Potassium Selectivity

Although a member of the Kir family, the KscA potassium cation channel pore region resembles that of a Kv sequence especially in view of the crucial Trp residue which is conserved in all members of the Kv family. In Kjr channels the Trp residue is actually replaced by leucine and phenylalanine and hence the Trp-Tyr hydrogen bonds that provide the framework for K+ selectivity, are not applicable to Kjr channels where a different sort of contact is required. 

Immediately below the selectivity filter, which comprises the narrowest part of the ionophore, the channel opens up into a cavity (10 A in diameter) that holds a pool of water to stabilize and shield an ion from what would otherwise be the most hydrophobic part of the membrane. Additional stabilization is provided by the C-termini end of the four pore helices which have substantial negative electrostatic potential due to the helix dipole and point right at the pool.9 The hydrophobic lining of the pool is also probably conducive to the high flux of ions by providing a low-resistance pathway. 

To date, the most extensively studied natural ionophore is gramicidin,10 a polypeptide antibiotic isolated from the bacterium Bacillus brevis. Indeed, the idea of a channel-like structure for ion transport was inferred from its study. However, unlike channel proteins, which are exclusively composed of L-amino acids, each alternate amino acid in gramicidin has D-stereochemistry. It is composed of 16 residues, 15 of which are amino acids. The structure is summarized below, in which Xxx has the following identities gramicidin A (gA), Trp gB, Phe gC, Tyr gD, a mixture of gA, gB, gC, -80 5 15. 

The development of biophysical techniques to make possible measurements of single ion channels in membrane bilayers has been fundamental to the advances in the understanding of how natural ionophores, particularly ion channel proteins, operate at the molecular level. The two principal techniques are the planar lipid 

This technique essentially comprises two aqueous compartments containing electrolyte connected by a pinhole across which a bilayer of phospholipid molecules is spread. An electrical potential is applied across the bilayer and in the presence of an ionophore the resultant current flowing between the two chambers is measured as a function of time.12 The method is often used to assess the activity of synthetic peptides, and has been used to measure the channel activity of several non-peptide channels. Of particular significance is its use in allowing the measurement of the single channel properties of isolated channel-forming proteins that have been reintroduced back into planar bilayer membranes.13 

---

Voltage-gated potassium (Kv) channels are membrane-inserted protein complexes, which form potassium-selective pores that are gated by changes in the potential across the membrane. The potassium current flow through the open channel follows by the electrochemical gradient as defined by the Nernst equation. In general, Kv channels are localized in the plasma membrane. 

E. Lindner, K. Toth, M. Horvath, E. Pungor, B. Agai, I. Bitter, L. Toke, and Z. Hell, Bis-crown ether derivatives as ionophores for potassium selective electrodes. Fres. Z. Anal. Chem. 322, 157-163 (1985). 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

H.A. Arida and R.F. Aglan, A solid-state potassium selective electrode based on potassium zinc ferro-cyanide ion exchanger. Anal. Lett. 36, 895-907 (2003). 

D. Engel and E.W. Grabner, Copper hexacyanoferrate-modified glassy carbon a novel type of potassium-selective electrode. Ber. Bunsenges. Phys. Chem. 89, 982—986 (1985). 

C. Gabrielli, P. Hemery, P. Liatsi, M. Masure, and H. Perrot, An electrogravimetric study of an all-solid-state potassium selective electrode with Prussian blue as the electroactive solid internal contact. J. Electrochem. Soc. 152, H219 (2005). 

There are two general classes of naturally-occurring antibiotics which influence the transport of alkali metal cations through natural and artificial membranes. The first category contains neutral macrocyclic species which usually bind potassium selectively over sodium. The second (non-cyclic) group contains monobasic acid functions which help render the alkaline metal complexes insoluble in water but soluble in non-polar solvents (Lauger, 1972 Painter Pressman, 1982). The present discussion will be restricted to (cyclic) examples from the first class. 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

Potentiometric titration has been applied to the determination of potassium in seawater [532-534], Torbjoern and Jaguer [533-544] used a potassium selective valinomycin electrode and a computerised semiautomatic titrator. Samples were titrated with standard additions of aqueous potassium so that the potassium to sodium ion ratio increased on addition of the titrant, and the contribution from sodium ions to the membrane potential could be neglected. The initial concentration of potassium ions was then derived by the extrapolation procedure of Gran. 

