Amylose chain conformation

Figure 3. Projection into the xy plane of an arbitrary coordinate system of one particular amylose chain conformation consistent with the energy surface of Figure 2. Circles represent the glycosidic oxygens. These are connected by virtual bonds spanning the sugar residues for clarity the residues are not shown.

Figure 4. Projection into the yz plane of the same amylose chain conformation...

Figure 7. Projection of an amylosic chain conformation consistent with the energy...

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... 

Amylose complexes (wet precipitates) were prepared with fluoro-benzene, 1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane, bromo-form, and ferf-butyl alcohol. The conformation and packing of the amylose chains complexed with halogen-substituted hydrocarbons are the same as found in the complex with tert-butyl alcohol, namely,... 

Figure 5. Molecular drawings of a) one single strand of an amylosic chain in the left-handed conformation, having a six-fold symmetry, repeating in 2.1 nm. b) the double-helix generated by the association of two single strands, through two-fold symmetry operation, c) and d) Space-filling plots of the double helix, projected along and perpendicular to the fiber axis, respectively.

The nonbonded energy (van der Waals) is computed for isolated helical amylose chains as a function of the dihedral angles (, relative orientations of the glucose residues in the polysaccharide chain. In conformity with x-ray data, different helical conformations ere proposed for different crystalline modifications of amylose. 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

Both in theory and in practice there exist eight gluco-pyranose homopolymers, and some of the molecular conformations of three of these, i.e. cellulose and amylose (l.,2., 3,4), and (1+3)-8-D-glucan (5.,6.,.7) have been established by x-ray analysis. Although (1+3)-a-D-glucan is among the five homopolymers previously unsolved by x-ray diffraction, possible chain conformations were predicted with computers to be an extended ribbon (8.,9.) a single helix (9.), or a double or triple helix (10). 

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. 

Other WAXD starch patterns, known as V-types, are associated with the amylose fraction. In V-amylose, the chain conformation is a left-handed single helix with six residues per turn (V6) for complexes with aliphatic alcohols and monoacyl lipids with ligands bulkier than a hydrocarbon chain, helices of seven or eight glucose residues per turn are feasible.31 Aliphatic chains within amylose helices are rather locked... 

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