Mannan, polymorphism

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

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. 

A survey of the various parameters which have a role in directing the crystallization of a given polysaccharide toward one or another polymorph, presents the temperature of crystallization as being the most influential polysaccharides such as cellulose (3), mannan (4), dextran (5,6), etc. are particularly sensitive to changes in crystallization temperatures as they yield completely different crystals at low or high temperature. 

A final and unexpected polymorphism parameter is found in the molecular weight of the sample to be crystallized. This is in particular described in the case of mannan and glucomannan polymers(4,8) where crystals of mannan I are usually obtained with low degree of polymerization (DP) material, whereas only mannan II is found when higher DP are recrystallized. 

A similarity can be established between the polymorphism of mannan and that of cellulose, as with both polysaccharides, one observes a molecular weight influence on the polymorphism, even though this influence is more pronounced in the case of mannan than in the case of oe llulose. It was shown that with mannan and glucomannan polymers, the mannan I structure was obtained more easily with low molecular weight material, whereas mannan II corresponded to higher molecular weight (4,8). A close resemblance... 

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