our laboratory on sorption of mannans onto bleached kraft pulp (BKP) showed that sorption of a high-molar-mass GM was not affected by the presence of a layer of presorbed low-molar-mass GM, and vice versa (31). Furthermore, sorption onto mechanical pulp fibers, which already contain high amounts of hemicelluloses and probably little, if any free cellulosic surfaces, has also been achieved with certain mannans. This point will be further discussed in the next chapter. 

Benzyl derivatives of (1 6)-a-D-glucan, (l->6)-a-D-mannan, and (l-> 6)-a-D-galactan have been studied in 1,4-dioxane. These derivatives have complex and interesting c.d. spectra due to the ww transition of the chromophore with resolved vibrational structure. However, a conformational interpretation of these interesting spectra is not possible at this time. 

The difference in action patterns between these p-mannanases is shown clearly by fractionating and characterising the reaction products (Table I) (5). These products are a consequence of the abihty of the enzyme to cleave in the vicinity of D-mannosyl residues substituted by D-galactose, as well as the length of the 1,4-P-D-manno-oligosaccharide chain required by the enzyme for binding. The favoured conformation of the (l-4)-p-D-linked mannan chain is a flat... 

It is noteworthy that most of the chemical shift values for all three polymers may be closely approximated ( ) by calculations based on data for monomeric reference compounds. These findings illustrate, therefore, the general validity of studies on low molecular weight model compounds for einalysis of spectra of carbohydrate polymers. Many examples of equally satisfactory comparisons of this kind are to be found in studies on other polysaccharides (11,23). These polymers include glucans (l6), mannans (2k, 2 ), limit dextrins (26), lichenin (2j), agarose (28) and various polysaccharides of fungal and microbial orgins (e.g., 7,8,29-31). Observed departures from expectation have been attributed to specific conformational influences ( 8). 

Limited, X-ray diffraction evidence suggested that the chain is a 2(1.52) helix, with an uncoordinated variation about this average conformation. As the projected height of the trisaccharide repeat-unit is 1.52 nm, the average advance per saccharide is 0.507 nm, similar to that in cellulose and D-mannan. 

There are at least four crystalline forms of cellulose, based on different packing of the primary chain (Blackwell, 1982), and three forms of granular starch, based on the packing of double helices (Noel et al., 1993). The differences are largely in the unit-cell dimensions and the crystallization and precipitation temperatures. One form of starch, precipitated with alcohol, is in a symmetrical molecular arrangement and is readily dispersible in cold water (Kerr, 1950). Mannan and dextran yield different crystals at low and high temperatures, and there was not only a polymorphic difference, but a conformational difference in cellulose (Quenin and Chanzy, 1987). Curdlan appears to have three polymorphs—anhydrous, hydrated, and annealed. 

The hemicelluloses are polymolecular, random-coil, often branched hetero-glycans interspersed with cellulose microfibrils and starch granules in vivo. Glucose, xylose, and arabinose are the prominent sugars, although lesser quantities of galactan, mannan, etc., and minor concentrations of acetic, ferulic and uronic acids, etc., are in evidence. Heterogeneity extends to dimensions and conformation. 

Zugenmaier P (1974) Conformation and packing analysis of polysaccharides and derivatives, I. Mannan. Biopolymers 13 1127-1139... 

The basic conformational motif of these molecules appears to be the two-fold helix, maintained by intra-chain 05 H03, familiar from cellulose. Electron diffraction studies on single crystals of mannan I revealed an antiparallel arrangement in an orthorhombic unit cell containing four equivalent mannoses. The polymer chain adopted a two-fold helix with (jo=-81°, F = -161° jy 79°) and 161°. In addition to the intra-chain 05 H03 hydrogen... 

P-Mannan is water-insoluble and solubilizing it requires very strong alkali, cuprammonium or derivatization. X-ray and electron diffraction show that mannan has a ribbon-like conformation, similar to cellulose, with the 2-OH axial instead of equatorial. In seeds (palm, coffee, caraway) a few (<5%) of a-Gal groups are attached to the hydroxymethyl and in seaweeds a small percentage (<5) of Glc residues may occur in the chain. A pure P(l-4) mannan has been extracted from cell walls of the siphonous green alga Codium latum as the methylol mannan with paraformaldehyde dimethylsulphoxide. It was purified by size exclusion chromatography in dimethyl sulphoxide and recovered by the addition of water or methanol and could be re-dissolved in hot dimethyl sulphoxide. Infra-red and n.m.r. spectroscopy and periodate oxidation confirmed the unbranched P(l-4) structure [185]. 

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