Cation ionophores

Cation-selective ionophores are the most successful in polymeric ISEs and selectivi-ties exceeding ten orders of magnitude became quite common. The cation-ionophore binding occurs dominantly due to Lewis interactions and could be understood in terms of hard and soft acid and bases theory (HSAB). While hard base oxygen atoms originate from ester, ether or carbonyl functionalities, and interact with hard acid alkaline cations, the softer sulfur or nitrogen atoms better bind with transition metal ions. Cation... 

Fig. 18a.8. Chemical structures of some commonly known cation ionophores used in the design of ion-selective electrodes.

Gad, S.C., Reilly, C., Siino, K.M. and Gavigan, F.A. (1985). Thirteen cationic ionophores Neurobehavioral and membrane effects. Drug Chemical Toxicol. 8 (6)451-468. 

Table 4 lists some antibiotics that function as ionophores, including some that bind Group IIA cations. Ionophores may be classed as channel formers or carriers by several approaches. A carrier ionophore is dependent on membrane fluidity and so cannot function below the transition temperature of the phospholipids in the membrane, as these are now frozen . The channel formers are much less dependent on membrane fluidity. A channel-forming ionophore cannot function if it fails to span the membrane, and so if an ionophore ceases to function in thicker membranes then it is probably a channel former. The kinetics of ion transport may provide a good indication of the type of ionophore. If an ionophore functions at a rate of more than 104 ions s 1 then it must be a channel former, as this level of ion flux cannot be provided by the diffusion of a carrier complex across the membrane. 

Gad, S. C. Reilly, C. Siino, K. Gavigan, F. A. Witz, G., (1985) Thirteen cationic ionophores their acute toxicity, neurobehavioral and membrane effects Drug Chem. Toxicol. 8, 451-468. 

The compounds calcimycin (A23187, 168) and nocobactin (187) contain the common feature of an oxazole ring but otherwise differ widely in functionality they are grouped together on the basis of their being cation ionophores. Calcimycin (A23187, 168) occurs in Streptomyces chartreusensis, from which it may be isolated as the mixed magnesium-calcium salt (89, 90). The structure of the free acid, a crystalline solid, was determined spectroscopically to be 168... 

Chaney MO, Demarco PV, Jones ND et al (1974) Structure of A23187, a divalent cation ionophore. J Am Chem Soc 96 1932-1933... 

Fluorescence microphotolysis can be used to measure diffusion in single cellsand nuclear envelope permeability. Pyranine probes have been used to measure internal pH changes in Escherichia coli membrane vesicles. Probe measurements of intralysosomal pH in living cells and perturbation of pH has been described by Ohkuma and Poole.Fluorescence polarization of six membrane probes in embryonal carcinoma cells have been measured by a cell sorter,Fluorescence of pancreatic islets labelled with fluorescein diacetate has been used to show the effects of cations, ionophores, and hypoglycaemic sulphonylureas. ... 

Green, D.P.L. 1978). The induction of the acrosome reaction in guinea pig sperm by the divalent metal cation ionophore A23187. J.Cell Sci. 52 137-151. 

There is compelling evidence to suggest that Ca + serves as a common mediator of platelet functional change. Thus, addition of the divalent cation ionophore, A23187, to a platelet... 

Furthermore, when the platelets were pretreated with ATP, which acts as a specific ADP antagonist (28), platelet shape change and the change in Ca-CTC fluorescence were both inhibited. It can also be seen that stimulation of platelet shape change by the divalent cation ionophore, A23187, was associated with a similar redistribution of intraplatelet Ca2. These findings provided evidence that platelet activation induced by ADP is mediated through an internal release of membrane bound Ca2+. 

Cation-Ionophore Interactions Quantification of the Factors Underlying Selective Complexation by Means of Theoretical Computations... 

Elks, M. L., Youngblood, W. W., and Kizer, J. S., 1979, Synthesis and release of serotonin by brain slices Effect of ionic manipulations and cationic ionophores, Brain Res. 172 461-469. 

SSHG measurements have also been performed at ion-selective membrane electrodes (ISE) in presence of the cations Li", and Na dissolved in the aqueous phase and several ionophores like dibenzo-24-crown-8 in a poly(vinyl chloride) (PVC) membrane [49]. In these experiments, the nonlinear optical signal has been observed as a function of the analyte concentration in the aqueous phase and has been ascribed to the oriented cation-ionophore complexes at the aqueous phase/liquid membrane. Langmuir isotherms were built from the data and the results suggested that a saturation of the interface was achieved at high analyte concentration. 

One approach that has been used to control stoichiometries is steric hindrance, as it is similarly discussed above for cation ionophores (Section 4.3). One of the nicest examples of this approach is the one of picket fence porphyrins. 

Cation Ionophore Slope (mV decade" ) Linear range (M) Most important interfering ions (log Ks i > -2) Detection limit (M) References... 

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