Ionophore cation complexes

Under UV light irradiation, cis-trans photoisomerization of the ionophore in the membrane occurs. We assume that the cis and trans isomers are both present in the membrane and the cis isomer forms a 1 1 (ionophore-cation) complex with a stability constant, Am.cb- In this case, the surface charge density, and the phase boundary... 

These structural features are maintained more or less by all the ionophores in their complexed forms. However, mention should also be made of exceptions to these general structural principles. Thus, in the case of some ionophores, cation complexes of other than 1 1 stoichiometry have also been described in which the formation of sandwich -type arrangements is favored... 

Figure 5 also shows the effect of the ionophore concentration of the Langmuir type binding isotherm. The slope of the isotherm fora membrane with 10 mM of ionophore 1 was roughly three times larger than that with 30 mM of the same ionophore. The binding constant, K, which is inversely proportional to the slope [Eq. (3)], was estimated to be 4.2 and 11.5M for the membranes with 10 mM and 30 mM ionophore 1, respectively. This result supports the validity of the present Langmuir analysis because the binding constant, K, should reflect the availability of the surface sites, the number of which should be proportional to the ionophore concentration, if the ionophore is not surface active itself In addition, the intercept of the isotherm for a membrane with 10 mM of ionophore 1 was nearly equal to that of a membrane with 30 mM ionophore 1 (see Fig. 5). This suggests the formation of a closest-packed surface molecular layer of the SHG active Li -ionophore 1 cation complex, whose surface concentration is nearly equal at both ionophore concentrations. On the other hand, a totally different intercept and very small slope of the isotherm was obtained for a membrane containing only 3 mM of ionophore 1. This indicates an incomplete formation of the closest-packed surface layer of the cation complexes due to a lack of free ionophores at the membrane surface, leading to a kinetic limitation. In this case, the potentiometric response of the membrane toward Li+ was also found to be very weak vide infra).

To evaluate the contribution of the SHG active oriented cation complexes to the ISE potential, the SHG responses were analyzed on the basis of a space-charge model [30,31]. This model, which was proposed to explain the permselectivity behavior of electrically neutral ionophore-based liquid membranes, assumes that a space charge region exists at the membrane boundary the primary function of lipophilic ionophores is to solubilize cations in the boundary region of the membrane, whereas hydrophilic counteranions are excluded from the membrane phase. Theoretical treatments of this model reported so far were essentially based on the assumption of a double-diffuse layer at the organic-aqueous solution interface and used a description of the diffuse double layer based on the classical Gouy-Chapman theory [31,34]. 

Figure 16(a) (O) shows the EMF responses of a 1,2-dichloroethane membrane containing anionic sites (KT/ C1PB). A Nernstian response was obtained. An SHG response to KCl was observed at activities of the latter above 10 M [Fig. 16(b), O]-These results can be interpreted in the same way as for ionophore-incorporated PVC liquid membranes, for which we have shown that the concentration of oriented cation complexes at the liquid-liquid interface can explain both the observed SHG signal and EMF response. The present SHG responses thus suggest primary ion concentration dependent charge separation at the interface of the 1,2-dichloroethane membranes incorporated with ionic sites. 

J.-P. Bourgeois, P. Seiler, M. Fibbioli, E. Pretsch, F. Diederich, L. Echegoyen, Cyclophane-Type Fullerene-dibenzo [18] crown-6 Conjugates with trans-1, trans-2, and trans-3 Addition Patterns Regioselective Templated Synthesis, X-ray Crystal Structure, Ionophoric Properties, and Cation-Complexation-Dependent Redox Behavior , Helv. Chim Acta 1999, 82,1572-1595. 

Wipff, G., Troxler, L. (1994) MD Simulations on Synthetic Ionophores and on their Cation Complexes Comparison of Aqueous / non-Aqueous Solvents, in Wipff, G. (eds.), Computational Approaches in Supramolecular Chemistry, Kluwer, Dordrecht, pp 319-348. 

The structures of synthetic pyran-based ionophores have been investigated. m-2-Alkyl-3-oxytetrahydropyrans form useful subunits for the preparation of new types of ionophores with C2 symmetry <2005T8177>. X-Ray crystallography of some of these compounds provided useful information on solid-state conformational preferences that can be related to the cation-complexation properties in solution. In a related study, the synthesis of 18-32-membered cyclic pyran-based compounds of the type shown (50 and 51) was described <1996TL343, 1995JA12649>. Studies of these compounds focused on structural elements important for control of the shape and cation-binding ability and the structures of several of the compounds were determined by X-ray crystallography. 

For the seemingly simplest case of spherical metal ion complexation, the hole-size fit often, but not necessarily, holds. Fig. 2.5 illustrates the classical case where the cavity diameter of an ionophore determines the selectivity of cation complexation according to its radius [31]. As long as sufficient contact between the metal ion and the donor atom of the hgand is possible, the complexation free energy will be just a linear function of the number of such interactions and their donor quality... 

A large number of optodes developed for the selective detection of inorganic anions and cations, so-called ion-selective optodes (see Table 13.2), consist of polymer membranes that contain transducers. The latter are mostly physically admixed, but in some cases they are covalently bound to the polymer matrix. Most of these optodes [7, 8] are based on poly(vinyl chloride), plasticized with DOS, BBPA, DOP, o-NPOE or other plasticizers (see Chart 13.3). Typically, membranes are composed of 33 wt.% PVC, 66 wt% plasticizer, and 1 wt% ionophore (analyte-complexing agent) and lipophilic salt (ion-exchanger). Other polymers occasionally employed in hydrophobic optodes include polysiloxanes and poly(vi-... 

Painter. G.R. Pressman, B.C. Cation complexes of the monovalent and polyvalent carboxylic ionophores Lasalo-cid (X-537A). monensin, A23187 (calcimycin). and related antibiotics. Met. Ions Biol. Syst. 1985.19. 229-294. 

Generally, two different modes of transmembraneous transport have been established the carrier and the channel mechanism. The ionophores considered here act by the carrier mechanism. They form discrete antibiotic cation complexes at one interface of the membrane which then migrate across the membrane to the other interface where the metal ion is released. This kind of transport is displayed by the depsipeptide-type antibiotics which form positively charged complexes with metal ions. This is also true for the macrotetrolide nactins whereas the open-chain polyether antibiotics of the nigericin family mainly lead to electrically neutral metal ion complexes by dissociation of their carboxyl group. For the latter type of carriers, the ion transport of metal ions is coupled with a transfer of protons in the opposite direction. 

As we have seen, X-ray studies of the ionophorous antibiotics and their cation complexes were able to explain many of the steric factors that determine the selectivity patterns shown by these ligands. However, more systematic investigations on the relationship between host-cavity size and guest-ion radius could only be carried out using simpler synthetic ligands as models. In 1967, Pedersen reported the synthesis and complexing properties of a new class of compounds named crown ethers which are able to mimic effectively their natural counterpieces. 

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