Amylose iodide

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. 

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. 

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

The reaction between starch and iodine (or iodine-iodide mixtures) to form an inclusion compound was first reported in 1814 by Colin and de Claubry 131) and has since become familiar to all chemists through its applications in analytical chemistry. Its deep blue colour (kmax 620 nm) has been known for years to result from a linear arrangement of polyiodide within a canal formed by a helical coil of amylose. The helical amylose structure will trap other molecules 132,1331 and other hosts will stabilise polyatomic iodide guests134> 135). 

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. 

It is now realised that amylose-polyiodide compounds can also involved Va- and Vh-forms 155 . Recent studies on the Vh-compound 154) have shown that the early structural ideas described above are essentially correct but that rather than P2l2i2i the monoclinic space group P2X is applicable. An almost linear arrangement of iodine atoms was present in the helical canals but the length of the iodide chain could not be determined. In addition to this guest, eight water molecules were present in the unit cell and once again these were present in the interstitial sites (see Fig. 14). 

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... 

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. 

For simple, linear polymers such as amylose, the positions of the glycosidic bonds are determined by treating the intact polysaccharide with methyl iodide in a strongly basic medium to convert all free hydroxyls to acid-stable methyl ethers, then hydrolyzing the methylated polysaccharide in acid. The only free hydroxyls present in the monosaccharide derivatives so produced are those that were involved in glycosidic bonds. To de-... 

Figure 16-6 (a) Schematic structure of the starch-iodine complex. The amylose chain forms a helix around l6 units. [Adapted from A T. Calabrese and A. Khan, "Amylose-lodine Complex Formation with Kl Evidence for Absence of Iodide Ions Within the Complex." J. Polymer Sci. 1999, A37,2711.] (fc>) View down the starch helix. Showing iodine inside the helix.8 [Figure kindly provided by R. D. Hancock, [rower Engineering, Sett Lake City.]... 

Figure 4-8 (A) Structure of the helical complex of amylose with I3 or I5. The iodide complex is located in the interior of the helix having six glucose residues per turn. (B) Model of a parallel-stranded double helix. There are six glucose units per turn of each strand. The repeat period measured from the model is 0.35 nm per glucose unit. Courtesy of Alfred French.

Senti and Witnauer1 have provided the only information yet available on the stoichiometry of formation of polysaccharide adducts. Their studies of addition compounds of amylose in aqueous ethanolic media showed that the combining ratio of D-glucose residue to salt is a function of salt concentration, and that the minimum ratios are approached as the salt concentration is increased. Beyond a certain concentration of salt, the ratio becomes almost constant. The anion plays an important role in determining the magnitude of the minimum ratio for an amylose adduct. Potassium bromide and potassium iodide give adducts of minimum ratio 2 1, whereas potassium acetate and potassium propionate give 1 1 adducts. A study of the composition of the potassium acetate adduct as a function of salt concentration indicated that two, relatively stable adducts are formed, the 1 1 and the 2 1. 

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 location of the metal cation (or cations) in alcoholates has been investigated by the method of substitutive methylation for the sodium derivatives of methyl a-D-glucopyranoside, amylose, and cellulose. The method assumes that methylation by either dimethyl sulfate or methyl iodide occurs only at an anionic oxygen atom. Lenziu methylated the mono-sodium alcoholates (prepared in boiling, butanolic sodium hydroxide by the... 

M s in DMSO-water in the presence and absence of amylose. Thus quenching is reduced about 30-fold for iodide by amylose incorporation of the stilbene chromophore. While it is somewhat uncertain as to what precisely the nature of the quenching of stilbene by iodide is, it is reasonable to assume that the reduced quenching constants imply a more difficult approach of the iodide ion to the complexed stilbene than to the free. We are currently exploring many aspects of reactivity of amylose-incorporated chromophores. We find for example that amylose is able to extract totally insoluble hydrophobic stilbene molecules into water and we are presently trying to obtain crystal structural data on the complex molecule. The dynamics of complex formation and dissociation are currently under investigation. 

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]. 

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... 

According to a recent article, complexation of strictly anhydrous amylose (which avoids the formation of an iodide anion) would give a complex of molecular iodine (Murdoch 1992). 

The parallel between the cyloamyloses and V-amyloses helped to explain the well-known blue colour of starch-iodine complexes a series of intensely coloured complexes of cyclohexaamylose, iodine and metal iodides had chains of iodine atoms in the central cavity of stacks of cyclohexaamyloses. The blue colour was thought to arise from charge transfer interactions amongst the I2 molecules and the linear and ions trapped in the central cavity. 

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