Of ionophore

Dietary adrninistration of ionophores is coupled with the use of anaboHc steroid implants to maximize rate and efficiency of gain in growing catde. Effects of ionophores and anaboHc steroid implants are generally additive. 

The total world market for the use of ionophores for feed efficiency improvement in mminants is approximately 80— 90 million. The United States is the largest market. Lasalocid and monensin are the only members of this class cleared for use. Outside the United States, salinomycin is used in limited quantities. Worldwide usage is about 1.5 million kg. 

Pressman, B., 1976. Biological applicadons of ionophores. Annual Review of Biochemistry 45 501-530. 

Table 8. Stability constants and competitive transport ability of ionophores (50-57) for potassium and sodium cations...

A certain crown ether having additional coordination sites for a trasition metal cation (71) changes the transport property for alkali metal cations when it complexes with the transition metal cation 76) (Fig. 13). The fact that a carrier can be developed which has a reversible complexation property for a transition metal cation strongly suggests that this type of ionophore can be applied to the active transport system. 

To achieve the transport of ions against their concentration gradients, the reversible change in the nature of ionophores at the both interfaces of a membrane is necessary, and for this object, many ingenious devices in the structure of ionophores and the transport systems have recently been developed. 

Painter, G. R., Pressman, B. C. Dynamic Aspects of Ionophore Mediated Membrane Transport, ibid., p. 83M10... 

Dynamic aspects of ionophore mediated membrane transport. G. R. Painter and B. C. Pressman, Top. Curr. Chem., 1982,101, 83-110 (105). 

The measurement of change in the surface potentials of aqueous solutions of electrolytes caused hy adsorption of ionophore (e.g., crown ether) monolayers seems to he a convenient and promising method to ascertain selectivity and the effective dipole moments of the ionophore-ion complexes created at the water surface. 

It was found in later work that it is precisely the idea of ionic hydration that is able to explain the physical nature of electrolytic dissociation. The energy of interaction between the solvent molecules and the ions that are formed is high enough to break up the lattices of ionophors or the chemical bonds in ionogens (for more details, see Section 7.2). The significance of ionic hydration for the dissociation of electrolytes had first been pointed out by Ivan A. Kablukov in 1891. 

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

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).

Anionic effects were observed by FT-IR-ATR spectrometry with a membrane containing not ionophore 2 but a different kind of ionophore, ETH 129 [13]. When the lipophilic counteranion, SCN, was used for the primary ion solutions, the spectra from both the complexed cation and corresponding counteranion were seen, of which the stoichiometric ratio was nearly equal to that of ETH 129 complex-SCN salt at relatively high concentrations of the primary ion solutions. With KTpCIPB in the membrane. 

FIG. 6 Dependence of the square root of the SHG intensity ( I(2a>)) for membrane 2 without KTpCIPB (a) with KTpCIPB (b) on K+ ion concentrations in the adjacent aqueous solution containing KCl (O) and KSCN ( ), respectively. Inset The corresponding observed EMF to KCl and KSCN. The concentrations of ionophore 2 and KTpCIPB were 3.0 X 10 M and 1.0 x 10 M, respectively for both SHG and EMF measurements. The data points present averages for three sets of measurements. Error bars show standard deviations. (From Ref. 15.)... 

Use of ionophore-incorporated membranes leads thus to the same conclusions as described above for the ionophore-free membranes. Here too, the SHG measurements suggest that a permanent, primary ion-dependent charge separation at the liquid-liquid interface, and therefore a potentiometric response, is only possible when the membrane contains ionic sites. 

The above SHG studies exhibited several most important facts in the response mechanisms occurring at the surfaces of ionophore incorporated liquid membrane ISEs. 

We recently synthesized several reasonably surface-active crown-ether-based ionophores. This type of ionophore in fact gave Nernstian slopes for corresponding primary ions with its ionophore of one order or less concentrations than the lowest allowable concentrations for Nernstian slopes with conventional counterpart ionophores. Furthermore, the detection limit was relatively improved with increased offset potentials due to the efficient and increased primary ion uptake into the vicinity of the membrane interface by surfactant ionophores selectively located there. These results were again well explained by the derived model essentially based on the Gouy-Chapman theory. Just like other interfacial phenomena, the surface and bulk phase of the ionophore incorporated liquid membrane may naturally be speculated to be more or less different. The SHG results presented here is one of strong evidence indicating that this is in fact true rather than speculation. 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

The polarity of the interior of the channel, usually lower than in the case of ionophores, often prevents complete ion dehydration which results in a decrease in the ion selectivity of the channel and also in a more difficult permeation of strongly hydrated ions as a result of their large radii (for example Li+). 

Gomez-Puyon, A., and C. Gomez-Lojero, The use of ionophores and channel formers in the study of the function of biological membranes, in Current Topics in Bioenergetics, Vol. 6, p. 221, Academic Press, New York, 1977. 

It is now recognised that a wide range of organic molecules, collectively termed ionophores 185,186) or complexones 187), are able to facilitate ion (usually cation) transport. Two major mechanisms have been revealed for this process, namely the involvement of transmembrane ion carriers and transmembrane pores or channels (see Fig. 19). The majority of ionophores studied to date are natural antibiotics and their synthetic analogues which are, on a biological scale, comparatively small molecules lending themselves to study outside the biological system. In contrast far less is known about the molecular structures involved in normal transport processes. Such molecules are likely to be more complex or present in small amounts and may require... 

Theoretical insight into the interfacial charge transfer at ITIES and detection mechanism of this type of sensor were considered [61-63], In case of ionophore assisted transport for a cation I the formation of ion-ionophore complexes in the organic (membrane) phase is expected, which can be described with the appropriate complex formation constant, /3ILnI. 

Solvent polymeric membranes conventionally consist of ionophore, ion exchanger, plasticizer, and polymer. The majority of modem polymeric ISEs are based on neutral carriers, making the ionophore the most important membrane component. Substantial research efforts have focused on the development of highly selective ionophores for a variety of analytes [3], Some of the most successful ionophores relevant to biomedical applications are depicted in Fig. 4.1. 

Size-related problems may become important for all microsensors. Leakage of sensing materials from a small membrane may lead to rapid deterioration of sensor properties [104], While the lipophilicity of membrane components cannot be increased infinitely, immobilization of ionophore and ion exchanger in the polymer by covalent attachment or molecular imprinting along with utilization of plasticizer-free membranes could help solve the leakage problem. 

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