Valinomycin

Valinomycin is a dodecadepsipeptide, that is, it is made of twelve alternating amino acids and esters to form a macrocyclic molecule.

Valinomycin is obtained from the cells of several Streptomyces strains, one of them Str. Tsusimaensis.

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 highly selective for potassium ions over sodium ions within the cell membrane. It functions as a potassium-specific transporter and facilitates the movement of potassium ions through lipid membranes "down" an electrochemical potential gradient. The stability constant K for the potassium-valinomycin complex is 106 and for the sodium-valinomycin complex only 10. This difference is important for maintaining the selectivity of valinomycin for the transport of potassium ions (and not sodium ions) in biological systems.

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 its shape and size this molecular duality is the main reason for its binding properties. For polar solvents valinomycin will mainly expose the carbonyls to the solvent and in nonpolar solvents the iso-propyl groups are located predominantly on the exterior of the molecule. This conformation changes 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 is not actually locked into configuration because the size of the molecule makes it highly flexible, but the potassium gives some degree of coordination to the macromolecule.

Application
Valinomycin acts as ion-exchange agent in Potassium selective electrode.