Open-chained ionophores

Table 5. Stability constants of complex formation between murexide and several open chain ionophoric ligands with alkali cations...

Fig. 5 Synthetic, pyran-containing open-chained ionophores.

A second source of inspiration for studying the open-chained equivalents of crown ethers was the observation that a number of naturally occurring antibiotics enhance cation transport and bear a structural similarity to open-chained crown ethers. A number of groups have examined neutral synthetic ionophores and a variety of novel cation carriers is now available. This is discussed in Sect. 7.4, below. 

A second group of ionophores are considered to promote the formation of cylindrical channels through the membrane. The cation diffuses through the channel from one membrane surface to the other. The known channel-forming ionophores (the open-chain peptide derivative, gramicidin A, is one example) are non-cyclic species and, as such, lie outside the scope of this discussion. 

Other components of the design [138—142] include the choice of the crown size or even the use of open-chain metalloclefts, not necessarily polyether armed. The issue of special importance for extraction and transport applications is the selection of substituents ensuring a necessary lipo-philicity-solubility balance. For example, use of a cyclohexano moiety as shown above, instead of the more common o-phenylene bridge between nitrogens, enhances solubility in the membrane phase. The modification of the polyether chain with binaphthyl or calixarene substituents provides high membrane transport rates due to increased ionophore lipophilicity [138,142]. Some representative examples (initial fluxes, in 10 mol cm h , through o-nitrophenyloctyl ether-impregnated Accurel membrane 1 M source urea [138]) are as follows. 

Any classification scheme is necessarily arbitrary, but ionophores are often divided into two groups naturally occurring and synthetic. A subclassification distinguishes on the basis of open-chained (noncyclic) versus cyclic. Examples of the latter typically exceed the former because binding site preorganization and geometric complementarity are usually easier to achieve or ensure in a cyclic structure when the goal is to complex a spherical ion. 

Metal ion complexes of ionophores can be considered as host-guest complexes in which the guest entity is of spherical shape and entrapped in a cavity-like structure formed by the cyclic or open-chain host molecule. This cavity site can either be... 

Generally, two different modes of transmembraneous transport have been established the carrier and the channel mechanism. The ionophores considered here act by the carrier mechanism. They form discrete antibiotic cation complexes at one interface of the membrane which then migrate across the membrane to the other interface where the metal ion is released. This kind of transport is displayed by the depsipeptide-type antibiotics which form positively charged complexes with metal ions. This is also true for the macrotetrolide nactins whereas the open-chain polyether antibiotics of the nigericin family mainly lead to electrically neutral metal ion complexes by dissociation of their carboxyl group. For the latter type of carriers, the ion transport of metal ions is coupled with a transfer of protons in the opposite direction. 

Carboxy ionophores, produced by various streptomyces cultures and sometimes called ionophores of the nigericin subclass. They are open-chain polyethers containing tetrahydrofurane and/or tetrahydropyrane rings one end of the chain is terminated by a carboxy group and the other by one or two hydroxy groups. 

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