Ionophores sandwich complexation

The enniatins and beauveracin are cyclic hexadepsipeptides and so are 18-membered macrocycles built up of alternating amino acid and carboxylic acid residues. They exhibit lower selectivity than (137) and consequently are more broad spectrum ionophores. The structure of (138)KI has been solved544 but it was not possible to conclude, with certainty, whether the metal is entrapped in the central cavity, as in related cyclic polyether structures (Figure 14a), or whether it occupi.es a site between two adjacent ligand molecules to give an infinite sandwich (147) as has been found in the RbNCS complex of a synthetic ldlldl isomer of (138).545 Adducts of 1 2 sandwich and 2 3 club sandwich stoichiometry have been proposed for Cs+ complexes of (138),546a and the 1 2 K+ sandwich complex of (138) has been proposed as the species which transports K+ across lipid layers as it shields the metal effectively from solvent interactions.5461 ... 

The sodium cation forms 1 1 complexes with 60 — 64 and the potassium ion forms 1 2 intermolecular sandwich complex with the same ferrocenyl ionophores. Electro-... 

The well-developed and highly adaptable synthetic chemistry of ferrocene, together with its accessible ferrocene/ ferrocenium (Fc/Fc ) redox couple has led to its abundant use in redox-active receptors. The first organometallic cation receptors to be studied were the ferrocene-modified crown ethers of Saji 1 and Beer 2-4. The electron-rich crown ethers provide the cation-binding site—they are potent ionophores for group 1 and 2 metal cations. NMR (in methanol/acetone solution) and solid-state X-ray crystal structure studies demonstrated that 2-4 bind Na, K, and Cs. The binding stoichiometry varies depending on the receptor and cation used. Both Na and form 2 1 cation receptor complexes with 3, as does Na with 2. The ability of ferrocene to swivel around the CpFeCp axis also allows the formation of a 1 1 intramolecular sandwich complex, as seen in 2 Na, 2 K, 2 Cs and 3 K. ... 

Ovchinnikov. Yu.A. Sandwich complexes as a functional form of the enniatin ionophores. FEES (Fed. Eur. Bio-chem. Soc.) Lett. 1973. 36. 65-71. 

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... 

These theoretical advances have led to the development of potentiometric methods for quantifying fundamental membrane processes. The stoichiometry of the ionophore-ion complexation in the membrane phase can be determined by studying the effects of ionic sites on potentiometric selectivity. Such a study also reveals whether an ionophore serves as a neutral or a charged ionophore (64). Formation constants of the complexes with the corresponding stoichiometry can be determined from the unbiased selectivity coefficients or more directly by the sandwich method. Quantitative information about the complexation processes in the membranes, which eventually limits practical performances of the electrodes, will be useful for future design of selective ionophores. 

The mechanistic problems associated with the transport of cations (especially Na and K+) through membranes have continued to receive attention. The structures of macrocyclic alkali-metal complexes and the thermodynamics of their formation have been reviewed, as has the general question of the selectivity of the carriers towards the metals. The role of hydration energy has been considered and it has been suggested that, at least in the case of the enniatin ionophores, sandwich... 

M.M. Shultz, O.K. Stefanova, S.S. Mokrov, and K.N. Mikhelson, Potentiometric estimation of the stability constants of ion-ionophore complexes in ion-selective membranes by the sandwich membrane method theory, advantages, and limitations. Anal. Chem. 74, 510-517 (2002). 

Conversely, the fundamentals for the UDL he on the coextraction of counterions into the membrane therefore, the membrane is no longer permselective (Donnan failure) [9]. Ideally, when the ionophores are saturated by ions, the ion-ionophore complex functions as an ion-exchanger and the membrane shows an anion Nernstian response. The UDL can be estimated from the membrane composition, formation constant and coextraction coefficients obtained from the so-called sandwich membrane method [73]. 

Stoichiometries different from 1 i have been found for some ionophores which appear to form sandwiches or stacking structures with ions. For example, dibenzo-18-crown-6 forms 1 1 complexes with K, 1 1 and 2 1 complexes with Rb, and 2 1 and 3 2 complexes with Cs [65]. It is unlikely that this type of complex has any relevance for transport studies. 

Figure 33 Determination of the stability of ionophore complexes with the sandwich membrane method Only the left hand membrane contains ionophore.

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