Amylose reaction with iodine

Since several synthetic polymers also develop a blue color upon reaction with iodine, it is likely that they have a helical structure similar to that of amylose. Therefore it is probable that the aforementioned complexes of synthetic polymers with starch can exist in the form of a double helix. 

Starches differ in their chemical composition and, except in rare instances, are mixtures of two structurally different polysaccharides, amylose and amylopectin. The proportions of these present in natural starches depend upon the source, although in most starches amylopectin is the main component, amounting to about 70-80 per cent of the total. An important qualitative test for starch is its reaction with iodine amylose produces a deep blue colour and amylopectin solutions produce a blue-violet or purple colour. 

The quantity of amylose may be determined in starches or other mixtures by a characteristic reaction with iodine. Amylose combines with iodine to form a deep-blue complex, which is responsible for the color of starch indicators. Amylopectin solutions are colored blue-violet or purple. The intensity of the amylose-iodine complex can be measured in a spectrophotometer, or titrimetric measurements can be made of the amount of iodine taken up in forming the amylose-iodine complex. 

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. 

Starch is composed of macromolecular components, a-amylose and (i-aim -lose. The former reacts irreversibly with iodine to form a red adduct. (i-Aim losc. on the other hand, reacts with iodine forming a deep blue complex. Because this reaction is reversible, [3-amyl0sc is an excellent choice for the indicator. The undesired alpha fraction should be removed from the starch. The soluble starch that is commercially available, principally consists of (3-amylose. (3-Amylose is a polymer of thousands of glucose molecules. It has a helical structure into which iodine is incorporated as I5. ... 

Both the amylose and amylopectin components of starch form complexes with iodine, but early studies showed that there is no connection between the iodine reaction and the reducibility of starch fractions.63 The complex of amylose is pure blue, whereas the complex of amylopectin is blue-violet.5864 Thus, the varying amylose-to-amylopectin ratio can be one of the factors responsible for the various shades of blue color exhibited by various varieties of starch. Amylopectin takes up less iodine than does amylose. Also the course of complex formation uptake is different, as is evident65 from Fig. 2. 

The reaction of starch with iodine is a common identity test for starch. A dilute solution of iodine stains starches a blue to bluish red color. It is believed that the amylose portion complexes with iodine by forming a helix around it. This blue color has been used both as a qualitative and quantitative test for starch in various systems. 

More commonly, titrations involving iodine are performed with a suspension of starch as an indicator. The deep blue color that develops in the presence of iodine is believed to arise from the absorption of iodine into the helical chain of j8-amylose (see Figure 20-2), a macromolecular component of most starches. The closely related a-amylose forms a red adduct with iodine. This reaction is not easily reversible and is thus undesirable. In commercially available soluble starch, the alpha fraction has been removed to leave principally j8-amylose indicator solutions are easily prepared from this product. 

First described in the literature in 1814, the blue reaction of iodine with starch is the best-known example of a polymerization process induced by supramolecular stabilization. Structural studies indicate that amylose (the linear fraction of starch) forms a helix with six glucose residues per turn to include guest iodine molecules. Inside this helix, iodine molecules form a polymeric chain with a periodicity of 3.1 A, which is much shorter than the nonbonded distance between iodine atoms (4.3 A) but greater than the single I-I bond distance (2.7 A). The amylose-iodine reaction is widely used in analytical chemistry and was utilized for the solubilization and purification of carbon nanotubes. 

An important test for the presence of starch is the reaction that occnrs between iodine, I2, and the coiled form of amylose. The product of the reaction is deep bine in color (see I Fignre 7.16) and is thonght to consist of the amylose helix filled with iodine molecules (see Figure 7.15B). This same iodine reaction is also widely used to monitor the hydrolysis of starch. The color gradnally fades and finally disappears, as sfarch is hydrolyzed by ei-fher acid or enzymes to form dextrins (smaller polysaccharides), fhen maltose, and finally glucose. The disappearance of the blue iodine color is thought to be the result of the breakdown of the starch helix. 

Fig. 43. Amylose. The molecular shape of amylose explains some of its chemical and biochemical properties. With iodine, amylose turns intensely blue. This venerable reaction has been explained only recently. It was found that iodine molecules enter the hollow center of the coil created by the glucose units. In such enclosure compounds iodine exhibits changed physical characteristics, such as the strong absorption of light. There are enzymes that slit open this tube and free fragments, each of which contains six glucose units (Section 6).

The chromatogram zones colored by iodine can be fixed later by treatment with a 0.5-1 Vo aqueous starch (amylose) solution. This yields the well known, deep blue iodine-starch inclusion complex which is stable over a prolonged period. This reaction... 

Amylose consists of unbranched al 4-linked chains of 200-300 glucose residues. Due the a configuration at C-1, these chains form a helix with 6-8 residues per turn (1). The blue coloring that soluble starch takes on when iodine is added (the iodine-starch reaction ) is caused by the presence of these helices—the iodine atoms form chains inside the amylose helix, and in this largely non-aqueous environment take on a deep blue color. Highly branched polysaccharides turn brown or reddishbrown in the presence of iodine. 

The I2-starch reaction is the basis of a spectrophotometric assay at 640 nm with amylose standards. A similar Congo red method is less precise than the iodine method, but it has the advantage of insensitivity to the DP, molecular size, and shape. The Congo red assay can therefore be supple-... 

The helical structure of amylose also serves as the basis for an interesting and useful reaction. The inside of the helix is just the right size and polarity to accept an iodine (I2) molecule. When iodine is lodged within this helix, a deep blue starch-iodine complex results (Figure 23-19). This is the basis of the starch-iodide test for oxidizers. The material to be tested is added to an aqueous solution of amylose and potassium iodide. If the material is an oxidizer, some of the iodide (I-) is oxidized to iodine (I2), which forms the blue complex with amylose. 

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