Crystal Structures of a-Cyclodextrin-3-lodopropionic Acid (1: 1) Complex Pentahydrate and Hexakis(2, 3, 6-tri-O-methyl)-α-cyclodextrin-lodoacetic Acid (1: 1) Complex Monohydrate

Abstract

Crystal structures of a-cyclodextrin-3-iodopropionic acid (1: 1) complex pentahydrate ( I )and hexakis(2, 3, 6-tri-O-methyl)-α-cyclodextriniodoacetic acid (1: 1) complex monohydrate OD were investigated by the X-ray method. The crystal of I is orthorhombic, and the space group being P 2₁2₁2₁ with Z=4; the cell dimensions are a=13.808 ( 2 ), b 39.581 ( 4), and c=9.685 ( 1 ) Å, The crystal of II is monoclinic, and the space group being P 2, with Z=2; the cell dimensions are a=11.590 ( 2 ), b=23.285 ( 4 ), c=13.901 ( 2 ) Å, and 8 =106.98 ( 1 )°. The structures of both complexes were determined by the heavy atom method combined with the rigid-body least-squares technique, and refined by the block-diagonal least-squares method to the final R-values of O.096 for 4056 reflections ( I ) and 0.10 for 3634reflections On. In the complex I, α-cyclodextrin is in the shape of a distorted hexagon, within which 3-iodopropionic acid is included. The iodine atom is located at the secondary hydroxyl side of the cavity, while the carboxyl group located at the primary hydroxyl side forms the hydrogen bond with the secondary hydroxyl group of neighboring a-cyclodextrin. a-Cyclodextrin molecules are arranged in a zigzag mode, thus forming a typical “cage-type”structure. Five water molecules, which are located outside the cavity, form many hydrogen bonds among them and with hydroxyl groups of a-cyclodextrin. The hexakis(2, 3, 6-tri-Omethyl)a-cyclodextrin (methyl-α-CyD) molecule is in the shape of a distorted hexagonal cone. The cavity, which is wider than that of a-cyclodextrin, includes iodoacetic acid and water molecules. The methyl-α-CyD and iodoacetic acid molecules are linked by the hydrogen bonds via water molecule inside the cavity. The position of iodoacetic acid is disordered, and iodoacetic acid occupies two positions within the cavity with the same occupancy. Methyl-α-CyD molecules are stacked along the crystallographic a axis in a head-to-tail mode to form a “channel-type” structure.