Structure of the inclusion complex of beta-cyclodextrin with 1,12-dodecanedioic acid using synchrotron radiation data; a detailed dimeric beta-cyclodextrin structure

A detailed crystal structure study of the dimeric inclusion complex of beta-cyclodextrin (betaCD) with 1,12-dodecanedioic acid is presented [IUPAC name: beta-cyclodextrin-1,12-dodecanedioic acid (2/1)]. The structure was solved with synchrotron high-resolution data (0.65 A) at 100 K [crystal data: P1, Z= 1, a = 18.153 (7), b = 15.456 (8), c = 15.251 (4) A, alpha = 102.81 (2), beta = 113.13 (2), gamma = 99.90 (3)degrees, V = 3,673 (3) A3, R = 0.0474 for 25,134 unique reflections with I > 2sigma(I)]. Moreover, the room-temperature structure is used for comparison [crystal data: P1, Z = 1, a = 18.220 (3), b = 15.488 (3), c = 15.409 (3) A, alpha = 102.903 (6), beta = 113.122 (5), gamma = 99.708 (5)degrees, V = 3735.2 (12) A3, R = 0.0828 for 8,235 unique reflections with I > 2sigma(I)]. Combining the high-resolution data and the low-temperature made possible the location of the disordered guest molecule, 1,12-dodecanedioic acid, inside the wide cavity of the macrocycle formed by two betaCD monomers. Moreover, almost all the H atoms of the betaCD macrocycle and many of the water molecules have been located in the low-temperature structure. Thus, for the first time, it has been possible to show in detail, up to now only given by neutron diffraction data, that two betaCD monomers self-assemble through O3...O3 intermolecular hydrogen bonds to form the betaCD dimer, as well as describe the hydrogen-bonding scheme between the dimer's hydroxyl groups among themselves and with water molecules in the lattice. The long guest threads through two host molecules forming a [3]pseudorotaxane. Its polar carboxyl groups, fully hydrated at the primary faces of the betaCD dimers, influence their packing so that those faces are exposed to the solvent. This is in contrast to the packing of the beta-cyclodextrin complexes of the corresponding aliphatic monoacids, where the dimeric complexes form channels in order to isolate the terminal methyl group from the water environment of the lattice.