Crystal structure and hydrogen-bonding system of cholic acid hemihydrate, C24H40O5�1/2H2O

Abstract

Crystals of the hemihydrate of this bile acid are hexagonal, space groupP6522, a=b=13.736(8)A,c=85.06(6)A,V=13,899A3. 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, toR=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 axisin 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.