Starch double helix

Fig. 1.7. Native starch double helix (from French and Murphy, 1977).

For more about starch see The Other Double Helix— The Fascinating Chemistry of Starch in the August 2000 issue of the Journal of Chem ical Education pp 988-992... 

Chain Building. Both crystalline polymorphs of starch have the same fiber repeat of 1.05 nm and are built with the same unit a parallel stranded double-helix. Each strand has six glucose residues per turn in 2.1 nm and the two strands are related by a two-fold axis of symmetry this creates the apparent 1.05 nm fiber repeat. The chirality of the helices has been postulated to be either right-handed (12.13) or left-handed (14.15. ... 

Understanding the Crystalline Polymorphism in Native Starch. From this study, it is clear that the A and B forms have in common not only a double-helix but a... 

Figure 4.3 The building block structure of potato amylopectin clusters. Branched building blocks (encircled) are mainly found inside amorphous lamellae (A) of semi-crystalline rings in starch granules. Double helices (symbolized as cylinders) extend from the building blocks into the crystalline lamellae (C). Enlargements of a double helix segment, in which the single strands are parallel and left-handed, and a building block are shown to the right.

Figure 5.6 (a) Molecular drawing for the double helix found in A and B starches. Each single strand of... 

Figure 5.11 Representation ofthe double helix of crystalline starch after modeling a branching point between two strands. Schematic cluster model of amylopectin molecule incorporating the double helical fragments. (Reproduced with permission from reference 45)...

Fig. 1.—Representation of double-helix packing and unit cells in A and B crystalline starches (a and b, respectively). Dashed lines represent hydrogen bonds. Water molecules have been omitted. (Reprinted with permission from A. Imberty, A. Buleon, Vinh Tran, and S. Perez, Staerke, 43 (1991) 375-384.)...

Since several synthetic polymers also develop a blue color upon reaction with iodine, it is likely that they have a helical structure similar to that of amylose. Therefore it is probable that the aforementioned complexes of synthetic polymers with starch can exist in the form of a double helix. 

Figure 8.4 Concentric arrangement in the starch granule of crystalline amylopectin clusters shovsTi to the left. In the middle the cross-sectional packing of double-helices in these clusters are shown (A, B are different branch types). The double helix arrangement of glycose units is shown to the right.

Interpretation of the WAXS patterns of native starch is often difficult because of the low crystallinity, small size, defects and the multiple orientations of the amylopectin crystallites (Waigh et al, 1997). Two main types of X-ray scattering patterns have been commonly observed (A and B). Potato starch has been shown to crystallize in a hexagonal unit cell in which the amylopectin molecules twist in a double helix (the B structure) (Lin Jana Shen, 1993). Between adjacent helices a channel is formed in which 36 water molecules can be located within the crystal unit cell. By means of heat treatment this structure can be transformed into a more compact monoclinic unit cell (the A structure) (Shogren, 1992). Amylose (the linear and minor component of starch) can be crystallized from solution in the A and B structures (Buledn etal, 1984), yielding X-ray diffraction patterns similar to those of amylopectin but with higher orientation. 

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