Depsipeptides ionophoric cyclic

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

Enniatin family, cyclic depsipeptides produced by strains of Fusarium. Members are enniatin A, cyclo-(-D-Hiv-Me-Ile-)3, enniatin B, cyclo-(-D-Hiv-MeVal-)3, and enniatin C, cyclo-(-D-Hiv-MeLeu-)3, enniatin D, cyclo-(-D-Hiv-MeLeu-D-Hiv-MeVal-D-Hyv-MeVal-), enniatin E, cyclo-(-D-Hiv-MelLe-D-Hiv-MeLeu-D-Hiv-MeVal-), and enniatin F, cyclo-(-D-Hiv-MeLeu-MeIle-D-Hiv-Melle-) which all contain D-a-hydroxyisovaleric acid (o-Hiv) besides various L-methylamino acids. Enniatins act as ionophores to form complexes with K+ ions which allow their transport across membranes. 

The name was coined to describe the activity of compounds. such as valinomycin, a naturally occuning cyclic depsipeptide that transports ions across membranes efficiently and selectively. A number of natural ion transport agents have been identified. They regulate the concentrations of the predominant biological cations (Na", K , Mg, and Artificial ionophores were first... 

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