Crystal Structure of a Head-to-Head Bis(cholic)-Urea Dimer

A head-to-head dimer of cholic acid linked at C3 carbon atoms by a urea bridge was synthesized and the crystal structure was studied. The crystal belongs to the monoclinic crystal system, space group I₂. The packing shows a bilayer structure with alternating hydrophobic and hydrophilic layers. The horizontal and vertical planes of the two cholic residues are almost parallel to each other, the vertical ones being perpendicular to the bilayers. The hydrogen bond network is two-dimensional as it does not propagate in the direction perpendicular to the bilayers. It is noteworthy that the nitrogen atoms of the urea bridge do not participate in that network. The carbonyl urea group and only one of the ester of the two side chains form hydrogen bonds with two other dimers through the steroid hydroxy O7-H and O12-H groups. This means that each dimer forms eight hydrogen bonds but only four are different.