Transport Group IIA ions

Group IIA ion transport,. 569 Stabilization image dyes photography. 112 silver halides... 

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. 

The ability of crown ethers to bind selectively to particular Group IA and Group IIA metal ions, because of the relationship between hole size and metal ion radius, has led to considerable interest in them in relation to membranes (models for selective ion transport), antibiotics (similar polyether structure), organic synthesis [solubilization of inorganic reagents leading to milder routes for oxidation (122), nucleophilic substitution (123), fluoridation (90)] and extraction of alkali... 

Some 20 years ago it was observed that certain antibiotics could induce the movement of aqueous K+ ions into the mitochondria of cells, but not that of aqueous Na+ ions. These antibiotics, many of which are naturally occurring, are termed ionophores, i.e. neutral molecules which can mediate the transport of the essential groups IA and IIA cations across biological membranes.76 The essential features of an ionophore are a highly polar interior, a hydrophobic exterior and conformational flexibility. Many are cyclic peptides, the coordination properties of the cyclic molecules are considerably different to those of the linear peptides. These differences are outlined in Chapter 20.2. 

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