Cholic acid as host for long linear molecules: a series of co-crystals with n-alkylammonia

Abstract

Co-crystals of cholic acid (CA) with n-alkylammonia (n = 10, 12, 14, 16) represent the first examples of CA's co-crystals with molecules of comparable size. In one of the compounds a completely new type of bilayer arrangement was found. The host–guest ratio in crystals was found to be 1 : 1 in three cases and 2 : 1 in one case. In 1 : 1 complexes CA molecules are assembled in bilayers and n-alkylammonia guests are included in the hydrophobic zones between those layers in a kind of sandwich-type structure. In the 2 : 1 complex the CA molecules include guest molecules in a different way: bilayers are not parallel but instead they cross and form one-dimensional hydrophobic channels into which guest molecules are included. In the case of 1 : 1 co-crystals the bilayers of CA are separated much more than in previously reported structures in order to accommodate large n-alkylammonia molecules. In spite of the different bilayer arrangements the characteristic spacing in the plane of CA bilayers is remarkably similar to the majority of other CA crystal structures known to date.