Amylose right-handed conformations

Figure 8. A comparison of the single-stranded (top) and double-stranded (bottom) ,

Recent crystal structure proposals for A- and B-amylose (14,15) consist of parallei-stranded, right-handed 3buFle helices (Figure 5) packed in an anti-parallei fashion. Each strand is comprised of six a-p-glucose units in the stable 40 conformation. The unit-cell of A-amylose is orthorhombic with ... 

The polysaccharide chains of crystalline B-amylose exist as double stranded helices which have a right-handed six-fold helical conformation of 20.8 A. The helices are anti-parallel and have an open channel in the hexagonal array which is filled with water molecules. Two helices (i.e. twelve D-gluose residues) exist in each unit cell which has the dimensions a = b = 18.50 A and c, the fibre repeat distance, 10.40 A with the angle y being 120°. Similar studies on the crystalline polymorph A-amylose have shown that it also exists as a right-handed double helix in which the chains are parallel. The helices pack in an antiparallel fashion into an orthorhombic unit cell (a = 11.90 A, ft = 17.70 A, and c = 10.52 A). The water content of A-amylose was only about 25% of the value for B-amylose. 

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