Molecular and Crystal Structure of 7-Fold V-Amylose Complexed with 2-Propanol

Single crystals of amylose V 2-propanol, i.e., amylose cocrystallized with water and 2-propanol, were prepared from synthetic linear amylose. When observed under frozen hydrated conditions, these crystals yielded base-plane electron diffraction diagrams containing more than 100 independent diffraction spots, whose intensities were used to solve the crystal structure of this complex. The molecular and crystal structure of amylose V 2-propanol clearly indicated that the amylose molecules were organized in 7-fold left-handed helices, with 2-propanol and water molecules located as guests both within and between the helices. The V 2-propanol unit cell contains four helices, distributed in two antiparallel pairs of parallel helices organized along the P 2 1 2 1 2 1 symmetry. The helices are organized along alternating motifs of four helices in a close-packed hexagonal arrangement together with four others in a nearly square organization surrounding a central column of water and 2-propanol. Whereas the location of the amylose helices is well established in the unit cell, the coordinates of the guest molecules could not be defined with certainty, most likely due to a positional disorder. A tentative model of the guest molecule distribution is presented, which consists of two 2-propanol and two water molecules within the helical cavity together with four water molecules and two 2-propanol molecules between the helices. The mobility of the guest and its description as a continuum, rather than at fixed crystallographic positions, explain why so many structures isomorphous to V 2-propanol can be obtained with different guest molecules, while yielding similar electron diffraction diagrams.