Ion transport agents

Organic molecules forming complexes with sodium ions in water, such as monensin, speed up ion transport by factors of about 10, molecules that form tunnels by factors of 10 ° and more. Scheme 2.7.1 depicts two typical structures of ion transport agents, namely monensin and filipin. Gramicidine, on the other hand, is thought to form a pore in membranes. The only clear-cut distinction between an ion pore and an ion transporter is that a pore can be closed and reopened by stopper molecules, whereas ion transport will always occur as long as complexation in membranes and decomplexation in water can occur. 

The name was coined to describe the activity of compounds. such as valinomycin, a naturally occuning cyclic depsipeptide that transports ions across membranes efficiently and selectively. A number of natural ion transport agents have been identified. They regulate the concentrations of the predominant biological cations (Na", K , Mg, and Artificial ionophores were first... 

Fig. 44. Schematic examples of facUitated transport of gases and metal ions. The gas-transport example shows the transport of oxygen across a membrane using hemoglobin (HEM) as the carrier agent. The ion-transport example shows the transport of copper ions across the membrane using a Uquid...

Iron transport agents may belong to the protein or non-protein class. In the former group are found the animal proteins transferrin (25), lactoferrin (26) and conalbumin (27). The low molecular weight iron carrying compounds from microorganisms, the siderochromes, may occur with or without a bound metal ion. Typically, severe repression of biosynthesis of these substances can be expected to set in at an iron concentration of ca. 2 x 10-5 g atoms/liter (28). Most, but not all, of these substances can be described as phenolates or hydroxamates (4). 

Carbonic anhydrase inhibitors such as acetazolamide act in the proximal tubule. These drugs prevent the formation of H+ ions, which are transported out of the tubular epithelial cell in exchange for Na+ ions. These agents have limited clinical usefulness because they result in development of metabolic acidosis. 

The acute toxic properties of all the organochlorine pesticides in humans are qualitatively similar. These agents interfere with inactivation of the sodium channel in excitable membranes and cause rapid repetitive firing in most neurons. Calcium ion transport is inhibited. These events affect repolarization and enhance the excitability of neurons. The major effect is central nervous system stimulation. With DDT, tremor may be the first manifestation, possibly continuing to convulsions, whereas with the other compounds convulsions often appear as the first sign of intoxication. There is no specific treatment for the acute intoxicated state, and management is symptomatic. 

In the future, salt-binding receptors will be employed in various separation and sensing applications. The work described in this chapter demonstrates that ditopic receptors, with an ability to bind the salts as contact ion-pairs, have particularly attractive properties as extraction and transport agents. Another future direction is the utilization of salts as molecular glue to assemble complex supramolecular structures that have dynamic properties and the capability to behave as molecular machines. 

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