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.