Valinomycin potassium binding

Valinomvcln. To observe the effect of the potassium binding polypeptide valinomycin on lipid bilayer resistance, a phospholipid bilayer membrane was formed under PBS containing 1 mM KCl, and valinomycin was added with stirring to the front chamber. While lipid bilayers are normally highly impermeable to charged species, K" " ions bound to the lipophilic valinomycin diffuse freely through the hydrophobic core of a bilayer (11,12). Valinomycin... 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

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. 

Because the stability constant of its complex with potassium is much greater than that with sodium, valinomycin is a relatively specific potassium ionophore. In contrast, the mushroom peptide antamanide has a binding cavity of a different geometry and shows a strong preference for sodium ions.388,390 The structure of the Na+-antamanide complex is also shown in Fig. 8-22B. The Streptomyces polyether antibiotic monensin (Fig. 8-22D),389,391 a popular additive to animal feeds, is also an ionophore. However, its mode of action, which involves disruption of Golgi functions, is uncertain 392... 

Materials with selective binding or transport properties will have a major impact on sensor design and fabrication. Selectivity in either binding or transport can be exploited for a variety of measurement needs. This selectivity can be either intrinsic, that is, built into the chemical properties of the material, or coupled with selective carriers that allow a non-selective material to be converted into a selective one (see the section on recognition chemistry). An example of the latter is the use of valinomycin as a selective carrier in a polyvinyl chloride membrane to form a potentiometric potassium ion sensor. Advances in the fields of gas separation materials for air purification and membrane development for desalinization are contemporary examples illustrating the importance of selective materials. As these materials are identified, they can be exploited for the design of selective measurement schemes. 

Fig. 6.15 (a, b) Neutral ionophore valinomycin with the cavity for binding potassium ion. (c) Charged ionophore di n-octyl phenyl phosphonate used in calcium ion-selective electrodes... 

One-electron reduction of (NH3)5Ru-4-(l 1 -dodecenyl)py + by externally added reductants followed biphasic kinetics when the complex was bound at both interfaces of PC liposomes, but only the fast step was observed when binding was limited to the external surface [111]. The slow step was first order and independent of the identity and concentrations of added reductants. The rate constant, k = 10 s", was unchanged upon adding either the potassium ionophore, valinomycin, or the proton carrier, carbonyl cyanide m-chlorophenylhydrazone, to the medium, indicating that, although electrogenic, the transmembrane reduction step was not rate-... 

We have already introduced the hole-fit concept for a macrocyclic ligand in Chapter 4 (see Figure 4.38), which effectively means that the most stable complexes form where the internal diameter of the ring cavity matches the size of the entering cation. The effect can be significant for example, the natural antibiotic valinomycin is a macrocycle that binds potassium ion to form a complex 104 times more stable than that formed with the smaller... 

Fig. 30. Complexation of potassium by valinomycin [77]. Hydrogen bonding in the com-plexing agent helps assure a torus-like structure for metal ion binding...

Figure 15-6b shows how the electrode works. The key in this example is the ligand, L (called an ionopkore), which is soluble inside the membrane and selectively binds analyte ion. In a potassium ion-selective electrode, for example, L could be valinomycin, a natural antibiotic secreted by certain microorganisms to carry ion across cell membranes. The ligand L is chosen to have a high affinity for analyte cation and low affinity for other ions. 

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