Amylose double-helical form

Because the varieties of starch that contain only amylopec-tin are also crystalline, exhibiting the same diffraction patterns as starches containing amylose, there is a strong likelihood that the extensively branched amylopectin molecule also crystallizes in a double-helical form. In turn, this implies that linear sequences in amylopectin remain sufficiently long to... 

X-ray diffraction studies support the double-helical structure but suggest a parallel orientation of the amylose chains.81 Since amylose has not been obtained as single crystals the diffraction data do not give a definitive answer. However, if double helices are formed by adjacent branches in amylopectin and glycogen the two strands would be parallel. Starch granules also contain amorphous starch which appears to contain single helices, possibly wrapped around lipid materials.82... 

The unit cell dimensions of all crystalline amyloses that have been determined in some detail, are listed in Table I. Also included are some intermediate forms between the va and Vjj amyloses (Ji.) and some V-amylose complexes with n-butanol, which, although not yet completely determined, have been added to illustrate the range of variability in unit cell dimensions. In the case of the Va-BuOH complex, a doubling of one unit cell axis was detected after a careful study of electron diffraction diagrams of single crystals ClO). A consequence of the doubling is that the unit cell now contains four chains, instead of the two normally found in amylose structures. Cln a strict sense, the A- and B-amyloses should also be considered as four-chain unit cells, but their double-helical structure still results in only two helices per cell) (13,1 ). 

Amylose is synthesized by granular-bound starch synthase, whereas amylopectin is synthesized by soluble starch synthase (Chapter 4).334,339 Because amylose is synthesized by the granular-bound starch synthase in a progressive manner,340 the amylose molecule is likely confined in the granule and has little opportunity to interact and form double helices with other starch molecules to facilitate branch formation. Branching reactions do occur on some amylose molecules, but at a much lower frequency than with amylopectin, and result in slightly branched amylose molecules. 

Dilute Systems. We will first consider what will happen in an amylose solution. Actually, amylose is very poorly soluble in water at room temperature (although it is well soluble in some salt solutions, notable KC1 cf. Table 6.1). It readily forms helices in water, of which at least part are double helices. These helices tend to align, forming parallel stacks that may be considered microcrystallites. X-ray diffraction shows the chain packing to be similar to that of B type crystallites in native starch. As much as 70% of the amylose may become crystalline. It depends on amylose concentration what the consequences will be. An amylose solution at 65°C has a chain overlap concentration c (see Section 6.4.2) of about 1.5%. If a more dilute solution is cooled, precipitation of amylose will occur. 

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).

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