Inclusion complexes with iodine

The molecular uniformity of constituting components of a nb/lcb glucan fraction of potato starch was investigated with Sepharose CL 2B (Fig. 16.16) as well as with Sephacryl S-1000 (Fig. 16.17). Therefore, each of the subsequently eluted 3-ml fractions was analyzed on their potential to form inclusion complexes with iodine, a sensitive test for the presence of nb/lcb glucans. Results are shown in Fig. 16.17 in terms of branching index, the ratio of extinction of pure iodine solution and of nb/lcb glucan/iodine complex the higher the index, the more pronounced the nb/lcb characteristics. 

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. 

Ed. 2000, 77,178 D. Diaz, I. Vargas-Baca, and J. Graci-Mora, P-Cyclodextrin Inclusion Complexes with Iodine, J. Chem. Ed. 1994, 71, 708. 

CDs can form inclusion complexes with iodine, which makes them candidates for iodine-sorption from nuclear waste gases. In model experiments, it was shown that the aqueous solutions containing CDs and crosslinked CD polymers were selective and effective iodine absorbers [86]. Especially the cr-CD derivatives (methylated and crosslinked) have high sorption capacity. On the basis of the results, the binding of elemental and organic iodine emitted into the air by chemical and nuclear power plants can be made effectively by immobilizing iodine vapor in aqueous CD solutions or in CD polymer gel beds. Such new sorbents can be employed in the air filtration systems (Table 8.4). 

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. 

The s-triazines undergo chlorination at nitrogen to yield reactive N-chloro derivatives which oxidize iodide to iodine in the second step. This then forms an intense blue iodine-starch inclusion complex with starch. 

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... 

Chen, S. and Wang, Y. 2001. Study on P-cyclodextrin grafting with chitosan and slow release of its inclusion complex with radioactive iodine. J. Appl. Polym. Sci. 82 2414-2421. 

The resulting Br3 /a-CD complex is subject to UV detection at 265 nm. Using this system, the concentration of bromate in a bottled water sample was determined at 6ngl and the calibration had a linear-regression coefficient of 0.996. A similar method was also used to determine trace amounts of separated iodate and nitrite ions in drinking water. Iodate and nitrite in their individual chromatographic peaks form iodine when HI is introduced, after which iodine reacts with excess 1 to form 13 , which in turn forms an inclusion complex with CD. In this process (7-10), CD shifts the equilibrium in (9) to the right, the total concentration of triiodide ion is increased, and therefore, the sensitivity of detection is improved. 

Primary and secondary amines and amides are first chlorinated at nitrogen by the chlorine released by the gradually decomposing calcium hypochlorite. Excess chlorine gas is then selectively reduced in the TLC layer by gaseous formaldehyde. The reactive chloramines produced in the chromatogram zones then oxidize iodide to iodine, which reacts with the starch to yield an intense blue iodine-starch inclusion 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... 

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 ). 

Substances containing active chlorine or bromine oxidize iodide ions — if necessary under the influence of UV light - to iodine, which reacts with starch to yield the well-known intense blue starch-iodine inclusion complex. 

FtG. 5. The crystallographic structure of the inclusion complex of a-cyclodextrin with iodine (15). 

Usually, iodine is not detected as its usual brown form but as a blue inclusion complex (cage complex) with starch. The sulfite method, besides being labor-intensive, has the disadvantage of yielding accurate data in cell-free systems only, as living cells interfere with the processes in Eqs. (8.36)—(8.38) necessary for the analysis. 

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... 

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