Ionophore general structure

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... 

Numerous structurally diverse compounds have been found to act as ionophores within biological membranes The general structural features of several representative ionophorous agents are illustrated in Fig. 2. Heteroatoms which have been identified as liganding moieties by X-ray crystallography are filled in. 

In this study, the target molecules are acyclic oligoamides with the generalized structure shown in Figure 2. Within this structure, constituents R, X, Y, and Z, as well as the chain length, may be systematically varied. In addition, incorporation of appropriate substituents into X and Y may be utilized to enhance the propensity for formation of a pseudocyclic cavity. Effectiveness of these new ionophores as metal ion carriers can be evaluated by proton-driven cation transport from a weakly acidic aqueous solution through a liquid membrane into a more acidic aqueous solution (see Figure 3). 

We now proceed to more complicated ionophores in order to testify the validity of this extrathermodynamic relationship and its hypothetical interpretation as an attempt to understand the nature of supramolecular interactions more generally and deeply. The thermodynamic parameters are plotted in Figures 16-19 for long glymes, (pseudo)cyclic ionophore antibiotics, lariat ethers with donating side-arm(s), and bis(crown ethers), whose structural changes upon complexation are schematically illustrated in Figure 20. 

Figure 7.1 The general mode of action of ionophores in ion transport, (a) A channel formed by two gramicidin A molecules, N-terminus to N-terminus. (b) The sequence of events in the operation of a carrier ionophore such as valinomycin. Valinomycin is a cyclic peptide consisting of three repeating units with the structure shown...

Both in their general pattern of structure and mode of cation binding, 222 and 223, as well as many other naturally occurring ionophores, are similar to crown... 

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... 

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