Amylose double helical structures

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 ). 

The double-helical structures of native A- and B-amyloses are found in the fourth group. It is interesting that in both h as well as the d and dyg spacings, they are comparable with the structure of amylose triacetate I (ATAI). In part, this may arise because the packing of the bulky acetate substituents in ATAI is similar to the close-packing of two amylose chains into a double helix. In the latter, one chain may act as the "substituent" for the other chain. At any rate, all three structures contain similar, cylindrical-shaped helices. Somewhat unexpectedly, the distances cL and d-yo are very close for the two native polymorphs, even though their unit cells and packing are... 

Both amylose and amylopectin take part in the retrogradation process and network structure formation and up to four different steps of ordering have been postulated [30]. The first one, which takes place in a few hours, has been ascribed to gelation of the solubilized amylose in the intergranular space forming double helices structures. The second and the third one, for which days are needed, have been ascribed to chain reordering and crystallization of amylopectin. 

The crystal structure of A-amylose is made up of parallel-stranded, ng/tr-handed double helices which pack in an antiparallel manner to give an orthorhombic unit celll67). About eight water molecules are present per unit cell and these guests are present in the interstitial sites (Fig. 15). 

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. 

Figure 20-3 Proposed structure of a molecule of amylopec-tin in a starch granule. The highly branched molecule lies within 9 nm thick layers, about 2 / 3 of which contains parallel double helices of the kind shown in Fig. 4-8 in a semicrystalline array. The branches are concentrated in the amorphous region.113 114 121 Some starch granules contain no amylose, hut it may constitute up to 30% by weight of the starch. It may he found in part in the amorphous bands and in part intertwined with the amylopectin.122...

Since its introduction several years ago, the virtual bond, constrained optimization method has proved very useful in studies of polysaccharide crystal structure. Notable among the successes that can be ascribed to it are the structural determinations of the double-helical amylose (.11), the cellulose polymorphs of different chain polarities (.12, 13), and of a number of other polysaccharides and their derivatives. As described in a review of amylose structures elsewhere in this volume, the use of this refinement method has produced structural detail that has previously been unavailable (ll). These results have provided much-needed... 

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