Amyloses with iodine

Structure of the helical complex of amylose with iodine (I2). The amylose forms a left-handed helix with six glucosyl residues per turn and a pitch of 0.8 nm. The iodine molecules (I2) fit inside the helix parallel to its long axis. 

The interactions of slightly substituted carboxymethyl- and diethylamino-ethyl-amyloses with iodine in the presence of iodide have been investigated spectrophotometrically as a function of iodide concentration, temperature, and polymeric charge. ... 

The enzymatic polymerization of G-l-P using 6 and 9 as primers was performed to produce the two types of amylose-grafted polyacetylenes (7) and (10) (Schemes 5a and b). The complex formation with iodine is a well-known characteristic property of amylose [23]. The colorless solutions of 7 and 10 in DMSO turned to violet after the addition of a standard iodine-iodide solution to the polymer solution, as the same color change in the complex formation of amylose with iodine. Values of the A-max of UV-Vis spectra of the iodine... 

Starches can be separated into two major components, amylose and amylopectin, which exist in different proportions in various plants. Amylose, which is a straight-chain compound and is abundant in potato starch, gives a blue colour with iodine and the chain assumes a spiral form. Amylopectin, which has a branched-chain structure, forms a red-purple product, probably by adsorption. 

Cultures of B. subtilis were introduced into the stems of young potato plants by Suit and Hibbert104 in an attempt to bring about replacement of starch by another polysaccharide. Sections of some of the resulting potatoes gave little or no color with iodine, and were provisionally designated starchless potatoes. However, based on analogy with recent developments in starch chemistry, it seems probable that the starchless potato was free from amylose, and contained only amylopectin. 

It is now more or less believed that the iodine is held as an absorption complex within the helical chain of the macromolecule f3-amylose i.e., a component of most starches. However, another component, a-amylose, is undesirable because it produces a red-colouration with iodine which is not readily reversible, and... 

Amylopectin Amylopectin is similar to amylose except that the glucose chain has branches. These branches involve linkages at the -CH2OH position ( 6), which makes them a 1 —> 6 linkages. Amylopectin is water-soluble it also interacts with iodine to form a reddish-purple complex. Typically, amylopectin is ten times the size of an amylose molecule. Digestion requires (3-amylase (1 4 linkages) and a second... 

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. 

Figure E2.3.1 Plot of absorbance at 600 nm against percentage amylose (w/w) for mixtures of potato amylose and amylopectin with iodine. The absorbance of 0% amylose is due to the l2 affinity of the long outer branches of amylopectin.

Largely because the 0-6 hydroxyls of all six residues of one helix turn are in equivalent positions in the V, V -iodine and vDMS0 structures gg in Vh and Vh-iodine, gt in VDMsq)> thus forming symmetric intramolecular hydrogen bonds, all residues in these structures are equivalent. However, in Va-amylose, with its mixture of 0-6 positions, molecular sixfold symmetry is not present in the helix and instead, a 2q screw axis along the helix axis exists, thus combining three residues of one half-turn into the asymmetric unit. Nonetheless, the helix backbone still resembles a six-fold helix. 

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. ... 

Starch can form an intense, brilliant, dark blue-, or violet-colored complex with iodine. The straight chain component of starch, the amylose, gives a blue color while the branched component, the amylopectin, yields a purple color. In the presence of iodine, the amylose forms helixes inside of which the iodine molecules assemble as long polyiodide chains. The helix-forming branches of amylopectin are much shorter than those of amylose. Therefore, the polyiodide chains are also much shorter in the amylopectin-iodine complex than in the amylose-iodine complex. The result is a different color (purple). When starch is hydrolyzed and broken down to small carbohydrate units, the iodine will not give a dark blue (or purple) color. The iodine test is used in this experiment to indicate the completion of the hydrolysis. 

The capacity of starch to stain blue-black with iodine suggests that some of the amylose is present in the starch in the V-form. The lipids present in cereal starch would bind to amylose if it were in the V-form, and yet X-ray analysis does not show the presence of the V-polymorph in cereal starches (i.e., most of the amylose would be in the amorphous form). The conclusion is that although a significant part of the amylose is probably in the helical form, the three-dimensional order necessary to give a crystalline diffraction pattern is absent. Indeed, the crystalline nature of starch is now attributed to the presence of amylopectin and not to amylose. Starch from waxy mutants contains only amylopectin (and no amylose), but this starch has the same degree of crystallinity and the same X-ray pattern as the regular starches that contain both components. 

Inclusion complexes of amylose are rather well defined, and a consistent theory of such complexes is available that explains amylose complexes with iodine, fatty acids, alcohols, and other guest molecules.4,5 This subject is surveyed in this article because of the growing interest and importance of such complexes in pharmacology and in the food industry. It is probable that starch in its biological sources (tubers, granules) exists in the form of native complexes with proteins, lipids, mineral salts, and water. 

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