Amylose helix

Three potential helical structures of polysaccharides, (a) Single helix with twofold screw axis (cellulose) (b) double helix (amylose) and (c) triple helix (/ , 1-3 glucan) (according to Rees [4]). The particular structure depends largely on the constraints imposed by hydrogen-bonds and rotational freedoms around the glycosidic intermonomer bonds (i. e., O Fig. 3, energy barriers to rotation)... 

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Fig. 8. Amylose inclusion compounds scheme of the amylose helix showing the inclusion channel. The guest compound is shaded (58).

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. 

FIGURE 7.22 Suspensions of amylose in water adopt a helical conformation. Iodine (b) can insert into the middle of the amylose helix to give a bine color that is characteristic and diagnostic for starch. 

As a thickener (as opposed to a gel), it is amylose that has the main function. The long water-soluble chains increase the viscosity, which doesn t change much with temperature. Amylose chains tend to curl up into helixes (spirals) with the hydrophobic parts inside. This allows them to trap oils, fats, and aroma molecules inside the helix. 

Fic. 10.—Parallel packing arrangement of 6-fold, A-amylose (8) molecules, (a) A stereo side view of less than 2 turns of a pair of double helices 10.62 A (=al2) apart. The two strands in each helix are distinguished by open and filled bonds, and the helix axis is also drawn, for convenience. Note that atom 0-6 mediates both intra- and inter-double helix hydrogen bonds. 

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

Fig. 12.—Packing arrangement of shallow, 6-fold, V-amylose (10) helices, (a) Stereo view of two unit cells approximately normal to the fee-plane. The helix at the center (filled bonds) is antiparallel to the two helices at the comers in the back (open bonds). Intrachain hydrogen bonds (3-OH - - 0-2 and 6-OH 0-3) are shown in thin lines.

Lewis and Johnson compared the c.d. spectra of amylose and cyclomaltohexaose, and showed that amylose is helical in aqueous solution. Cyclomaltohexaose is chromophorically equivalent to amylose, and it is known to assume a pseudohelix having zero pitch, and thus, no helical chirality. The conformation of amylose is clearly different from that of cyclomaltohexaose, as their c.d. spectra are very different (see Fig. 9). The difference in conformation was considered to be a matter of helical chirality. To confirm this, these workers measured the c.d. spectrum of an amylose-1-butanol complex presumed to have the V-form of helical conformation with the 1-butanol complexed in the channel of the helix. The c.d. spectrum of the complex is identical to that of amylose in aqueous solution. 

Although the dipolar and resonating nature of the interaction of amylose and iodine is well established, Schlamowitz173 regards the iodine in a starch complex as being in a predominantly non-polar form, and Meyer and Bern-feld174 refute the helix theory and consider that adsorption of iodine occurs on colloidal micelles in amylose solutions. Most of the experimental facts which Meyer presents can, however, be satisfactorily explained on the helical model. 

The unit cell is tetragonal, with a = b = 10.7 A (1.07 nm) and c = 16.1 A (1.61 nm). The amylose helix is left-handed, with four D-glucose residues per turn. Both ions are located in a water-like environment. The atoms 0-2, 0-3, and 0-4 from D-glucose residues on adjacent chains coordinate around K+. The R factor is 41%. 

The space group is P2i2i2 . The unit cell is pseudotetragonal, with a = b = 19.17 A (1.917 nm), and c = 24.39 A (2.439 nm), with two antiparallel chains per cell. The amylose chain is a left-handed 6(—1.355) helix, with three turns per crystallographic repeat. One molecule of dimethyl sulfoxide for every three D-glucose residues is located inside the helix. An additional 4 molecules of dimethyl sulfoxide and 8 of water are located in the interstices. The interstitial dimethyl sulfoxide is the source of additional layer-lines that are not consistent with the 8.13 A (813 pm) amylose repeat. The overall R factor is 35%, and, for the layer lines with amylose contribution alone, it is 29%. 

From the positive Cotton effect observed in the amylose case and the negative Cotton effect in the SPG case, it is clear that the oligosilane adopts opposite screw sense helical conformations in the two cases.337 It now appears possible to associate an d/-screw sense helix with a positive sign Cotton effect (and vice versa), although other experimental confirmations of this are desirable. 

FIGURE 5.8 Unit cells (outlined in each diagram) and helix packing in A and B polymorphs of starch. Reprinted from Carbohydrate Research, Vol. 61, Wu and Sarko (1978b), The double helical molecular structure of crystalline A-amylose, Pages 27-40, with permission from Elsevier. 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

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