The Hydrated Potassium Complex of the Ionophore Monensin A

The crystal structure of the potassium monensin A dihydrate has been determined (orthohombic, space group P2₁2₁2₁, a = 16.485 (2) Å, b = 19.189 (2) Å, c = 12.661 (1) Å, Z = 4). Examination of the conformation of the K⁺ complex compared with that of the previously reported structures of several complexes, particularly the Na⁺ complexes, reveals several features which correlate with the observed selectivity of monensin for Na⁺over K⁺. Coordination of the K⁺ion involves changes in the conformation of a spiro-fused ring, the conformation of which is highly conserved in the structures of several Na⁺ complexes. Preliminary molecular mechanics calculations are consistent with the conclusion that the coordination of the K⁺ion involves a distortion of the spiro-fused ring from its low-energy conformation. Furthermore, the coordination of the K⁺ion is less uniform than that observed in the Na⁺ complexes.