Models of Amylopectin

After extensive acid-catalyzed hydrolysis, the crystallites of starch granules are detached. With mild attrition, the starch granules break into small particles.307 The size of the small-particle starch varies with the conditions of the acid hydrolysis  

The large proportion of short chains (Bl- and A-chains) present in A-type starch amylopectin is located within one cluster, with their non-reducing ends free and unrestricted. These short chains have more freedom to move around and are likely 

The porous internal structures of A-type starch granules agree with known features for example, A-type starches exhibit faster chemical penetration and derivatization reactions,325 have weak points, are more susceptible to enzyme-catalyzed hydrolysis,38 

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FIGURE 5.1 Cluster model of amylopectin. A and B denote nomenclature of branch chains, 0=reducing end, c.l. = chain length in degree of polymerization. Reprinted from Carbohydrate Research, Vol. 147, Hizukuri (1986), Polymodal distribution of the chain lengths of amylopectin, and its significance, Pages 342-347, with permission from Elsevier. 

Maltese cross (Blanshard, 1979). The crystallinity of starch is caused essentially by amylopectin pol)Tner interactions (Banks and Greenwood, 1975 Biliaderis, 1998 Donald, 2004 Hizukuri, 1996). An illustration of currently accepted starch granule structure is given in Fig. 5.5. It is believed that the outer branches of amylopectin molecules interact to arrange themselves into "crystallites" forming crystalline lamellae within the granule (Fig. 5.5 Tester et al., 2004). A small number of amylose polymers may also interact with amylopectin crystallites. This hypothetical structure has been derived based on the cluster model of amylopectin (Hizukuri, 1986 Robin et ah, 1974 Fig. 5.1). 

Amylopectin molecules are large flattened disks consisting of (1 —4)-linked a-glucan chains joined by frequent 6)-branch points. Many models of amylopectin structure have been proposed. The most satisfactory models are those proposed by Robin et al.,18 Manners and Matheson,19 and Hizukuri 20 all are known as cluster models. Reviews by Morrison and Karkalis21 and Hizukuri22 discuss in detail the chemical and physical aspects of the starch granule and its components, amylose and amylopectin. 

Figure 5.11 Representation ofthe double helix of crystalline starch after modeling a branching point between two strands. Schematic cluster model of amylopectin molecule incorporating the double helical fragments. (Reproduced with permission from reference 45)...

Debranching studies of starch lends further support to the idea that a blocklet level of structure exists. Hizukuri15 demonstrated that B-chains of amylopectin can participate in more than one crystalline amylopectin side chain cluster, and proposed a revised model of amylopectin structure, classifying the B-chains according to the number of side chain clusters in which they participate. From this work, it is evident... 

Figure 6.8 A cluster model of amylopectin proposed by Hizukuri with A, and B1-B3 chains. The chain carrying the reducing end (0) is the C chain, (1 —>4)-a-D-glucan chain a-(1 —>-6) linkage.263...

The polymodal distribution of unit chains and their proportions in amylopectin support the cluster model. The cluster model of amylopectin suggests a repeat structure made up of one population of A chains, another of relatively short B chains (Bj), and three populations or more of long B chains (B2, B3, B4, B5). The various populations of chains appear to be separated with a periodicity of 12, so that A chains have been assigned to the group of chains with DPn 6-12, B] chains = DP 13-24, B2 chains = DP 25-36 and (Bn)n>3 to chains > 37.225 The weight-percent... 

Fig. 11.—Successive Actions of Phosphorylase, beta-Amylase, Isoamylase, and befa-Amylase on the Meyer Model of Amylopectin and Glycogen. [The symbolism is basically that of Fig. 9, modified to show the action of the various enzymes as follows - - - removed by pho horylase and heto>amylase , branch points hydrolyzed by isoamylase and-, immune to beta amy]ase after isoamylase action.]...

Figure 22.13 A molecular model of amylopectin, a plant starch. Each sphere represents a glucose monomer. The monomers are linked in linear chains by a(l — 4) bonds, and branches occur every 24 to 30 glucose units via j8(l — 6) bonds. Glycogen, or animal starch, has the same composition and structure except that branching occurs every 8 to 12 glucose units. 

The concept of amylopectin forming double helices easily integrates into the currently-accepted cluster model, with the short linear chains of the branches being intertwined into double helices, while the branch points are located in the more amorphous regions between the clusters of double helices. Understanding that parts of amylopectin molecules are capable of forming double helices explains the apparent anomaly that a branched polymer is the source of structural order within granules. 

Figure 5.16 Schematic model for the arrangement of amylopectin in potato starch. Crystalline layers containing double helical linear segments in amylopectin molecules form a continuous network consisting of left-handed helices packed in tetragonal arrays. Neighboring molecules are shifted relative to each other by half the helical pitch. (Adapted with permission from reference 3)...

Figure 6.12 Proposed cluster model of smooth pea amylopectin (a) the mode of interconnection of small structural units to build up dextrin b1. Arrows trace the sub-pieces (included in boxes) of the larger dextrins (b) the fine structure of the dextrins showing how the clusters of short chains (—) are interconnected by long B-chains.261...

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