Valinomycin (VN)

Valinomycin is a dodecadepsipeptide, that is, it is made of twelve alternating amino acids and esters to form a macrocyclic molecule. The twelve carbonyl groups are essential for the binding of metal ions, and also for solvation in polar solvent. The isopropyl and methyl groups are responsible for solvation in nonpolar solvents.  Along with its shape and size this molecular duality is the main reason for its binding properties. K ions must give up their water of hydration to pass through the pore. K+ ions are octahedrally coordinated in a square bipyramidal geometry by 6 carbonyl bonds from Val. In this space of 1.33 Angstrom, Na+ with its 0.95 Angstrom radius, is significantly smaller than the channel, meaning that Na+ cannot form ionic bonds with the amino acids of the pore at equivalent energy as those it gives up with the water molecules.