Design ionophore/receptor

Hydration energies are still important since ion extraction process is involved and it is typically more difficult to design ISEs for hydrophilic ions than it is for hydrophobic ones. On the other hand, it is often a difficult to design selective receptors for large, bulky ions. Consequently, ionophore-based ISEs for potassium and calcium were realized early on, while selective sensors for magnesium, lithium, sodium, and small anions such as chloride, carbonate, and phosphate have been developed more recently or are still topics of current research. Ionophore-based ISEs for bulky anions such as perchlorate are not really known. 

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

Possible measures that are expected to improve the potentiometric selectivity are (1) use of hosts that form stronger complexes, (2) modification of the host to avoid ionophore self-association, and (3) an improved choice of the membrane solvent to avoid strong solvation of the hosts in the membrane. Evidence for the importance of (2) and (3) has been obtained from C NMR spectra of 12. While the properties of 1 1 host-guest complexes are very often of primary interest in supramolecular chemistry, the above results show that use of receptors for sensing purposes must be based on a receptor design that goes beyond this viewpoint. 

Other approaches to self-assembling receptors have been reported in recent years. A self-assembling, trimeric palladium complex based on the bis(benzimidazole) ligand (17) was designed by Williams and coworkers [4]. The complex contains a hydrophobic cavity that in the X-ray structure has included a molecule of acetonitrile. In a different context, Schepartz and McDevitt [70] have used the chelation of nickel(n) by A,7V -bis(salicylaldehy-de)ethylenediamine (salen) derivatives to control the position of K -binding glyme chains, and it has been shown that these self-assembled ionophores influence alkali metal transport across liquid membranes [71]. Also, Shinkai and coworkers [72] and Schneider and Ruf [73] have used metal chelation to induce an allosteric effect on binding at a second site. 

Dieldrin (1), y-BHC (2) and picrotoxinin (3,A) have been shown to influence the presynaptic events on the American cockroach (Periplaneta americana) central nervous system (CNS) and thereby to stimulate excitatory neurotransmitter release. As to the cause for such stimulation, we have proposed that these agents specifically interact with the putative picrotoxinin receptor closely associated with the chloride ionophore in the y-aminobutyric acid-chloride iono-phore complex (designated as the GABA receptor system in this paper) at the presynaptic region, and that such interaction causes inhibition of chloride ion uptake. This uptake is regulated by GABA to modulate the presynaptic membrane potential (4-8). 

Our quest in ionophore and receptor design emerged from one of the seemingly intractable problems of contemporary biochemistry the selective recognition of the ammonium cation (NH/) [141], Much of the problem, as discussed in earlier section is due to the nearly equivalent sizes of NH/ and the potassium cation (K ) [67]. One also learns from our discussion of the cation-7i interaction, that receptors providing improved dispersion stabilization are more selective forNH/ [67,71,72]. 

Thus, cation water clusters favour internal structures in contrast to the surface strucmres favoured by anionic water clusters. This critical difference in the structural preferences of hydrated cation and anion clusters provides important cues for the design of cation- and anion-specific ionophores and receptors. Indeed, we note that most cation receptors have spherical structures, while almost all anion receptors do not have compact spherical structures but have a vacant space around the anion binding site without full coordination (which might be exceptional for the F ion with strong electronegativity for which the excess electron is strongly bound to F due to its small ion radius). However, as the temperature increases, the hydration structure tends to be more spherical due to entropy effects. 

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