Polymorphism of dehydrocholic acid:: crystal structure of the β-phase and guest-mediated solid phase conversion

Dehydrocholic acid may be obtained in two polymorphic forms: alpha and beta. The complete characterization and the crystal structure of the beta-phase (obtained by unconventional powder diffraction methods) are described and discussed. Conversion of the alpha-form into the thermodynamically more stable beta-polymorph may be obtained through a room temperature solvent-mediated process. On the other hand, transformation of the beta-form into the alpha-form is realized through two distinct pathways: via a temperature-mediated process or as a guest-mediated solid state conversion, without the intervention of an amorphous state. R-(+)-Methyl p-tolyl sulfoxide is the guest of choice for this unusual beta-to-alpha transformation.