Amylose Clathrates

Iodide and iodate ions react under the influence of protons to yield iodine molecules which react with amylose to yield a blue clathrate complex ... 

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. 

Starch-guest molecule compounds in inclusion complexes are usually nonstoichiometric species. On the other hand, in the case of amylose such compounds are stoichiometric, but their composition is not repeatable. Starch complexes may consist of partly physical mixtures, adsorbates, and true inclusion complexes formed by direct involvement of dipolar interactions, host-guest hydrogen bonds, and/or clathration-like interactions within the starch matrix. 

The intensely blue color is due to a charge-transfer complex formed by the triiodide ion and starch. It results from the inclusion of tri-iodide ions into the straight-chain fractions of a-amylose of starch, which form a helix. The formed clathrate exhibits a narrow band of charge transfer near 620 nm (Fig. 18.2). 

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