Guest-induced inversion of an asymmetric host layer in inclusion crystals of cholic acid

Abstract

Cholic acid, a typical asymmetric host, forms inclusion crystals with six 1,2,3-trisubstituted benzenes and five 1,2,4-trisubstituted benzenes as guests. All the eleven crystals have a bilayer structure composed of hydrophilic and lipophilic layers. The guest molecules are included in one-dimensional (1-D) cavities within the lipophilic layers, where the host molecules array in an antiparallel mode. On the other hand, in the absence of host–guest hydrogen bonds, the host arrangements in the hydrophilic layers change depending on the guest shape; 1,2,3- and 1,2,4-trisubstituted benzenes give the parallel and antiparallel arrangements that are categorized into shv and shv# patterns, respectively. The former crystals have a size-fit relationship between the guest molecule and the host cavity lesser than usual inclusion crystals involving the latter crystals. Therefore, the shape-fit relationship plays an important role in the formation of the host arrangements with the shv pattern. Moreover, the crystal structures of cholic acid with 1,2,3-trisubstituted benzenes are similar to those of deoxycholic acid with the identical guests. This is a rare example that hosts with a different number of hydroxyl groups in their molecular structure form an isomorphous crystal structure.