Amylose repeat unit

Five articles on polysaccharide helices solved prior to 1979 have appeared in the volumes published between 1967 and 1982.2-6 The first was a review on X-ray fiber diffraction and its application to cellulose, chitin, amylose, and related structures, and the rest were bibliographic accounts. Since then, X-ray structures of several new polysaccharides composed of simple to complex repeating units have been successfully determined, thanks to technological advances in fiber-diffraction techniques, the availability of fast and powerful computers, and the development of sophisticated software. Also, some old models have been either re-... 

Maltose repeating unit of starch (amylose) D-Glucose joined in a-1,4-linkages... 

The space group is P2i2i2 . The unit cell is pseudotetragonal, with a = b = 19.17 A (1.917 nm), and c = 24.39 A (2.439 nm), with two antiparallel chains per cell. The amylose chain is a left-handed 6(—1.355) helix, with three turns per crystallographic repeat. One molecule of dimethyl sulfoxide for every three D-glucose residues is located inside the helix. An additional 4 molecules of dimethyl sulfoxide and 8 of water are located in the interstices. The interstitial dimethyl sulfoxide is the source of additional layer-lines that are not consistent with the 8.13 A (813 pm) amylose repeat. The overall R factor is 35%, and, for the layer lines with amylose contribution alone, it is 29%. 

Fig. 5a. Repeating unit of amylose. b. Repeating unit of amylopectin...

The active fraction of starch is amylose, a polymer of the sugar ce-D-glucose. with the repeating unit shown in Figure 16-5. Small molecules can fit into the center of the coiled. 

Iodine forms a well-known blue complex with the amylose form of starch. From resonance Raman and I Mossbauer spectroscopy it has been shown that the color is caused by a linear array of I5 (I2I I2) repeating units held inside the amylose helix. 

Figure 1.49 Repeat unit structure (a) of amylose (primary storage form of glucose in cells) (b) cartoon showing how amylose chains exist in a hollow helix (V-form) (secondary structure) conformation in the presence and inclusion of a guest molecule such as polyiodide. Such helices become destabilised in the absence of a guest molecule and combine to form double hollow helix structures (A-form) as illustrated (see Fig. 1.50). Colour code, carbon (black), oxygen (red) and hydrogen (white) (illustration from Voet, Voet Pratt, 1999 [Wiley], Fig. 8-10).

Figure 16-3 (a) Structure of the repeating unit of the sugar amylose found in starch, (b) In the starch-iodine complex, the sugar chain forms a helix around nearly linear le units. [V. T. Calabrese and A. Khan, y. Polymer Sci. 1999, A37, 2711.]... 

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