Potassium ions selective 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. 

To illustrate which components are necessary to prepare an ISE membrane, let us again go back to a simple extraction experiment as it was similarly described in Section 3.1.1. Consider an aqueous potassium chloride solution equilibrated with an immiscible organic phase containing an electrically neutral ionophore for K+, that is, a receptor compound that binds the potassium ion selectively. How does the phase boundary potential between these two phases depend on the KCl concentration in the aqueous phase Upon equilibration of the two phases, some KCl will be present in the organic phase (Figure 5). For low amounts of KCl in the system, the potassium ions in the organic phase will be present in the form of ionophore complexes, and there will be an excess of free ionophore, L. In comparison to the concentration of the ionophore complex, the organic phase concentration [K+] of free potassium ions that are not bound by the ionophore is very low and can be calculated from the formation constant, of the potassium ion complex, [LK+] ... 

Figure 12.11 Schematic diagram of the ion pore of the K+ channel. From the cytosolic side the pore begins as a water-filled channel that opens up into a water-filled cavity near the middle of the membrane. A narrow passage, the selectivity filter, links this cavity to the external solution. Three potassium ions (purple spheres) bind in the pore. The pore helices (red) are oriented such that their carboxyl end (with a negative dipole moment) is oriented towards the center of the cavity to provide a compensating dipole charge to the K ions. (Adapted from D.A. Doyle et al.. Science 280 69-77, 1998.)...

Potassium hexachloromolybdate, 3, 1230 Potassium hexacyanoferrate discovery, 1, 3 Potassium ions biology, 6, 559 selective binding biology, 6, 551... 

Sodium hexakis(formato)molybdate, 3, 1235 Sodium hypochlorite alkene epoxidation manganese catalysts, 6,378 Sodium ions biology, 6, 559 selective binding biology, 6, 551 Sodium molybdate, 3, 1230 Sodium peroxoborate, 3,101 Sodium/potassium ATPase, 6, 555 vanadate inhibition, 3, 567 Sodium pump, 6, 555 mechanism, 6, 556 Sodium pyroantimonate, 3, 265 Sodium salts... 

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. 

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... 

Finally, the sensory behaviour of (18) is not as good as that of its smaller ring cousin (14). On the positive side, however, the cation-binding constants remain quantitatively predictable and fluorescence enhancement selectivity peaks at potassium ion. The smaller fluorescence enhancement factor of 3 for (18) with potassium ion can be attributed to lower intrinsic charge density and more charge dispersal to aliphatic oxygen centres than those discussed in the previous paragraph. 

Lower rim substitution can also be used to link cahxarene molecules. An elegant example was produced by linking two calix[4]arenes with four ethylene bridges to produce what was termed as a calix[4]tube (102). The synthesis was templated by the potassium ion, and the product was found to bind this cation selectively. 

Amphoteridn is a complex amphoteric polyene antibiotic that binds to cell membranes and forms a pore through which ions can pass, with consequences that include loss of potassium ions from within the cell. Since the antibiotic binds more readily to fungal cell membranes than mammalian, its action is relatively selective. It can potentiate the action of certain other antifungals. and it may be used with flucytosine. Also, it confers antifungal activity on rifampicin (normally antibacterial). As it has an appreciable renal toxicity, it needs to be used with caution in some patients. Nystatin is a polyene antibiotic similar in structure to amphotericin, often used for local treatment. 

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