Salts, in addition to causing the soil to be basic, can have deleterious effects on analytical procedures. For example, significant error can occur if a potassium-selective electrode is used to determine potassium in a high-sodium soil (see Chapter 9). As discussed in Chapter 14, other salts could cause inaccurate results when atomic absorption analysis of a soil extract is carried out. 

J. Pick, K. Toth, E. Pungor, M. Vasak, and W. Simon, A potassium-selective silicone-rubber membrane electrode based on neutral carrier, Anal Chim Acta 64, 477-480 (1973). 

Table 2 Structures of Potassium Selective Cryptands Cryptofix 222 and Cryptofix 222BB and the Stability Constants of the Matching Cryptates...

Perhaps the most used substance in biophysical ion-transport experiments is valinomycin, (VIII), which is also the basis of potassium-selective electrodes (63), It is a cydododecadepsipeptide (64), [D-valine,... 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Sachleben et al. observed that for bis(alkoxy)calix[4]arene monocrown ethers, reducing the size of the alkoxy substituent from octyl to allyl increased the cesium extraction by 10%-30% and the cesium-to-potassium selectivity by 20%-40%, with little impact on cesium over sodium selectivity. A standard modeling approach was used to analyze the complementarity of the calix-crown cavity toward potassium and cesium. MM3 optimizations were performed by modifying the K+ and Cs+ complexes, replacing the 1,3-dioxybenzene-substituent with tert-butoxy, methoxy, or hydrogen groups. 

The membrane used to activate this potassium-selective IWAO [134] consists of a potassium bulk optode based on 0.5 wt % chromoionophore ETH 5294, 1.0 wt% ionophore valinomycin, 0.5 wt% ionic additive potassium tetrakis(4-chlorophenyl)borate (KtpClPB), 31.0 wt % polymer PVC, 67.5 wt % organic solvent and plasticizer bis(2-ethylhexyl)sebacate (DOS) [142], This commercially available optode not only acts as an example of the development of an enhanced ion-selective IWAO, but also serves to validate the previously remarked features, because results can be compared with the ones obtained with membranes of the same composition and thickness in a con-... 

Of particular relevance to chemical sensor technology are the novel results of the electrochemical competition experiments. When an equimolar mixture ofNa+/K+orNa + /K + /Mg2 + cations is added to electrochemical solutions of (26), the ferrocene/ferricinium redox couple shifts anodically by an amount approximately the same as that induced by the K+ cation alone. This observation, together with the FABMS competition experimental findings, suggests that (26) is a first-generation prototype potassium-selective amperometric sensor, capable of detecting the K+ cation in the presence of Na+ and Mg2+ ions. 

A number of ferrocene cryptand molecules (66-74) ((29)—(31)) have been reported in the literature and it is only relatively recently that their electrochemical coordination properties have been disclosed. We have synthesized potassium-selective metallocene cryptands (72) (30) and (31) the electrochemistry of the former in the presence of K+ guest cations proved disappointingly irreversible (75). Hall and co-workers (76) have used cyclic voltammetry to investigate the coordination of... 

Matthews SE et al (2002) Calix[4]tubes A new class of potassium-selective ionophore. J Am Chem Soc 124 1341-1353... 

Wright Al et al (2001) Novel resordn[4]arenes as potassium-selective ion-channel and transporter mimics. Chem Eur 17 3474-3481... 

In the case of a neutral non-ionic chelating agent we have neutral carrier-selective electrodes transport is achieved by selective complexa-tion of certain ions. The best-known electrode of this kind is the potassium-selective electrode, whose membrane consists of a valinomycin macrocycle immobilized in phenylether. The important criterion appears to be the size of the cavity in the centre of the macrocycle and interferences are from cations with similar hydrated ionic radius, such as Rb+ and Cs+. 

Figure 17. (i) Beer s potassium selective calix[4]tube. (ii) Cragg s oxacalix[3]arene. 

Chen, G. Q., Cui, C., Mayer, M. L., and Gouaux, E. (1999). Functional characterization of a potassium-selective prokaryotic glutamate receptor. Nature 402, 817-821. 

---