Amylose left-handed conformations

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.

Figure 7. A section of the

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... 

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... 

Amylose has a marked tendency to adopt a single coiled helical conformation (10.26) and this has been shown to occur in the variety known as W amylose . It contains a left-handed helix with six glucose units per turn, with pitch height 8 A, and an internal channel diameter of 5 A. Both... 

The structures of V - and V -amylose are similar. In both cases the amylose chains form left-handed helices pack in an antiparallel manner in the space group V -amylose i " was originally believed to have a sixfold helical conformation but further refinement has shown only twofold symmetry due to different rotational positions of glucose hydroxymethyl groups. The structure contains significant amounts of water. This refinement locates these guest molecules in interstitial sites between neighbouring helices of amylose... 

A combination of stereochemical structural refinement and. T-ray diffraction analysis has been used to determine the crystal stracture of potassium hydroxide-complexed amylose, obtained by heterogeneous deacetylation of amylose acetate. The complex crystallizes as an orthorhombic unit cell with p2,2,2 symmetry and dimensions a = 8.84,6 = 12.31, and c = 22.41 A. The conformation of the amylose chain is a distorted left-hand helix with six o-glucose residues per turn. Each three-residue asymmetric unit is complexed with one molecule of potassium hydroxide and three molecules of water. Extension of the amylose chain probably arises because of the co-ordination of the K ion. 

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