CRYSTAL-STRUCTURE AND HYDROGEN-BONDING SYSTEM OF CHOLIC-ACID HEMIHYDRATE, C24H40O5.1/2H2O

Crystals of the hemihydrate of this bile acid are hexagonal, space group P6(5)22, a = b = 13.736(8)angstrom, c = 85.06(6)angstrom, V = 13,899 angstrom3. The asymmetric unit contains two independent steroid molecules and one water molecule, the latter disordered over two nonequivalent positions. Z = 12[2(C24H40O5) . 1(H2O)], F(000) = 5496. Structure solution by direct methods; refinement by least-squares, to R = 0.072. The complex hydrogen-bond system comprises: (a) three standard ordered O-H . . . O hydrogen-bonds; (b) what is probably a symmetrically hydrogen-bonded carboxylic acid dimer, with a twofold rotation axis in the plane of the two carboxyl groups; (c) helical hydrogen-bonding chains about each 65 axis, disordered over four possible arrangements. In these helical chains, the six independent hydrogen-bonds can as a group be in either of two systems of nearly equivalent ''flip-flop'' arrangements: O-H ... O <-- --> O ... H-O. Each helical system includes water, which can occupy either of two sites; thus, there is further disorder involving two sets of nonequivalent hydrogen-bonds with water as donor and acceptor. Many aggregation features here differ markedly from those in the crystal structures of either anhydrous cholic acid or cholic acid monohydrate.