Amyloses inclusion complexes with iodine

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. 

It has been shown that amylose forms inclusion complexes with iodine and many kinds of organic compounds in aqueous solution. Thus, the most probable model for the interaction of aunylose with TNS is an inclusion complex. Since cyclodextrins form inclusion complexes with TNS and enhance TNS fluorescence in aqueous solution ( ), it is interesting to compare fluorescence decay parameters for TNS-atmylose system with those for TNS-cyclodextrin systems. The values of lifetimes for amylose are the same as those for Y-cyclodextrin. However, the relative content of the long lifetime for amylose is much smaller than that for Y-cyclodextrin.Y -Cyclodextrin is expected to form both 1 1... 

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. 

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. 

As with iodine, starch forms an inclusion complex with bromine vapor.205 Depending on the starch variety, different colors are developed by the complex. Maize and wheat produce an ochre color, rice produces a light-buff color, potato and sago develop a pale-yellow color, and cassava forms a cream color.69 Iodine cyanide (and bromine)-amylose complexes are brown-black and dark brown, respectively.206 The adsorption of chlorine and iodine proceeds according to the Freundlich isotherm. A discontinuity on the Freundlich isotherm plot is reported, which possibly results from the swelling of starch granules454... 

The iodine-binding capacity of starch is dependent on the degree of polymerization (DP). Amylose forms with iodine a helical inclusion complex with an intense blue colour, which possesses an absorption maximum at wavelengths between 620 and 680 nm. Amylopectin has much less iodine-binding capacity because of its branched character, leading to a red-violet colour with absorption maximum of 540 nm 31. 

There seems little doubt that the dissolved complex is an inclusion compound in which iodine resides within the annular cavity of a more or less regular helical amylose chain. This picture emerged early from the work of Rundle and coworkers on the crystalline (10) and dissolved (11) complex and has not been seriously challenged by any more recent studies. Despite the acknowledged helical character of the dissolved complex, it is very clear from hydrodynamic evidence that the complexed polymer does not adopt a rigid, rod-like conformation indeed, the hydrodynamic volune of the polymer decreases upon complexation with iodine (2. 3, ) The spectroscopic properties of the dissolved complex imply that the bound iodine is arrayed in linear sequences, but neither the... 

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... 

An aqueous 1 % starch (amylose) spray can be sprayed on later to intensify the color contrast between the chromatogram zones and the layer background the well known blue-colored iodine starch inclusion complexes are formed. This later treatment with starch solution should only be carried out when the iodine excess has evaporated from the layer background so that only traces of iodine remain in the chromatogram zones. Otherwise the whole chromatogram will be colored dark blue (test at a corner of the chromatogram ). 

It should be noted that the different structures of amylose and amylopectin confer distinctive properties to these polysaccharides (Table II). The linear nature of amylose is responsible for its ability to form complexes with fatty acids, low-molecular-weight alcohols, and iodine these complexes are called clathrates or helical inclusion compounds. This property is the basis for the separation of amylose from amylopectin when starch is solubilized with alkali or with dimethylsulfoxide, amylose can be precipitated by adding 1-butanol and amylopectin remains in solution. 

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... 

Thus the blue inclusion complex becomes visible only when all the hydrogen sulfite has been consumed. According to studies by / . C. Teitelbaum, S. L. Ruby and T. ]. Marks the blue inclusion compound consists of the amylose component of the starch and the polyhalogenide anion Is", this was established by comparing the Raman and I Mofibauer spectra of the blue-black amylose-iodine complex with those of the adduct between trimesic acid hydrate and H Is , the structure of which is known (see figures). 

In its primary structure, the AGU are existing in the conformation c (chair conformation). The valence angles between the AGU are favoring a helical conformation, formed by 6-8 AGU, as the energetically most suitable state. The normal state in solution is that of a disturbed helix. An ideal stable helix conformation is formed and stabilized when hydrophobic molecules (iodine, aliphatic alcohols and acids) are allowed to penetrate into the molecular channel. The formation of such inclusion complexes is a typical property of a. and may be compared best with the inclusion behavior of - cyclodextrin. Insoluble complexes with organic solvents are used to precipitate amylose from starch solutions during fractionation. 

Beginning with polyvinyl alcohols and PVP we found that more than 20 hydrophilic polymers formed colored inclusion or clavic compounds with I3 in aqueous solution " . Rates of formation were studied and promotion by salts and lower temperatures was discovered. Doubt was cast upon the old theory that amylose, cellulose, and polyvinyl alcohols (PVA) require helical conformations for the insertion of iodine. While studying PVA-iodine complexes a novel method of making PVA fibers from foams was discovered . 

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