Crystalline Lattice

The two types, allomorphic A and B, differ essentially by stacking propellers and the number of water molecules in the crystal lattice. Type A has a monoclinic lattice type (a = 2.124 nm, b = 1.172 nm, c = 1.069 nm, and γ = 123.5 degrees) and belongs to the space group B2, which implies that the repeating unit of the helix is a trimer glucose (maltotriose) repeated two times per helical turn. Each propeller has six neighbors and the mesh contains only four water molecules distributed equivalently (Imberty et al., 1988). The crystalline B type has a hexagonal lattice with parameters a = b = 1.85 nm and c = 1.04 nm. In this structure, each helix has only three neighbors but this stacking generates a large canal that contains 36 water molecules per unit cell (Imberty and Perez, 1988).

More recent work by Takahashi et al. (2004) tends to confirm this structure with a hexagonal packing belonging to the P61 space group and slightly different lattice parameters a = b = 1.852 nm, c = 1.057 nm. The repeating unit of the helix is in this case a dimer (maltose) repeated three times per turn. The allomorphic types A and B can also be differentiated in nuclear magnetic resonance (NMR) 13C CP/MAS (cross-polarization/magic angle spinning). The carbon signal C1 (involved in the glycoside bond) of glucose is a triplet for A type and a doublet for B type. Indeed, the dihedral angles from both sides of the C1 carbon take three slightly different values in the case where the repeating unit is a maltotriose (Type A) and only two in the case of maltose (Type B) (Horn et al., 1987; Veregin et al., 1987a,b).

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