Starch-iodine complex helical structure

The helical structure of amylose also serves as the basis for an interesting and useful reaction. The inside of the helix is just the right size and polarity to accept an iodine (I2) molecule. When iodine is lodged within this helix, a deep blue starch-iodine complex results (Figure 23-19). This is the basis of the starch-iodide test for oxidizers. The material to be tested is added to an aqueous solution of amylose and potassium iodide. If the material is an oxidizer, some of the iodide (I-) is oxidized to iodine (I2), which forms the blue complex with amylose. 

In both starch and glycogen the glucose rmits of the main chains are linked with a-1,4 linkages. An extended conformation is not possible and the chains tend to rmdergo helical coiling. One of the first helical stmctures of a biopolymer to be discovered (in 19437 was the left-handed helix of amylose wormd arormd molecules of pentaiodide (I5 ) in the well-known blue starch-iodine complex (Fig. 4-8). The helix contains six residues per turn, with a pitch of 0.8 nm and a diameter of nearly 14 nm. Amylose forms complexes of similar structure with many other small molecules. ... 

Starch is composed of macromolecular components, a-amylose and (i-aim -lose. The former reacts irreversibly with iodine to form a red adduct. (i-Aim losc. on the other hand, reacts with iodine forming a deep blue complex. Because this reaction is reversible, [3-amyl0sc is an excellent choice for the indicator. The undesired alpha fraction should be removed from the starch. The soluble starch that is commercially available, principally consists of (3-amylose. (3-Amylose is a polymer of thousands of glucose molecules. It has a helical structure into which iodine is incorporated as I5. ... 

Since several synthetic polymers also develop a blue color upon reaction with iodine, it is likely that they have a helical structure similar to that of amylose. Therefore it is probable that the aforementioned complexes of synthetic polymers with starch can exist in the form of a double helix. 

It has been known for almost 200 years that starch gives a deep blue color when a solution of potassium iodide and iodine is added [47]. More than a century later it was suggested that the complex consisted of a helical polysaccharide, with triiodide in the center of the helix [48]. Using flow dichroism, it was demonstrated that the triiodide was stacked in a linear structure, as required for the helical model [49]. Another study of the optical properties of crystals of the amylose-triiodide complex showed it to be consistent with a helical structure [50] and X-ray diffraction showed the triiodide complex gave the dimensions of a unit-cell of a helix with six glucose residues per turn [51]. This confirmed a helical structure for the amyiose complex with triiodide that predated the helical models proposed by Pauling for polypeptides [52] and the double helical model for DNA by Watson and Crick [53] by 10 years. 

When starch is fractionated into its two components, usually by precipitating the amylose from solution by means of an organic solvent (such as an alcohol), a third type of structure is found this survives drying, and ultimately reverts to the B structure upon rehydration. This structure has been termed the V form, and it yields an x-ray pattern that is distinctly different from the other two types. Essentially the same pattern was observed for the amylose-iodine complex. Bundle and coworkers studied the various V amyloses obtained by complexing with alcohols or iodine, and, on the basis of powder diagrams, suggested unit-cell parameters for both the wet and dry (hydrated and anhydrous) states, as shown in Table I (seep. 422). From these data, Bear had suggested earlier that the "V structure of amylose is helical. (Historically, it is of... 

The starch-iodine(iodide) complex has been known for centuries. The presence of iodide, iodine and a sufficient amount of water [58] is necessary for the formation of the deep blue complex. Bundle [59] studied its structure by X-ray diffraction, and his results suggest a sixfold symmetrical helical conformation. Starch forms helical complexes not only with triiodide but also with many organics such as butanol or fatty acids, and this property can be used to separate amylose, which forms the helical complex, from other polycarbohydrates (amilopectins) which do not. Without complexing agents the helical conformation of amylose, called amylose-V, is stable only in the crystalline state. The structural parameters of the amylose-iodine(triiodide) complex were determined by Saenger etal. [60,61] in experiments on several model compounds. They found that six monomer units form a turn of the... 

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. 

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. 

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