Helical Inclusion Complexes of Amylose with Aromatic Compounds: Crystallographic Evidence for New V‑Type Allomorphs

Amylose, a mostly linear homopolymer of glucosyl units extracted from native starch granules, can form a large variety of so-called V-type host–guest complexes when cocrystallized with small hydrophobic molecules. Several allomorphs are known that contain six-, seven-, or eight-fold amylose single helices, the guest molecules being located in the helices, in-between, or both. The present report describes the crystal structure of model lamellar inclusion complexes prepared by crystallizing amylose from dilute aqueous solutions in the presence of three aromatic compounds, namely, napth-1-ol, salicylic acid, and quinoline. By adequately selecting the crystallization conditions (amylose chain length, mixing, and crystallization temperatures), each guest induced two allomorphs, characterized by transmission electron microscopy, X-ray and electron diffraction, and 13C CP/MAS solid-state NMR spectroscopy data. The three guests induced the formation of the tetragonal V8II allomorph, based on eightfold amylose single helices, whereas three other allomorphs were identified for the first time: V7III (with napht-1-ol) and V7IV (with quinoline), both based on orthorhombic unit cells containing sevenfold helices, and the monoclinic V8III (with salicylic acid). The results widen the family of known V-amylose allomorphs that may occur when starch is processed in the presence of various ingredients and additives.