Iodine color reactions with polysaccharides

The seeds of Tamarindus indica contain polysaccharides which can be extracted with hot water and purified by precipitation with alcohol. The polysaccharide is composed of D-galactose, D-xylose, and D-glucose (Fig. 10). These xyioglucans have been named amyloids since they give a color reaction with iodine similar to starch. The xyioglucans from Tamarindus are gel-forming polysaccharides which are stable in a wide pH range. This type of polysaccharide is mainly used in food industry, but, in some cases, it has been pharmaceutically employed as a substitute for pectin. 

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. 

Relationship between the Chain Length of Synthetic Polysaccharides, the Color of Their Reaction with Iodine, and the Position of the Absorption Peak in the Visible Spectra of Relevant Complexes145... 

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. 

A solution of 0.2-0.5 gm iodine in 100 ml aqueous 0.5% potassium iodide is often used as a reagent for certain polysaccharides in lichens (the I test). The reagent is susceptible to air oxidation and should be renewed when the brownish color fades. Isolichenin, but not lichenin, will give a blue color. The chemistry of the color reaction is probably the same as that for the well-known iodine test for starch. The reaction is reversible (the color disappears upon dilution with water). 

---