Valinomycin
Valinomycin is a dodecadepsipeptide, that is it is made of twelve alternating amino acids to form a macrocyclic molecule. It is a member of the group of natural neutral ionophores because it doesn't have a residual charge. It consists of enantiomeres D-Valine and L-Valine, D-Hydroxyvaleric acid and L-Lactic acid. Structures are alternated bound via amide and ester bridges. Valinomycin is discriminating sodium ions against potassium ions within the cell membrane. It is working as a "potassium pump". The stability constant K for the potassium-valinomycin comlex is 106 and for the sodium-valinomycin complex only 10. This selectivity is important in biological systems because valinomycin transports potassium ions across cell membranes.
Structure
From the chemical structure it can be seen that there are some prevailing features. The 12 carbonyl groups are essential for the binding of metal ions, and also for solvatation in polar solvent. The isopropyl groups and methyl groups are responsible for solvatation in nonpolar solvents. Along with it shape and size this molecular duality is the main reason for its binding properties. For polar solvents valinomycin will mainly "show" the carbonyls to the solvent and in an nonpolar solvent the iso-propyl groups will predominate on the exterior of the molecule. This has to change though when valinomycin is bound to a potassium ion. The molecule is "locked" into a conformation where the exterior is made up of the isopropyl groups. (It isn't actually locked into configuration because the size of the molecule a makes it highly flexible, but the potassium gives some coordination to the macromolecule).