Ionophores binding constants

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).

Attachment of carbonyl groups to crowns makes these products more akin structurally to the natural ionophore antibiotics such as valinomycin. The dioxo-derivative (179) of 18-crown-6 was prepared in 35% yield by condensation of tetraethylene glycol and diglycolic acid chloride in benzene at 50 °C for 48 hours (Izatt et al., 1977a and 1977b). This product gives binding constants for Na+, K+ and Ba2+ in methanol which are 102—104 times less stable than for the parent crown - the lower constants are a reflection of less favourable AH values for complexation in these... 

The ionophore should have a binding constant with the analyte that is neither too low, nor too high." ... 

Sodium permeabilities were found to be 62, 82, 126 and 158 ni /sec for 15, 22.5, 30 and 37.5 yM monensin respectively and lithium permeabilities were 12 uid 33 ni /sec for 400 and 800 yM monensin respectively. Thus, the permeabilities extrapolated to 1 yM of monensin for Ihe same don and lipid concentration are for Na 4.0 0.4 m /sec, for Li 0.035 4 0.005 nn sec. These results show that within the concentration range studied the sodium transport rate increases fairly linearly with the ionophore concentration, indicating that the dominant transporting species is a 1 1 complex of the sodium ionophore. The much higher value obtained for sodium either indicates that the complex association-dissociation processes determine the overall rate of transport or reflects the difference in the binding constants for these two ions. 

One such nonnatural function of natural ionophores is the complexation of organic ammonium cations for example, protonated amino acid esters. Since natural ionophores are chiral compounds, the process can be enantioselec-tive. Binding of ammonium cations by such receptors as monensin or lasalocid (Fignre 1) is efficient bnt lack the expected enantioselectivity. However, cyclic or podand-type derivatives of monensin like 1 or 2 (Figure 1) show significant enantioselectivity the ratios of binding constants of the R and S enantiomers of protonated methyl esters of phenylglycine, phenylalanine, and leucine to 2 are A r/ZCs = 5.1, 6.2, and 7.6, respectively. Crown-ether type derivatives like 1 show lower enantioselectivities. 

Figure 22 Increases in the stability of the complex between the target ion (here K+) and the ionophore increase selectivity bnt also reduce the npper detection limit of an ISE due to coion interference (binding constant of K+-ionophore complex a > b...

Effect of the ionophore-ionic site ratio on selectivity and determination of complex stoichiometries and binding constants... 

It is noteworthy that the calculated selectivity curves intersect with one another, which might not be expected intuitively. To explain this, consider the 1.5 1 ratio. On one hand, approximately half of the Ag+ is in the form of L2Ag+ complexes if the ionophore can form such a complex, and the free ionophore concentration and potentiometric selectivity are low. On the other hand, one-third of the ionophore is in its free form if only 1 1 complexes can form, and as a result of the relatively high free ionophore concentration the selectivity is high. The direct consequence is that for each curve (i.e., each set of binding constants) there are not multiple but there is... 

Clearly, the second relationship reflects the Hofmeister selectivity sequence (irais of higher lipophilicity are preferred over more hydrophilic ones), while the former predicts a selectivity that is additionally dictated by the binding constants between the ions and the ionophore. 

Fig. 9. Representation of the use of the modification of an ionophore s potentiometric response in order to detect antibody binding. A constant ion activity (in this case K ) must be maintained in the sample solution...

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

In biomedical applications, the ranges of ion concentration are higher by several orders of magnitude. For instance, the abovementioned calcium probes for living cells cannot be used because the dissociation constant is so low that they would be saturated. Special attention is thus to be paid to the ionophore moiety to achieve proper selectivity and efficiency of binding. For instance, at present there is a need for a selective fluorescent probe for the determination of calcium in blood which could work in the millimolar range in aqueous solutions so that optodes with immobilized probes on the tip could be made for continuous monitoring calcium in blood vessels. 

Because the stability constant of its complex with potassium is much greater than that with sodium, valinomycin is a relatively specific potassium ionophore. In contrast, the mushroom peptide antamanide has a binding cavity of a different geometry and shows a strong preference for sodium ions.388,390 The structure of the Na+-antamanide complex is also shown in Fig. 8-22B. The Streptomyces polyether antibiotic monensin (Fig. 8-22D),389,391 a popular additive to animal feeds, is also an ionophore. However, its mode of action, which involves disruption of Golgi functions, is uncertain 392... 

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