Transfer via Transmembrane Channels

The results obtained on coupled transport processes stress the role of cocarrier systems capable of transporting several substrates and driven by physical and chemical energy sources. 

Transmembrane channels represent a special type of multi-unit effector allowing the passage of ions or molecules through membranes by a flow or site-to-site hopping mechanism. They are the main effectors of biological ion transport. Natural and synthetic peptide channels (gramicidin A, alamethicin) allowing the transfer of cations have been studied [6.66-6.68]. 

Artificial cation channels could give fundamental information on the mechanism of cation flow and channel conduction [6.69, 6.70]. A solid-state model of cation transfer inside a channel is provided by the crystal structure of the KBr complex of 27c (Y = Y = CH3), which contains stacks of macrocycles with cations located alternately inside and above a macrocyclic unit, like a frozen picture of cation propagation through the channel defined by the stack [6.71]. 

A polymeric stack of macrocycles has been synthesized [6.72] and a cyclodextrin-based model of a half-channel has been reported [6.73]. Channel-type conduction of Na+ ions has been reported for a tris-macrocyclic ligand [6.74]. A derivative of the acyclic polyether ionophore monensin forms lithium channels in vesicles [6.75a], which may be sealed by diammonium salts [6.75b]. 

Electron channels, as transmembrane wires, represent the channel-type counterpart to the mobile electron carriers discussed above and will be considered in Section 8.3.2. Anion channels may also be envisaged. 

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