Helical Structures of Amylose

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. 

Thus, randomly coiled amylose binds I3 ions, and this interaction is responsible for the helical structure of amylose. Helical amylose arrests iodine and further I3 anions, a process which is cooperative. If the filling of one helix commences, then the filling of another helix proceeds once the former helix is full. The longest available helix has priority in the uptake... 

Studies by Rundle et al., which were reviewed by Szejtli and Augustat,382 point to the helical structure of amylose in aqueous solutions. The dimensions of the amylose helix are well recognized, and therefore the structural similarity of the turns of the helix to cyclomaltohexaose (a-cyclodextrin) is sound. The a-cyclodextrin-water complex contains two guest molecules of water inside the cavity.383 Therefore, it may be accepted that part of the constitutional water of starch is included inside the amylose helix. Indeed,... 

The secondary structure of the polysaccharides range from the helical structure of amylose (Figure... 

The problem whether or not a helical structure of amylose is retained in solution is nearly as old as the discovery of the V-amylose helix from X-ray data in 1943 (7 ) and has been the subject of extensive investigation and controversy. (For review see ( )). At present mainly two models are considered the "extended helix chain" 9) and the "randomly coiled pseudohelical chain" (10). According to Senior eind Hamori (9) the amylose chain conformation is characterized by loose, extended helical regions, which are interrupted by short, disordered regions. Hydrogen bonds between 0(2) and 0(3 ) of neighboring residues are... 

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... 

Figure 5 Simulated stereoview of helical structures of amylose (top) and cellulose (bottom) (Reprinted with permission from Ref. 6.)...

FIGURE 16.35 Three-dimensional helical structure of amylose. 

Extensive studies have been performed on the (1- 6)-)8-D-glucan (pustulan) and the (l- 4)-a-D-glucan (amylose). These are linear polysaccharides that may exist as helical polymers in aqueous solution, as demonstrated by c.d. spectroscopy. Characteristic of the helical structure of these glucans is a negative band at 182 nm, a crossover at 177 nm, and a more intensely positive band at shorter wavelengths (see Figs. 8 and 9). 

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). 

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... 

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.

The helical structure of amy-lose makes it possible for two ends of amylose fragments to bind together via glycoside bonds, (b) Top view showing interior of each loop. 

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. 

The double-helical structures of native A- and B-amyloses are found in the fourth group. It is interesting that in both h as well as the d and dyg spacings, they are comparable with the structure of amylose triacetate I (ATAI). In part, this may arise because the packing of the bulky acetate substituents in ATAI is similar to the close-packing of two amylose chains into a double helix. In the latter, one chain may act as the "substituent" for the other chain. At any rate, all three structures contain similar, cylindrical-shaped helices. Somewhat unexpectedly, the distances cL and d-yo are very close for the two native polymorphs, even though their unit cells and packing are... 

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 is the other carbohydrate-based feedstock. Approximately 10 Mt is produced annually from corn (maize), wheat and potato, out of a total agricultural production of 1.6 Gt a-1 carbohydrate equivalents. A minor fraction of starch is amylose, a linear a 1 ->4 polymer of glucose (Fig. 8.2b). The native structure of amylose is helical loose random coils are formed upon dissolution in water. The branched glucose polymer amylopectin is the major (approximately 75%) component of starch. 

X-ray diffraction analysis754 of a series of amylose complexes with lower and higher fatty acids revealed that the crystal structures depend on whether amylose was complexed in the dry or wet state. Both the 6, and 7i helical conformations of amylose were found in these complexes. The conformation appears to depend on the length of the hydrophobic moiety. Dry amylose forms crystalline complexes with a unit cell identical to that of the anhydrous 1-butanol-starch complex (lattice parameters a = b = 25.6 A). An orthorhombic unit cell was proposed for the 7i -helical structure of the wet complexes of monobasic acids (acetic, butanoic, pentanoic, hexanoic,... 

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