Amylopectin molecule size

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. 

A droplet-like structure, where a core of an almost amorphous amylopectin molecule is surrounded by complexed amylose molecules, has been confirmed by transmission electron microscopy (TEM) analysis of film slices [90], The droplet size is comparable with that of the microdispersion obtained by boiling. 

The characteristics of these cooked viscous solutions vary from starch to starch. After cooling to room temperature, the starch from roots are clearer and more fluid, while starch from the cereal grains yield a cloudy less fluid paste that tends to be jellylike. These characteristics are dependent upon the amylose and amylopectin content of the starch and upon the size of the amylose and amylopectin molecule. Some hybrids—waxy hybrids—of corn and sorghum have been developed which yield starch that is almost entirely amylopectin, while other hybrids have a high amylose content. Overall, the tendency to thicken or gel upon cooling, and to become opaque is caused by the presence of amylose. 

All cereals store amylose and amylopectin molecules in simple or compound granules (Chapter 4). Starch grauules from a single cereal differ from one another in size, shape, and other characteristics. The different granules possess different melting and swelling properties and respond differently to enzymatic hydrolysis. A... 

Methods which can be used to determine the size and shape of polysaccharides have been reviewed.107 (A critical survey of these has recently been given by Sadron108 and by Ogston.109) Special problems exist in the case of the undegraded starch components. In view of the branched nature of amylopectin and the large size of the amylose molecule, chemical methods of estimating size are inadequate, and it is questionable whether results are valid.38 The free components may also aggregate in aqueous solution. Study of derivatives is therefore more convenient, and the preparation of these is an essential preliminary to estimations of molecular size. 

Amylopectin Amylopectin is similar to amylose except that the glucose chain has branches. These branches involve linkages at the -CH2OH position ( 6), which makes them a 1 —> 6 linkages. Amylopectin is water-soluble it also interacts with iodine to form a reddish-purple complex. Typically, amylopectin is ten times the size of an amylose molecule. Digestion requires (3-amylase (1 4 linkages) and a second... 

Amyloses of five commercially important starches (wheat, corn, rice, tapioca and potato) were purified by fractional crystallization of their amylose-alcohol complexes and shown to be free of amylopectin.111,205 All five amyloses showed comparable iodine affinity, blue value and max of their polyiodide complexes (Table 10.5), which is consistent with their being (1 —4)-linked a-glucans with an average DPn of —500 or above.206 The average size of amylose molecules in cereal starches is smaller than... 

The branching enzyme from Bacillus stearomophilus decreased the molecular size of synthetic amylose. On studying the product of this reaction, it was found that BE had catalyzed the intramolecular transglycosylation to form a cyclic structure with a side chain. After removing the cyclic part of the molecule (using isoamylase) from the rest of the molecule, its cyclic nature was confirmed by the use of mass spectrometry. The authors proposed a new mechanism for the action of BE and suggested that plant BE may catalyze the cyclization of amylose and amylopectin. 

---