Potassium ions repulsion

Cell membranes contain selective ion channels that are highly discriminatory for potassium ions, sodium ions, calcium ions, and the proton. For instance, the highly selective potassium channels of nerves show selectivity for ions as Li < Na K > Rb > Cs, and calcium channels show selectivity as Mg Ca > Sr > Ba. Ion selection operates on the basis of size and repulsion, not... 

Figure 13.26. Two-Site Model for the Potassium Channel. The restricted part of the potassium channel has two energetically similar binding sites. The binding of a second potassium ion creates electrostatic repulsion to push the first ion out of the channel.

Fig. 18. A chemically driven supramolecular switch. In acetonitrile solution, the cyclophane CBPQT4+ resides preferentially around the better electron-donating thread 12. Addition of an excess of potassium ions to the solution results in a strong complex being formed between these cations and the crown ether portion of thread 12. Electrostatic repulsion between the cyclophane and the thread leads to the cyclophane residing upon the poorer tr-electron donating - yet electrically neutral -thread 13...

Although this first pseudorotaxane-based switch illustrates quite efficiently that electrostatic repulsion between the metalated thread 12-K+ and the cyclophane CBPQT4"1" is a reliable paradigm for inducing the dethreading of a complex, there was no opportunity to demonstrate the reversibility of the system - i.e., the potassium ions cannot be removed from the solution so that the switch can return to its original state. It is clear that any switching system has to be based upon a reversible system, in which the switch can be reset at will. Such a system is portrayed in Fig. 19. 

When the mica plates were well separated, i.e. more than 2 nm apart, the results fitted the DLVO theory of double-layer repulsions corresponding to a potential of - 78 mV, which amounts to 40% coverage of the mica with positively charged potassium ions. These long-range forces were those previously measured by Adams and Israelachvili. ... 

Table 10). The explanation (20) is that the two unidentate ligands can minimize their dipolar repulsion by going to opposite ends of the potassium ion surface. Such a relief of the dipolar repulsion is forbidden to the bidentate ligand. In addition, when the ethylenediamine attaches its first ligator atom (Nj ) to... 

Significantly, (a) a-sulfonyl carbanions of thiirane dioxides, generated from the latter in the presence of strong bases such as potassium f-butoxide and alkoxide ions , do epimerize to relieve steric repulsion between substituents as in 42 above and (b) the a-hydrogen in aryl-substituted three-membered sulfoxides (e.g. 46c) are sufficiently acidic to... 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

In many cases such as at water-mercury interfeices electrolytes are positively adsorbed. The application of the kinetic theory to surface films of molecules leads, as we have seen, to a ready interpretation of the lowering of the surface tension by capillary active nonelectrolytes. For electrolytes an additional fiictor has to be considered, namely the mutual interaction of the electrically charged ions adsorbed. As we shall have occasion to note the distribution of the adsorbed ions, both positive and negative, at an interface such as water-mercury is not readily determined, but it is clear from a consideration of the data of Gouy that mutual ionic electrical repulsion in the interface is an important factor. In the case of potassium iodide, for example, for very small values of F the Traube relationship... 

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