Amylose fine structure

Amylose is an instructive example, showing the relationships between different conformations and chemical properties (28). In the discussion of gelation mechanisms, the evaluation of the fine structure of the polysaccharides is a very important point. 

Table III shows the results of chemical analyses of amylose samples compared, where possible, with values of Mn. These indicate the presence of more than one nonreducing, terminal group in some of the amylose samples. In the case of potato starch, this result is thought to be attributable to the presence of contaminating amylopectin rather than to inherent branching in the molecule.106 Other methods of examining the fine structure of amylose, and the question of branching, will be dealt with later (see p. 381).

All chapters/subjects that were also in the previous edition have been updated. Chapters have been added on the biochemistry and molecular biology of starch biosynthesis, structural transitions and related physical properties of starch, and cyclo-dextrins. There are two chapters on the structural features of starch granules that present not only advances in understanding the organization of starch granules, but also advances in understanding the fine structures of amylose and amylopectin, both of which are based on techniques that have been developed since 1984. 

The greatly enlarged chapter on wheat starch presents advances in its production, the differences between large and small granules, the fine structures of wheat starch amylose and amylopectin, genetic and chemical modification of wheat starch, and its functionalities and uses, especially in food products. 

Although, on the basis of chemical studies, it was possible to ascertain the gross structures of the molecules of starch and glycogen, the use of highly purified enzymes has become established as one of the most versatile and definitive methods for the determination of the fine structure of these polysaccharides. The use of these techniques in the examination of the fine structures of glycogen, amylopectin, and amylose will now be discussed. 

Because of the lack of homogeneous, purified, cellulose-degrading enzymes, having a variety of action patterns, that will act on the native, unmodified polysaccharide, it has not yet been possible to investigate the fine structure of cellulose in a manner analogous to that used for amylose. The situation is complicated by the insolubility of the substrate and its resistance to degradation by any single enzyme. A system of enzymes, the function of at least one component of which is at present unclear, is required. For this reason, it has not yet been possible to examine the fine structure of cellulose enzymically. 

Qng, M. H., and J. H. V. Blanshard. 1995. Texture determinants in cooked parboiled rice. I. Rice starch amylose and the fine structure of amylop>ectin. Journal of Cereal Science 21 251-260. 

Interest in the genetic engineering of both plants and micro-organisms for the production of tailor made amylose, amylopectin and/or starches has also been reported. Furthermore, investigations on the enzymatic modification of starch and its major components, for example the introduction of additional branches composed of glucose and/or other monosaccharides and/or uronic acids as well as amino acids or peptides, to produce carbohydrates of possibly comparable functionality to galactomannans, pectin, gum arable, etc., has been initiated. Also studies on the metabolic fate of carbohydrates in food, the biosynthesis of starch, the fine structure of starch from different sources, the effect of electrolytes on the gelatinization of starch and the development of enzymic methods for starch analyses are still active. 

Solid-state cellulose can also be noncrystalline, sometimes called amorphous. Intermediate situations are also likely to be important but not well characterized. One example, nematic ordered cellulose has been described [230]. In most treatments that produce amorphous cellulose, the whole fiber is severely degraded. For example, decrystallization can be effected by ball milling, which leaves the cellulose as a fine dust. In this case, some crystalline structure can be recreated by placing the sample in a humid environment. Another approach uses phosphoric acid, which can dissolve the cellulose. Precipitation by dilution with water results in a material with very little crystallinity. There is some chance that the chain may adopt a different shape (a collapsed, sixfold helix) after phosphoric acid treatment. This was concluded because the cellulose stains blue with iodine (see Figure 5.12), similar to the sixfold amylose helix in the starch-iodine complex. 

Since amylopectin acetate produces only brittle films and plastics, its molecules probably have a non-linear structure which may be branched or coiled. A further striking difference between amylose triacetate and amylopectin triacetate is that the former can be obtained in the form of a highly fibrous mass, whereas the latter occurs only as a fine powder. 

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