A- Amylose

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. cx-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary stmcture. 

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. 

KOH-amylose complex, 346-347 A-Amylose, 340-342, 407-408 B-Amylose, 342-344, 409 V-Amylose, 345-346, 410 Analytical chemistry, 18 Anhydrides... 

Figure 13-12. Structureofstarch. A Amylose, showing helical coil structure. B Amylopectin, showing 1 - 6 branch point.

Fig. 15. Projection view of A-amylose on the a, b plane showing the guest water molecules (black dots) situated in the interstitial sites between helical double strands of amylose, which are represented diagrammatically as rings. The arrows illustrate the anti-parallel nature of the packing...

The crystal structure of A-amylose is made up of parallel-stranded, ng/tr-handed double helices which pack in an antiparallel manner to give an orthorhombic unit celll67). About eight water molecules are present per unit cell and these guests are present in the interstitial sites (Fig. 15). 

It is now more or less believed that the iodine is held as an absorption complex within the helical chain of the macromolecule f3-amylose i.e., a component of most starches. However, another component, a-amylose, is undesirable because it produces a red-colouration with iodine which is not readily reversible, and... 

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. 

Which is more apt to form a helix (a) amylose or (b) amylopectin ... 

This can be accomplished by more drastic chemical treatments where degradation also takes place and untwining occurs of the short segments (40). There is no support at present for these chains to re-intertwine (40). An untwining is envisaged by Sarko (16,41 ) in the case of A-amylose to V-amylose. 

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. ... 

Figure 5.2 Schematic diagram of (a) amylose and (b) amylopectin with a branch point at the 06 position.

Figure 5.4 X-ray powder diffractogram recorded for (a) A-type amylodextrins and (b) B-type amylodextrins grown as spherulites. X-ray fiber diffraction patterns (fiber axis vertical) for (c) A-amylose (fiber spacing 1.04 nm) and (d) B-amylose (fiber spacing 1.05 nm). (Reproduced with permission from references 30 and 31). Microcrystal of (e) A-starch and (f) B-starch observed by low dose electron microscopy. Inset the electron diffraction diagrams recorded under frozen wet conditions (e). (Reproduced with permission from references 32 and 34)...

Figure 4-19 Schematic Representation of the Action of Starch-Degrading Enzymes. (A) Amylose and amylopectin, (B) action of a-amylase on amylose and amylopectin, (C) action of a debranching enzyme on amylose and amylopectin, (D) action of amyloglucosidase and debranching enzyme on amylose and amylopectin. Source Reprinted from H.S. Olsen, Enzymic Production of Glucose Syrups, in Handbook of Starch Hydrolysis Products and Their Derivatives, M.W. Kearsley and S.Z. Dziedzic, eds., p. 36, 1995, Aspen Publishers, Inc.

If a little is good, a lot is better. a-Amylose is an unbranched glucose polymer. Why would this polymer not be as effective a storage form for glucose as glycogen ... 

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