Amylose solutions, fractional

Fractional Precipitation of Amylose-lodine Solutions. Fairly concentrated amylose solutions containing a mixture of two to five components (DP 90-56CXJ) were precipitated by successive addition of KI2 solution. The purified fractions were converted into tricarbanilate derivatives and were then separated over a high performance GPC column. The ratio of the components was obtained from careful evaluation of the peak areas using the concentration dependence of the single components and various mixtures of known composition. Figure 9 shows the elution curves of fractions a-d obtained from a two component system (DP 90 and DP 1800, 1 1). 

Amylose benzoate Fractional solution Nitromethane/methanol 31°C 2525,2526,2527... 

The fractionation of acid- and alkali-treated starches has been reported,64 but the products are obviously degraded. Further work is, however, necessary on the use of alkaline solutions for fractionating starch. Pacsu10 maintains that the addition of an amylose precipitant to an alkaline dispersion... 

Soluble Starch comprises principally of f3-amylose, with the a-fraction having been removed. Always, it is a practice to prepare indicator-solutions from this product exclusively. 

A second reason for the turn-over in the osmotic modulus may arise from a decrease in A2 until it becomes zero or even negative. This would be the classical situation for a phase separation. The reason why in a good solvent such a phase separation should occur has not yet been elucidated and remains to be answered by a fundamental theory. In one case the reason seems to be clear. This is that of starches where the branched amylopectin coexists with a certain fraction of the linear amylose. Amylose is well known to form no stable solution in water. In its amorphous stage it can be brought into solution, but it then quickly undergoes a liquid-solid transition. Thus in starches the amylose content makes the amylopectin solution unstable and finally causes gelation that actually is a kinetically inhibited phase transition [166]. Because of the not yet fully clarified situation this turn-over will be not discussed any further. 

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. 

Staudinger and Husemann determined the osmotic pressure of solutions of a potato starch acetate which had been fractionated into four parts by precipitation of its chloroform solution with ether. The molecular weights of the fractions ranged from 45,000 to 275,000. All of the fractions were soluble in chloroform, but fractions of low molecular weight were also soluble in acetone. For various concentrations of solute in either chloroform or acetone, the osmotic pressure did not increase in direct proportion to the solute concentration, but the deviation from van t Hoff s law was the smallest in the case of the acetone solutions. Osmotic pressure measurements on amylose and amylopectin tri-acetates dissolved in tetrachloroethane have been made by Meyer and co-workers, who have deduced molecular weights for these substances of approximately 78,000 and 300,000, respectively (see above discussion of the purity of these fractions). 

Consequently, the fractionation of heterogeneous polymers by the method of fractional precipitation mostly effects sharp, quantitative separations. Although, as a rule, chemically dissimilar polymers cannot be combined in one solvent since they will mutually precipitate each other, starch affords an example of the few exceptions to this rule, as its aqueous solutions actually contain two chemically dissimilar polymers, namely, amylose and amylopectin. Nevertheless, until 1950, there were no reports of any systematic studies on the effects caused by the gradual addition, to aqueous, starch dispersions, of water-soluble substances possessing non-solvent character for whole starch. Likewise, no work seems to have been done on establishing the relative difference in solubility between amylose and amylopectin in binary solvent mixtures. ... 

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