Eric Bertoft

Fractional precipitation of amylopectin alpha-dextrins using methanol

Abstract The oligosaccharides produced on hydrolysis of waxy-maize amylopectin by the alpha-amylase of Bacillus subtilis for 60 and 210 min were fractionated by precipitation with methanol and the fractions were analysed on Sepharose CL-6B. The smaller dextrins were comparatively well separated and the larger dextrins were obtained as mixtures. The d.p. of the products ranged from 67 to 1550. Maltohexaose was formed in addition to the precipitable dextrins.

Relationship between branching density and crystalline structure of A- and B-type maize mutant starches

Amylopectin from two double maize mutant starches of A-crystalline (wxdu) and B-crystalline type (aewx) was subjected successively to hydrolysis involving alpha and beta amylases, which isolated clusters and all branching zones of clusters (BZC). Enzymatic analysis together with ionic and size-exclusion chromatography revealed the structural features of the clusters and BZC and their role in starch crystallization. A-type clusters were larger (dp(n) > 80) and contained more (but shorter) chains than B-type clusters. The BZC of A-type starch was also larger, but with a shorter distance between the branching points than in B-type BZC. A-type clusters had a densely packed structure and B-type a poorly branched structure. Models for the structure of A- and B-type clusters are presented, and a hypothesis for the influence of cluster geometry on crystallization is proposed.

Composition of chains in waxy-rice starch and its structural units

Abstract φ,β-Limit dextrins of fractions of intermediate products obtained by alpha-amylolysis of waxy-rice starch were analysed for their unit chain distributions by gel-permeation chromatography. The proportion of long chains decreased in fractions of low-molecular-weight dextrins, but the ratio of A:B-chains, A:Ba-chains, and Ba:Bb-chains remained almost constant. High-performance anion exchange chromatography was used for a detailed analysis of the composition of B-chains. Chains with a length intermediate to the groups of short and long B-chains increased in small dextrins. The shortest B-chain in the φ,β-limit dextrins was maltotriose and was preferentially produced during alpha-amylolysis. Models are proposed showing the fine structure of the clusters of the amylopectin, that probably originated from different structural domains within the starch granules.

On the Building Block and Backbone Concepts of Amylopectin Structure

ABSTRACT The molecular structure of the starch components has been a focus of research since it was discovered that starch is transformed into sugar by treatment with acid or barley malt extract in the beginning of the 19th century. Only in 1940 was it definitely recognized that starch contains two glucan polymers, wherein amylose was defined as the linear component and amylopectin as the branched polymer. Since then, the structure of amylopectin has been exhaustively investigated, and during the years several models have been suggested, of which the cluster model has obtained the most acceptance. During the last decade, however, experimental results have been obtained that are not entirely in favor of this model, and an alternative, two-directional backbone model was proposed. This article discusses the latter model and its impact on our understanding of the structure, properties, and synthesis of the starch granule.

Building block organisation of clusters in amylopectin from different structural types.

Clusters of chains consisting of tightly branched units of building blocks were isolated from 10 amylopectin samples possessing the 4 types of amylopectin with different internal unit chain profiles previously described. It was shown that clusters in types 1 and 2 amylopectins are larger than in types 3 and 4, but the average cluster size did not correspond to the ratio of short to long chains of the amylopectins. The size-distribution of the building blocks, having one or several branches, possessed generally only small differences between samples. However, the length of the interblock segments followed the type of amylopectin structure, so that type 1 amylopectins had shortest and type 4 the longest segments. The chains in the clusters were divided into characteristic groups probably being involved in the interconnection of two, three, and four - or more - building blocks. Long chains were typically found in high amounts in clusters from type 4 amylopectins, however, all cluster samples contained long chains. The results are discussed in terms of the building block structure of amylopectin, in which the blocks together with the interblock segments participate in a branched backbone building up the amorphous lamellae inside growth rings of the starch granules. In such a model, amylopectins with proportionally less long chains (types 1 and 2) possess a more extensively branched backbone compared to those with more long chains (types 3 and 4).

