Amylopectin helical interactions

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,... 

In summary, therefore, substantial information regarding the location and state of amylose has been obtained. Individual amylose chains are believed to be randomly located in a radial fashion among the amylopectin molecules. The concentration of amylose (and lipid) increases towards the surface of the granule, with smaller (leach-able) amylose chains predominating near the surface. Amylose chains are believed to be in a single helical state, although a small proportion may be involved in lipid complexes.147 Some of the larger (non-leachable) amylose chains may be involved in double helical interactions with amylopectin.159... 

Do double helical interactions between amylose and amylopectin chains occur within the granule ... 

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. 

The outer branches of the amylopectin molecule assist in the formation of a helical inclusion complex with suitable lipids (Gudmudsson and Eliasson, 1990). Interactions depend on the starch origin. Mutant maize and mutant rice produce amylopectin with branched chains having about 15 residues rather than the normal 10 to 13 glucose residues. Instead of a normal A-pattem, mutant cereals gave B-pattems (Imberty et al., 1991). 

The competing ligand like SDS, appears to interact with the loose helical regions in amylose and amylo-pectin to form the corresponding compact helical complexes with SDS molecules trapped Inside the helices, when lodine(I3) is added to this system, a part of the SDS, which is loosely attached is displaced by iodine(I ). The absorption spectra of the resulting amylose-SDS-iodine-ccmplex/amylopectin-SDS-iodine-complex, when compared with their respective spectra in absence of SDS, show a reduction in... 

Amylose is a unique polysaccharide which forms a helical blue-coloured complex with iodine (I5). It also forms helical complexes with a variety of organic compounds such as 1-butanol, 1-pentanol, cyclohexanol, SDS etc. The interaction of SDS, a competing ligand with iodine (I ) complexes of amylose and amylopectin is studied spectrophotometrically. It is observed that the reduction in absorbance at 640 nm accompanied by the blue shift (640-570 nm) in the absorption spectrum is governed by the sequence of addition of the reagents, implying that this Interaction is closely associated with the coil—>hellx transition of the polymer chain. Perturbation of this complex with sodium thiosulphate and urea has revealed that the transition from helix—>coil is rather sluggish and hydrophobic interactions play an important role in the stability of this complex. 

Studies on starch demonstrate that starch will include aroma compounds the extent of inclusion will depend upon the starch (proportion of amylose to amylopectin), its processing history, and the aroma compound. Hau et al. [19] have presented a view of the extent of interactions one observes via static methods (measure of chemical interactions) between selected aroma compounds and starch (Figure 6.6). It is clear that the amount of interaction is quite significant for some compounds (nearly 80% of the hexanol is bound). The interaction is both compound and time dependent (although most compounds reached a plateau in binding after only 1 hr). It is postulated that free amylose forms a helical structure that has hydrophobic areas that will include certain aroma compounds. It should be noted that this research... 

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