Amylose fractionation

Thus, Freudenberg concluded that the cyclodextrins are not preformed in starch, but that their formation is made possible by the helicity of the starch chain. Freudenberg s hypotheses concerning the starch structure (that is, the amylose fraction) and the Bacillus macerans amylase mechanism have been confirmed by X-ray crystallography and chromatographic techniques. ... 

The reactions of potassium iodide in aqueous solutions are those of iodide ion, r. In iodometric titration I combines with iodine to form triiodide ion, I3. The latter adds to (i-amylose fraction of the starch to form a blue complex. 

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

Other WAXD starch patterns, known as V-types, are associated with the amylose fraction. In V-amylose, the chain conformation is a left-handed single helix with six residues per turn (V6) for complexes with aliphatic alcohols and monoacyl lipids with ligands bulkier than a hydrocarbon chain, helices of seven or eight glucose residues per turn are feasible.31 Aliphatic chains within amylose helices are rather locked... 

The amylose fraction from three different oat starches was characterized by high-performance size-exclusion chromatography (HP-SEC) and shown to be free of amylopectin.31 The degree of polymerization is given in Table 15.2. By treating amylose with isoamylase, it was found to be branched.31 The major fraction had a DP of 700, whereas a second fraction had a DP of 72. The amylose chain length was reported to decrease with increased amylose and starch-lipid content.30 The intrinsic viscosity of amylose has been reported to be 246-299 mL/g.7... 

Parameter Amylose fraction Amylopectin fraction Intermediate fraction... 

Generally, the phosphorus content in starches is associated with different pasting properties, and it confers a larger ion binding capacity. In wheat and com starch, phosphorus is present largely or wholly as adsorbed phos-phatides (extractable with boiling 85% methanol) associated preferentially with the amylose fraction. 

DP /DP = 1.5) is employed. It may be seen that (a) with natural amylose, DP is invariant during the hydrolysis, (b) in the case of the amylose fraction, an initial decrease in DP, is followed by invariance due to the regeneration of the exponential function, and (c) the value of DP for the synthetic amylose decreases constantly during the hydrolysis. The evidence to date, therefore, suggests that heta-amylase action is not single-chain, but either multi-chain or limited multiple attack—the experimental results cannot distinguish between the two. 

Pullulanase has great potential in structural investigations. In the case of amylose and its jS-limit dextrin, treatment with pullulanase has shown unambiguously (for the first time) that a-D-(l->6) branch-points form the natural barrier to heta-amylase action, that is, some amylose fractions have limited branching. ... 

Starch acetates normally contain 2 to 3% moisture. They have a strong binding power for water as is indicated by the observation that an unplasticized amylose (fraction A) triacetate film soaked in water will lose about one-third of its tensile strength. 

There can be little doubt that, during most preparative, fractionation methods, a certain amount of degradation is inadvertently introduced. As a result, the intrinsic viscosities of both fractions of starch will be found to be lower than the corresponding values for their native state. In order to make a comparison possible, some method of fractionation has to be developed which gives no degradation whatsoever. In this respect, the techniques outlined in this Section might all have a fair chance of success. Restricting attention to potato starch as a substrate, and furthermore to the intrinsic viscosity of its amylose fraction as measured in 1.0 AT potassium... 

Table VI shows the results of a number of these experiments, using different magnesium sulfate concentrations. The increase of fractionation efficiency with increase of the salt concentration is evident. Independent of the salt concentration, all experiments yield amylose fractions of maximum iodine-absorption capacity, as recrystallization with 1-butanol did not show any increase in the original values.

Table VII shows some results of the subfractionation (by particle size) of the amylose obtained from a single fractionation cycle. Increase of the starch concentration increases the average particle-size of the precipitated amylose fraction. Systems containing degraded starch (for example, soluble starch), irrespective of the rate of cooling, always yield amylose precipitates of very fine particle-size (about 0.2 micron).

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