Polysaccharides iodine interaction

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. 

It has been almost two centuries since the first reports on the interaction between iodine and starch to produce a blue color, and more than fifty years since the discovery of the complexation between aliphatic alcohols and starch that revealed this polysaccharide to have a linear and a branched component. A large and diffuse body of published work exists on the interaction of starch and its components with other molecules and with ionic substances. Two closely related chapters in this volume, by Tomasik (Cracow, Poland) and Schilling (Ames, Iowa), provide comprehensive accounts of the extensive literature on complexes of starch with inorganic and with organic guests. 

Some dyes have a specific interaction with certain polysaccharides e.g. Congo red interacts with (l->4) P-D, (l->3) P-D and (l->3)(l->4) p-D glucans and (l->4) p-D xylans (11, 12). Upon flooding the plate with the dye-solution, undegraded polysaccharide will be stained by the dye whereas degradation zones will remain uncoloured. Congo red was also applied directly in the medium with CMC (13). Another classical example is the blue coloured complex that is formed between intact amylose and Lugol s iodine solution. 

Some information also exists concerning the energetics of the coraplexation reaction. In particular, the cooperativlty of the iodine binding process has been accepted for many years (22, 23). Direct charge transfer interaction between iodine and the oxygen atoms of the polysaccharide annulus has been proposed (, 25),... 

Amylose is a unique polysaccharide which forms a helical blue-coloured complex with iodine (I5). It also forms helical complexes with a variety of organic compounds such as 1-butanol, 1-pentanol, cyclohexanol, SDS etc. The interaction of SDS, a competing ligand with iodine (I ) complexes of amylose and amylopectin is studied spectrophotometrically. It is observed that the reduction in absorbance at 640 nm accompanied by the blue shift (640-570 nm) in the absorption spectrum is governed by the sequence of addition of the reagents, implying that this Interaction is closely associated with the coil—>hellx transition of the polymer chain. Perturbation of this complex with sodium thiosulphate and urea has revealed that the transition from helix—>coil is rather sluggish and hydrophobic interactions play an important role in the stability of this complex. 

You might have used iodine to detect the presence of starch. The iodine molecule (1 and other forms such as 13 and I ") can fit in the helix of amylose. This complex (of iodine and starch) exhibits the characteristic deep purplish blue color. [The website of Dr. S. Immel of Darmstadt Technical University (http //caramel.oc.chemie. tu-darmstadt.de/ lemmi/graphics/polysaccharides.html) gives a variety of pictures of models of cellulose, amylose, and others as well as the interaction between them and water molecules and iodine]. 

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