The double-helical molecular structure of crystalline b-amylose

Abstract As was found with B-amylose (see preceding paper), the crystal structure of A-amylose is apparently based on parallel-stranded, right-handed double helices. The helices pack in antiparallel fashion into an orthorhombic unit-cell ( a = 11.90, b = 17.70, c (fiber repeat) = 10.52 A), which results in nearly hexagonal close-packing. The steric disposition of O-6 is either all gt or may be a mixture of gt and tg . The unit cell of A-amylose contains only about a quarter of the water molecules located in the unit cell of B-amylose. Aside from the differences in the amount of water, the A and B structures differ only in that the former has a denser packing-structure, whereas the letter is more open. The similarities in packing suggest that interconversions of A and B are possible. Experimentally, partial conversion of B into A has been observed.