Amylose, molecular structure

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

Molecular Structure. Most starches consist of a mixture of two polysaccharide types amylose, an essentially linear polymer, and amylopectin, a highly branched polymer. The relative amounts of these starch fractions in a particular starch are a major factor in determining the properties of that starch. 

Figure 8.4 Molecular structure of the amylose and amylopectin components of starch.

FIGURE 5.8 Unit cells (outlined in each diagram) and helix packing in A and B polymorphs of starch. Reprinted from Carbohydrate Research, Vol. 61, Wu and Sarko (1978b), The double helical molecular structure of crystalline A-amylose, Pages 27-40, with permission from Elsevier. 

Shibanuma et al.232 examined the molecular structures of starch isolated from three Japanese wheat varieties, one Australian standard white wheat and one US western white wheat. The data presented in Tables 10.9 and 10.10 again indicate that the properties and structural features of amylose and amylopectin are dependent on the starch source. The molecular sizes of amylose and amylopectin were larger in the US wheat compared to the corresponding starch fractions from the Australian and Japanese wheat starches. Among the five wheats, the two preferred for salt noodles in Japan, the Japanese variety Chihoku and the Australian standard white, contained a higher proportion of branched amylose and a lower number of chains per amylose... 

An amylose solution is colorless. The iodine solution is reddish-brown. Yet when you combine these two solutions, you observe an intense blue color. What changes in molecular structures give this coloration ... 

Fig. 3.1. Glucose and its two anomeric forms, which result from the formation of a ring on opening up of the carbonyl (—C=0) group. Note the several ways of drawing molecular structure. Maltose consists of two alpha-glucoses linked by a 1,4 bond, Amylose consists of a chain of such linkages. Glycogen, in addition, contains 1,6 links, which result in a branched structure. See Appendix 1, page 73, for further review of isomer terminology.

In many foods, both starch and protein can be encountered so that understanding interactions between them would be useful. The selectivity in interaction between proteins and starches is best seen in results of dynamic rheological studies. The results depend upon the molecular structure of protein, the starch state of the granules and the amylose/amylopectin ratio, the composition of protein and starch, as well as the phase transition temperatures are important factors influencing protein-starch interaction. Because proteins and starches are thermodynamically different polymers, their presence together may lead to phase separation, inversion, or mutual interaction with significant consequences on texture (Morris, 1990). 

Profound modifications and degradation of the molecular structure occur when starch granules, or their component amylose and amylopectin, are heated. The extent of the changes induced depends on the temperature and time involved, and, under extreme conditions, may result in a complete loss of carbohydrate character. 

CDs have special properties dependant on their molecular structures. For instance, their hydrophobic cavities can encapsulate organic and inorganic molecules with smaller molecular size to form various inclusion compounds in liquid- or sohd-state forms [3] while their hydrophilic shells can generate noninclusion complexes with larger molecular guests, such as amylose molecules and enzyme molecules [4,5]. Mainly based on the formation of the two kinds of complexes, CDs are widely used in many areas, including foods, pharmaceuticals, cosmetics and personal care industries. 

Lindner, K and W Saenger (1982). Crystal and molecular structure of cyclohepta-amylose dodecahydrate. Carbohydrate Research, 99,103-115. 

Figure 3 Molecular structures of a-o-glucose and the two major molecules that make up starch, amylose, and amylopectin. 

FIGURE 29.1 The molecular structure of starch containing (a) amylose and (b) amylopectin. 

These two forms of starch have very different properties, probably due to the ease of association of the hydroxyl groups among different molecules. The molecular structure of amylose is essentially linear with two to five relatively long branches. The average degree of polymerization of the branches is about 350 monomer xmits. 

Wu H.C.H., Sarko A., Xhe double-heUcal molecular structure of crystaUine B-amylose, Carbohydr. Res., 61,1978, 7-25. Wu H.C.H., Sarko A., Xhe double-heUcal molecular structure of crystaUine A-amylose, Carbohydr. Res., 61,1978, 27-40. Colonna R, Buleon A., Mercier C., RhysicaUy modified starches, in Starch Properties and Potential, Ed. GaUiard X, 1st Edition, John Wiley Sons, New York, 1987, pp. 79-115, Chapter 4. 

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