Calcium Complexation by Corticosteroids

Cortisone and hydrocortisone form calcium complexes of 2:1 steroid/Ca2+ stoichiometry in solution, with formation constants of 201 ± 1.7 and 184 ± 1.7 M−2, respectivelyroughly half the value seen for a simple aromatic α-hydroxy ketone, phenacyl alcohol (341 ± 10.0 M−2). X-ray crystallographic analysis demonstrates maintenance of this 2:1 stoichiometry in the solid state for the phenacyl alcohol and cortisone complexes, while the hydrocortisone complex crystallizes with a 1:1 stoichiometry in the form of linear chains, with the α-hydroxy ketone ligand and C(3) (A-ring) carbonyl groups binding two different calcium ions. In each complex, both in solution and in the solid state, the α-hydroxy ketone moiety serves as a bidentate chelator for calcium. Extensive hydrogen-bonded networks are present in each structure, linking various hydroxyl groups, coordinated and noncoordinated hydrate molecules, and the chloride counterions, in essence representing supramolecular complexation of the hydrated chloride anion.