Amylose iodine reaction

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. 

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. 

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. 

It was realized long ago that a number of organic compounds can obscure the blue reaction.97 This effect is caused by the formation of inclusion complexes by such compounds. Hence, the iodine reaction is widely used as the test for complexation of starch, mainly amylose, with many organic... 

Two methods were used one is the iodine method that was used to determine dextrinization or the ratio of hydrolysis of the starch, and the other is the phenolphifaalein method lhat was used to determine CD formation. Starch-dextrinizing activity was determined in accordance with Fuwa (19) and Pongasawasdi and Yagisawa (20) with slight modifications. The reaction mixture containing 100 (iL of diluted enzyme aliquot and 300 pL of 0.5% soluble starch prepared in 0.1 M acetate buffer, pH 5.5, was incubated at 55 °C for 10 min. The enzyme reaction was stopped by the addition of 4.0 mL of 0.2 M HCl solution. Then, 0.5 mL of iodine solution (0.3 g/L I2 and 3.0 g/L KI) was added to form an amylose-iodine complex with residual amylose. The final volume was adjusted to 10 mL with distilled water. The absorbance of the blue color of the amylose-iodine complex was measured by spectrophotometer at 700 nm, and a decrease in absorbance was verified, when compared to a control tube with heat-inactivated enzyme. One unit of enzyme activity was defined as the quantity of enzyme that reduces the blue color of the starch-iodine complex by 10% per minute. 

A second form of photo-degradation was postulated in the absence of oxygen it was studied by performing experiments in an atmosphere of nitrogen. The reaction is slow and leads to a decrease in the intensity of the amylose-iodine stain and to an increase in the reducing power. 

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. 

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 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... 

The reaction between starch and iodine (or iodine-iodide mixtures) to form an inclusion compound was first reported in 1814 by Colin and de Claubry 131) and has since become familiar to all chemists through its applications in analytical chemistry. Its deep blue colour (kmax 620 nm) has been known for years to result from a linear arrangement of polyiodide within a canal formed by a helical coil of amylose. The helical amylose structure will trap other molecules 132,1331 and other hosts will stabilise polyatomic iodide guests134> 135). 

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 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. 

Starch consists of two main components amylose (insoluble in cold water) and amylopectin (soluble in cold water). Amylose, which accounts for about 20 per cent by weight of starch, has an average molecular weight of over 10. It is a polymer of glucopyranose units linked together through a l,4 -linkages in a linear chain. Hydrolysis of amylose produces maltose. Amylose and iodine form a colour complex, which is blue/black. This is the colour reaction of iodine in starch, a confirmatory test for the presence of starch. 

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. ... 

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. 

The presence and content of phosphoric acid moieties in starch also does not affect the course of reaction.66 78 For several types of amylose, the iodine affinity varies between 18.5 and 20% in a 0.05 M solution of iodine.79... 

A method of determining airborne iodine has also been reported.241 Here, iodine is absorbed into 5% aqueous KI and spectrophotometrically determined at 590 nm in the form of its complex with starch. This method is selective with respect to bromine and chlorine, and the sensitivity of this method is 0.25 mg of I2 per m3 of air. The concentration of the, 31I isotope in water can be determined by a method involving isotope exchange in the starch-iodine complex.242 Flow-injection determination of ascorbic acid (0.1-40 mg/mL) has been proposed.243 Iodine is generated in the flow system as I3- ions, which are in turn exposed to starch to produce a steady signal at 350 and 580 nm. Ascorbic acid provides inversed maxima which are measured. This method is recommended for analysis of ascorbic acid in fruit juice, jam, and vitamin-C preparations. Use of the blue complex has also been reported for determination of sodium dichloro-isocyanurate in air.244 Obviously the blue reaction is applicable in the determination of amylose, amylopectin, and starch,245-252 as well as modified starches.245,253-255... 

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. 

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. 

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