Iodine amylopectin

Mokhnach and Rusakova11 have shown that amylose-iodine and also starch-iodine complexes absorb at 226, 288-290, 344-360, and 585-620 nm. The first band appears only when iodine is added to the solution simultaneously with KI, and it is absent when only iodine is added. The longest-wavelength absorption may be indicative of the molecular size of the carbohydrate portion of the complex, as demonstrated106 in Table VIII. Spectra of iodine-amylopectin complexes were measured by Archibald et al.146 The UV absorption spectrum of the starch-iodine complex is shown... 

Off-line iodine complexing is a frequently used method to characterize the SEC fractions. Amylose-iodine and iodine-amylopectin complex have a maximum absorbance at about 658 and 546 nm, respectively. Recently, Suortti and Pessa (63) demonstrated that is is possible to do iodine complexing detection on-line, such as that shown in Figure 4. Photodiode array (PDA) ultraviolet (UV) detectors are readily available now. PDA can take the UV spectrum of the eluant on the fly. The information is stored in the computer. After the run, a contour plot or three-dimensional plot can be shown on the screen or printed out. Iodine complexing with PDA detection can be a very useful technique for characterizing starch samples. 

Amylopectin differs from amylose in that its retr< adation from solution is slow. The setting of starch gels (cornstarch puddings) and the staling of bread are, however, due to amylopectin separation. With iodine amylopectin gives a purple to red color, and it may be distingui ed from amylose in a potentiometric titration with iodine, since it does not form a complex. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

Amylose [9005-82-7] (C Hio05) (/br use in iodine complex formation). Amylopectin was removed from impure amylose by dispersing in aqueous 15% pyridine at 80-90 (concn 0.6-0.7%) and leaving the soln stand at 44-45° for 7 days. The ppte was re-dispersed and recrystd during 5 days. After a further dispersion in 15% pyridine, it was cooled to 45°, allowed to stand at this temperature for 12hours, then eooled... 

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 obviously important that the fractionation products should be adequately characterized. The only accurate method for ascertaining the purity of the starch components, and also the amylose/amylopectin ratio in whole starch, is to determine potentiometrically the amount of iodine bound.8 38 Colorimetric methods which have been suggested37 38 are useful for comparative measurements, but are often not absolute. The yield of... 

Tapioca and maize amylopectins have been sub-fractionated by fractional precipitation from aqueous solution with increasing amounts of methanol,64 71 and potato amylopectin by preferential precipitation on electrodialysis of the iodine complex.72 When these three amylopectins were subjected to chromatography, and eluted with a neutral buffer, all were found to consist of several sub-fractions.70... 

It has not yet been possible to obtain samples of amylopectin which do not show some slight evidence of uptake of iodine by linear material in the early stages of an accurate potentiometric titration. Although this effect is presumably due to contaminating amylose, the presence of some long branches in the amylopectin cannot be excluded. Anderson and Greenwood190 have shown that in 0.01 M iodide solution, for concentrations of total free iodine less than 1 X 10-6 M, the amount of iodine bound by... 

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... 

Glycogen, animal starch, is similar to amylopectin, but it features more branching and tends to have a higher molecular weight. Glycogen occurs in the liver and muscle tissue. It interacts with iodine to produce a red color. 

The dependence of the mobilities of amylopectin and amylose on iodine concentration in the background electrolyte and applied temperature was studied by Brewster et al. (111). The method was used for the separation and identification of different plant starches, but no binding constants were calculated. 

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 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. 

Amylose, although water soluble, gives an unstable solution which irreversibly precipitates. It is mainly responsible for the deep blue coloration given by starch and iodine. Solutions of amylopectin are relatively stable. The iodine-binding capacity, on the other hand, is very low. A small amount of covalently bound phosphate normally appears with starch but its exact location within the molecule is not known. 

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.

The glucosidase activity alone was measured by 14C-glucose incorporation into polymer (16) and the transferase activity alone was measured by the change in the iodine spectrum of amylopectin (20). 

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