Crystal Structure of the Antibiotic Monensin Similarities and Differences between Free Acid and Metal Complex

The structure of monensin, C36H62O11, has been determined by X‐ray analysis of its crystalline monohydrate (orthorhombic, a = 15.15, b = 23.61, c = 10.65 Å, Z= 4, space group P212121). Phases were assigned by direct methods, making use of the ‘tangent formula’. Although the conformation of the free acid resembles that of the silver salt in being cyclic, there are differences in the hydrogen bonding pattern. These features are discussed in relation to the complexation of metal ions by monensin.