Amylose single helical structures

In an aqueous solution, amylose has a random coil structure with a variable amount of single helical structure composed of six, seven or eight glucose residues per turn of helix (Szejtli et al, 1967). When amylose undergoes retrogradation (precipitation from solution), the molecules associate together to form double hehces that further associate to give the precipitate. 

Nevertheless, other forms of crystallinity, different from the native ones, induced by the interaction of the amylose component with specific molecules, can be observed. These types of crystallites are characterized by single helical structures and are known as V complexes [66]. 

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 ). 

Three potential helical structures of polysaccharides, (a) Single helix with twofold screw axis (cellulose) (b) double helix (amylose) and (c) triple helix (/ , 1-3 glucan) (according to Rees [4]). The particular structure depends largely on the constraints imposed by hydrogen-bonds and rotational freedoms around the glycosidic intermonomer bonds (i. e., O Fig. 3, energy barriers to rotation)... 

In some polymers, the real complexation of a single helical amylose with the polymer backbone occurs, giving rise to supramolecular structures, whereas amylopectin remains in an amorphous state. Such complexes play an important role on the final properties of the starch-based polymer. 

Immediately after the processing (e.g., extrusion cooking) the starch does not show any residual crystaUinity (or values lower than one percent). However, some hours after the manufacture a new type of crystallinity will develop. The structures are formed by single helices of amylose and they can be divided into three types, called the V type (non-hydrated), the Vh type (hydrated), and the Eh type. The Eh structure is not stable and under the influence of moisture it changes during storage into the Vh type, although the total amount of amylose crystallinity remains the same [20-22, 30, 34]. 

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