Amylopectin orientation

Although it has been found that the separated amylose component can be readily orientated to yield fiber patterns, amylopectin usually gives poor or amorphous patterns. In the granule, however, amylopectin does exhibit crystallinity, since waxy maize starch gives a diffraction pattern and other waxy starches behave similarly.193 -195 (This suggests that the branch points in the amylopectin molecule may be in the amorphous part of the granule.)... 

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... 

Figure 5.8 Granule in cross-section showing the orientation of amylopectin double helices in the crystalline lamellae. The dashed line indicates the path followed to obtain the diffraction diagram using a microfocus x-ray diffraction beam having a diameter of 2p,m. (Adapted with permission from reference 42)...

Until recently, the location and state of amylose within granules was one of the most important questions remaining to be answered. Three main hypotheses for the location of amylose within starch granules have been put forward. The first hypothesis is that amylose is laid down tangentially to the radial orientation of amylopectin in order to minimize the amylose-amylopectin helical interactions.142 There is,... 

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. 

In native starches, branched amylopectine molecules form die crystalline phase and die eventually present amylose constitutes die amorphous state. Pairs of the branches with a degree of polymerisation of 26 fold into the native, parallel oriented double-helices, which build a planar-hexagonal packed crystalline phase. In the metastable B-structure, one of three sites of this packing are filled by water and in the stable A-structure the hexagonal... 

The neoamylose co-crystallises with amylopectine from solution. In the network formed by cocrystallisation of the two polymers the tie molecules are the long branches of the amylopectin and the network-branches are the crystallites. Such gels can be made by an extrusion process, melting the starch and the new polymer and shaped into oriented, elastic fibers and films. 

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