Amylose, complex with iodine, structure

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

Our earlier studies on the ligand Induced structural changes in amylose partially complexed with iodine have shown that competing ligand produces a... 

In both starch and glycogen the glucose emits of the main chains are linked with a-1,4 linkages. An extended conformation is not possible and the chains tend to undergo helical coiling. One of the first helical structures of a biopolymer to be discovered (in 1943)76 77 was the left-handed helix of amylose wound around molecules of pentaiodide (I5 ) in the well-known blue starch-iodine complex78 (Fig. 4-8). Tire 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.79... 

Structure of the helical complex of amylose with iodine (I2). The amylose forms a left-handed helix with six glucosyl residues per turn and a pitch of 0.8 nm. The iodine molecules (I2) fit inside the helix parallel to its long axis. 

Structural studies of amylose have, in turn, revealed a wide range of crystalline polymorphy, both in chain conformation and in crystalline packing. An example is the group of V-amyloses that exist as complexes with small organic molecules, water, or iodine. The latter complex is particularly interesting because it displays an intense blue color. The V-amyloses can be prepared by precipitation or drying from solution, and they crystallize readily. Consequently, their crystal structures are of interest in connection with any regenerated form of starch material. 

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. ... 

Using results of these kinds of studies, the characteristic structure of amylose can be differentiated from that of amylopectin. Amylose has a small number of branches and crystallizes and precipitates when complexed with 1 -butanol. The iodine affinity of amylose is much greater (i.a. 18.5 to 21.1) than that of amylopectin (i.a. 0.0 to 6.6),79,152-158,163,169-174 and the iodine affinity of amylose (3-limit dextrin is similar to that of the parent amylose. The average chain length of amylose (3-limit dextrins is much larger than that of the amylopectin (3-limit dextrin.160... 

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 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. 

Studies carried out in solution provide strong evidence that the helical amylose-iodine complex also exists in such circumstances.11014 1141 The structure of the iodine-amylose complex has also received theoretical treatment,142 and a three-dimensional free-electron model was developed for quantum mechanical calculations. Ultraviolet and visible spectra simulated in this manner reached close agreement with experiment observations. 

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. 

Solid-state cellulose can also be noncrystalline, sometimes called amorphous. Intermediate situations are also likely to be important but not well characterized. One example, nematic ordered cellulose has been described [230]. In most treatments that produce amorphous cellulose, the whole fiber is severely degraded. For example, decrystallization can be effected by ball milling, which leaves the cellulose as a fine dust. In this case, some crystalline structure can be recreated by placing the sample in a humid environment. Another approach uses phosphoric acid, which can dissolve the cellulose. Precipitation by dilution with water results in a material with very little crystallinity. There is some chance that the chain may adopt a different shape (a collapsed, sixfold helix) after phosphoric acid treatment. This was concluded because the cellulose stains blue with iodine (see Figure 5.12), similar to the sixfold amylose helix in the starch-iodine complex. 

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. 

Thus the blue inclusion complex becomes visible only when all the hydrogen sulfite has been consumed. According to studies by / . C. Teitelbaum, S. L. Ruby and T. ]. Marks the blue inclusion compound consists of the amylose component of the starch and the polyhalogenide anion Is", this was established by comparing the Raman and I Mofibauer spectra of the blue-black amylose-iodine complex with those of the adduct between trimesic acid hydrate and H Is , the structure of which is known (see figures). 

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 helical nature of the amylose-iodlne-complex, with six D-glucose units in the C-1 configuration per turn of the helix and the iodine molecules packed inside the lumen, parallel to the axis of the helix has been well established by X-ray diffraction studies (1 ). Electron microscopic studies of amylose-iodlne-complex in the form of fibrils have revealed rod like structures with 4o nm in diameter and the length depending on the degree of polymerization of the polymer chain, and the helices folded parallel to the long axis (2). The recent studies of this complex by Kaman resonance and iodine-1 29 H6ssbauer spectroscopy have provided evidence for the presence of IE species within the amylose helix (I-I—I —I-I) (3). 

With some of these polymers, the complexation of a single amylose helix with the polymer backbone can occur, which gives rise to supramolecular structures characterised by V-type crystallinity. This sort of modified amylose crystallinity is the same as mentioned in Section 8.4.4 and has been widely studied with small molecules such as alcohols, glycerol, dimethylsulfoxide, fatty acids or iodine [66]. 

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. 

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.]... 

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