Starch average chain length

Figure 6.10 (Continued) (c) average chain-length distributions of selected A-, B-, and C-type starch.79...

In the following experiment a thin paste of arrow-root starch was treated with malt a-amylase. The degree of hydrolysis was ascertained by titration with hypoiodite and n-glucose and maltose were determined by fermentative methods. After the fermentation, the average chain lengths of the a-dextrins was determined by titration with hypoiodite before and after complete hydrolysis with sulfuric acid. In Fig. 4, curve... 

Fig. 4.—Hydrolysis of arrow-root starch by malt a-amylase, 1, hydrolysis as calculated from reduction value II, average chain length of a-dextrins III, formation of maltose (and maltotriose) IV, formation of n-glucose.

In order to understand why the malt a-amylase acts so slowly on a normal hexaose (Fraction PDXII) we have tried to apply the Michaelis-Menten theory and have determined the affinity constants of the enzyme-substrate compounds. If the concentration of starch and dextrin are expressed as moles of maltose per liter we find the affinity constant, K, between malt a-amylase and starch to be about 200 and for /3-amylase about 170. The affinity constant a-amylase to a-dextrin is so small that no values could be obtained. The constant certainly is smaller than 12. Thus, it is clear that the slow action of the malt a-amylase on the dextrin is due to a very low affinity of the enzyme to the substrate. In other words, the enzyme can readily attach itself to a long chain, but only with difficulty to chains shorter than a certain value. In the dextrinization period of the action of the enzyme on starch, a mixture of a-dextrins is produced with an average chain length of about 7 units. It can be assumed, therefore, that the enzyme readily attaches itself to chains that contain more than about 8 units provided that the chains are normal and contain no branchings or other anomalies. When anomalies occur these... 

The average chain length of the branched chains in the starch fraction can then be calculated by the following ... 

There is evidence that some starches contain a small proportion (5-10%) of a. third polysaccharide. In potato and mbber-seed starches, this material is a short-chain amylopectin (average chain-length, 13-16 n-glucose residues) and not, as might be expected, a polysaccharide that is intermediate in structure between amylose and amylopectin. 

The average chain length of corn starch has been shown to be reduced by 7-irradiation as well as by acidic hydrolysis, the extent being dependent on the dose rate. There was no evidence for the formation of dialdehydes by ring cleavage of the D-glucose residues. 

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