Regular left-handed fragment of amylose: crystal and molecular structure of methyl-α-maltotrioside, 4H2O

The crystal structure of methyl-α-maltotrioside tetrahydrate C₁₆H₃₄O₁₆, 4H₂O), has been established by direct methods from 2269 independent reflections and refined to a final R value of 0.054. The crystal belongs to the orthorhombic system, space group P2₁2₁2₁ and has a unit cell of dimensions: a = 1.037 (1), b = 2.439 (1) and c = 1.065 (1) nm. The three glucose residues have the ⁴C₁ pyranose conformation and are α-(1–4)-linked. The conformation of the glycosidic linkage is characterized by torsion angles (φ, ψ) which take the values (82.2, −148.9) between the non-reducing and the middle residue and (82.8, −151.8) between the middle residue and the reducing one. The primary hydroxyl groups exist in a gauche-gauche conformation. This structure is also characterized by the lack of intramolecular hydrogen bonding between secondary hydroxyl groups belonging to contiguous residues. The molecules are held together by a complicated network of hydrogen bonds involving all the hydroxylic groups and the water molecules. the three dimensional arrangement corresponds to a regular alternation of antiparallel bilayers strongly linked by water molecules. A survey of the distribution of the glycosidic torsion angles in all known linear α-(1–4)-linked d-glucose residues, discloses the existence of three stable conformers. This crystal structure provides the first experimental evidence of a regular left-handed fragment of the amylosic chain in a highly hydrated neighbourhood. Furthermore, the helical conformation adopted by the trisaccharide gives rise to helical parameters which are close to those found experimentally for native A and B amyloses. The relevance of the present results to the rationalization of the polymorphic transformation of amylose, along with its crystallization habits is also discussed.