Amylose iodide solutions

It has not yet been possible to obtain samples of amylopectin which do not show some slight evidence of uptake of iodine by linear material in the early stages of an accurate potentiometric titration. Although this effect is presumably due to contaminating amylose, the presence of some long branches in the amylopectin cannot be excluded. Anderson and Greenwood190 have shown that in 0.01 M iodide solution, for concentrations of total free iodine less than 1 X 10-6 M, the amount of iodine bound by... 

Cellulose, starch, and their derivatives are commonly used as chromatographic stationary phases. They are, in principle, potential hosts for inducing CD activity in small molecules and could be used with effect for analysis in homogeneous media with chiroptical detection. An example might be the starch (amylose)-iodide complex [86]. Low aqueous solubility however is an obstacle to their general use in homogeneous solutions. Linear oligomers of maltose are more soluble than starch and could theoretically be used as alternatives to Cy, however they do not really compete in terms of the stability of the association complexes. 

Free iodine reacts with starch solution to give the blue-black amylose-iodine complex, which contains iodine in the form of an Ij" chain. The aiiiylopecliii also present in starch reacts with iodine to give a brown violet color. In concen Hated aqueous alkaline iodide solutions and in alcoholic solution iodine has an intense brown color at high dilutions this color changes to yellow because of... 

Amylose. A component (20-30%) of starch surrounded by amylopectin. A. is a linear a-l,4-glucan, Mr 50000-200000 (see figure at starch). Crystalline A. occurs in various polymorphic forms (A, B, C, and V-A.), that differ in conformation and crystal packing. A. is soluble in water and gives the characteristic blue color with iodine-potassium iodide solution (Lugol s solution) (formation of inclusion compounds, traces of iodide ions are necessary for occurrence of the blue color, formation of I5 ions I -1 I I -1). Because of its predominately unbranched structure, A. can be degraded to oligosaccharides both by a- and by /S-amylase. The screw-like (helical) conformation also allows the formation of inclusion compounds with alcohols. 

Starch is traditionally identified by the intense blue colour produced when amylose is reacted with an iodide solution. The iodide forms a complex with a surrounding amylose single hehx, which amylopectin cannot form. This reaction works only with soluble amylose so retrograded amylose will react weakly, if at aU. 

Figure 6.4. Conformational structures for amylose complexes A, random coil helix of amylose in solution B, amylose iodine-iodide complex C, lamellar structure for organic molecule-amylose complex.

The enzymatic polymerization of G-l-P using 6 and 9 as primers was performed to produce the two types of amylose-grafted polyacetylenes (7) and (10) (Schemes 5a and b). The complex formation with iodine is a well-known characteristic property of amylose [23]. The colorless solutions of 7 and 10 in DMSO turned to violet after the addition of a standard iodine-iodide solution to the polymer solution, as the same color change in the complex formation of amylose with iodine. Values of the A-max of UV-Vis spectra of the iodine... 

This helical arrangement of amylose, known as the V-form, may be precipitated from certain solutions (e.g. in butanol or DMSO) of amylose. Either hydrated (Vh-) or anhydrous (Va-) amylose absorbs I2 vapour to produce the blue compound with the necessary I" being produced in situ. Alternatively the compound may be formed from iodine-iodide mixtures in solution which allows the V-form to be produced and stabilised as the polyiodide compound 141 The compound was reported 142) to have the orthorhombic space group 7>212121. 

The reactions of potassium iodide in aqueous solutions are those of iodide ion, r. In iodometric titration I combines with iodine to form triiodide ion, I3. The latter adds to (i-amylose fraction of the starch to form a blue complex. 

Treatment of an amylose-alkali metal hydroxide adduct with a concentrated solution of an inorganic salt (such as potassium iodide or potassium acetate) in aqueous ethanol can result in the total displacement of hydroxide ion.28,87 This displacement indicates the existence of an equilibrium... 

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. 

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. 

It is the amylose component of starch that gives the blue color when KI/I2 solution is added. To study the iodine-iodide color of amyloses of different d.p. values, maltodextrin-amylose molecules, with various avg. d.p. values from 6 to 568 were prepared by Bailey and Whelan [62], using phosphorylase, a-D-glucopyranosyl-1-phosphate, and maltohexaose. The colors of the various sized maltodextrins (1 mg) were observed when 10 1 (w/w) KI/I2 solution was added. The first color to be observed was faint red for avg. d.p. 12 a red-purple color was observed for avg. d.p. 31 a purple color was observed for avg. d.p. 40 and a blue color was observed for avg. d.p. 45. The increase in the blue value was linear as a function of avg. d.p. up to avg. d.p. 60 the absorbance at 645 nm then slowly increased and reached a maximum at avg. d.p. of 400. The intensity of the iodine/iodide color in the low molecular weight range was dependent on the concentration of the iodine. When the concentration of the iodine was increased 10-fold, the intensity was increased 50% [62]. 

A standard test for amylose is to add a solution containing potassium iodide and iodine. If amylose is present, the solution turns a deep blue. What is the explanation for this effect ... 

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