Temperature-Driven Isosymmetric Reversible Phase Transition of the Hormone Estradiol 17β Valerate

The
hormone estradiol 17β valerate (E2V), used in hormone
replacement therapy, undergoes a structural phase transition at 251.1
K on cooling. The crystalline structures of the low and room temperature
phases were determined showing that neither the space group nor the
site symmetry and number of atoms are altered. These phases are related
by a large conformational reorientation of the valerate chain. In
addition, thermal analysis, solid state nuclear magnetic resonance,
and infrared spectroscopy show that the transformation is reversible,
discontinuous, and first order, evidencing the occurrence of an isosymmetric
phase transition, rare for organic compounds.