The molecular structure of waxy maize starch nanocrystals

The insoluble residues obtained by submitting amylopectin-rich native starch granules from waxy maize to a mild acid hydrolysis consist of polydisperse platelet nanocrystals that have retained the allomorphic type of the parent granules. The present investigation is a detailed characterization of their molecular composition. Two major groups of dextrins were found in the nanocrystals and were isolated. Each group was then structurally characterized using beta-amylase and debranching enzymes (isoamylase and pullulanase) in combination with anion-exchange chromatography. The chain lengths of the dextrins in both groups corresponded with the thickness of the crystalline lamellae in the starch granules. Only approximately 62 mol% of the group of smaller dextrins with an average degree of polymerization (DP) 12.2 was linear, whereas the rest consisted of branched dextrins. The group of larger dextrins (DP 31.7) apparently only consisted of branched dextrins, several of which were multiply branched molecules. It was shown that many of the branch linkages were resistant to the action of the debranching enzymes. The distribution of branched molecules in the two populations of dextrins suggested that the nanocrystals possessed a regular and principally homogeneous molecular structure.

Partial characterisation of amylopectin alpha-dextrins

Abstract Alpha-dextrins, obtained by fractional precipitation with methanol from hydrolysates of waxy-maize amylopectin after the action of Bacillus subtilis alpha-amylase, were analysed using sweet-potato beta-amylase and rabbit muscle phosphorylase. Those obtained after alpha-amylolysis for 60 min had similar phosphorolysis (39.5%) and beta-amylolysis limits (48.4%), and those obtained after 210 min had much lower limits. The limit values were higher for the smaller dextrins. The mean chain-length in the alpha-dextrins obtained after alpha-amylolysis for 60 and 210 min were 14.9 and 11.1, respectively, regardless of the d.p. The smaller chain-length in the latter alpha-dextrins was due to a shortening of the external chains. It is suggested ( a ) that the alpha-amylolysis involves two independent processes, namely, formation of maltohexaose from the external chains and of branched intermediate alpha-dextrins by fission of longer internal chains; and ( b ) that the phosphorolysis limit dextrins had three d -glucosyl residues on the external B-chains and four residues on the A-chains.

Investigation of the fine structure of amylopectin using alpha- and beta-amylase

Abstract The hydrolysis of waxy-maize amylopectin and its beta-amylolysis limit dextrin by the alpha-amylase of Bacillus subtilis was followed by gel filtration on Sepharose CL-6B. The mixture of alpha-dextrins was also treated with beta-amylase. The d.p. of these β-limit dextrins was similar to those of the products obtained when the amylopectin β-limit dextrin was the substrate. It is suggested that amylopectin and its β-limit dextrin are hydrolysed by the alpha-amylase by fission of longer internal chains between unit clusters. A model is presented which shows how these unit clusters can be connected to build up the amylopectin molecule. Maltohexaose was produced from the amylopectin, but not from the amylopectin β-limit dextrin, and it is concluded that the latter is produced solely from the external chains.

On the importance of organization of glucan chains on thermal properties of starch.

The relationship between the internal structure of amylopectin from diverse plants and thermal properties of the starch granules has been investigated. Correlations were found between structural parameters, such as number of building blocks in clusters, interblock chain length and length of external chains, and gelatinization parameters. Onset gelatinization temperature negatively correlated with number of building blocks (r=-0.952, p<0.01) and positively correlated with inter-block chain length (r=0.905, p<0.01). Enthalpy of gelatinization positively correlated with external chain length (r=0.854, p<0.01). These data showed that the internal structure is predictive of trends in thermal properties. A model is proposed based on the backbone concept of amylopectin structure that explains how the organization of chains in the semicrystalline lamellae of starch granules relates to the thermal properties.

Investigation of the fine structure of alpha-dextrins derived from amylopectin and their relation to the structure of waxy-maize starch

Alpha-dextrins, obtained by fractional precipitation with methanol of the products of the action of Bacillus subtilis alpha-amylase on waxy-maize amylopectin, were debranched with isoamylase and the distributions of the unit chains were analysed by gel-permeation chromatography. The large alpha-dextrins still contained long B-chains after hydrolysis for 60 min, but these were absent from the small dextrins with chain numbers of approximately 11 or less. The small dextrins contained increased amounts of chains with lengths intermediate of those of the long B-chains and the main part of the short chains. After hydrolysis for 210 min, almost all of the long B-chains had disappeared and the chains with intermediate lengths had been shortened further. The distributions of the unit chains of the internal chains, obtained by debranching of the phosphorolysis (phi)-limit dextrins, gave similar results and showed that the ratio of A- to B-chains was unchanged during the alpha-amylolysis. Models for the fine structure of the intermediate alpha-dextrins are proposed.