Amylopectin structural model

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. 

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... 

Several models have been proposed for amylopectin structure (2), but cxily the cluster models (3 4) have been accepted widely. The cluster model shown in Figure 1 has been presented based on the chain-length distributions of some amylopectins (5.). The chains are named as A, B, and C by Peat et al. ( ) that is, A chains link to... 

Fig. 4.36. Structural models (I, II) for amylopectin with parallel double helices. Ill is an enlarged segment of I or II (according to Banks and Muir, 1980)...

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). 

The investigations carried out by Professor French and his students were based on sound experimental approaches and on intuitive theoretical considerations. The latter often resulted in new experiments for testing a hypothesis. On the basis of theoretical considerations, Professor French proposed a model for the structure of the amylopectin molecule, and the distribution of the linear chains in this molecule. This model was tested by utilizing enzymes that selectively cleave the linear chains, and the results substantiated the theoretical deductions. He proposed a theory on the nature and types of reactions occurring in the formation of the enzyme - starch complex during the hydrolysis of starch by amylases. In this theory, the idea of multiple attack per single encounter of enzyme with substrate was advanced. The theory has been supported by results from several types of experiments on the hydrolysis of starch with human salivary and porcine pancreatic amylases. The rates of formation of products, and the nature of the products of the action of amylase on starch, were determined at reaction conditions of unfavorable pH, elevated temperatures, and increased viscosity. The nature of the products was found to be dramatically affected by the conditions utilized for the enzymic hydrolysis, and could be accounted for by the theory of the multiple attack per single encounter of substrate and enzyme. 

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 6.11 Structural cluster models for amylopectin as proposed by (a) French 67 (b) Robin et al. 285 and (c) Manners and Matheson.290 1 and 2 in the model represent crystalline and amorphous regions, respectively.

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...

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... 

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