Hydrogen bonding ionophores

In this section, om recent approaches based on the use of hydrogen-bonding ionophores [22,23,34,35] are described to achieve the phosphate selectivity in two-phase-transfer systems. 

Biihlmann, P. Amemiya. S. Nishizawa, S. Xiao. K.P. Umezawa, Y. Hydrogen-bonding ionophores for inorganic anions and nucleotides and their application in chemical sensors. J. Incl. Phenom. Mol. Recognit. Chem. 1998. 32, 151-163. 

Shioya, T., S. Nishizawa, and N. Teramae, Anion recognition at the liquid-liquid interface. Sulfate transfer across the 1,2-diehloroethane-water interface fadlitated by hydrogen-bonding ionophores, 7 Am Chem Soc, Vol. 120, (1998) p. 11534. 

A basic property of an ionophore is that it is capable of forming a structure with a lipophilic exterior and polar cavity, as depicted in the scheme of the structure of valinomycin in fig. 7.4. The ionophore cavity must contain less than 12 and preferably 5-8 polar groups. The final complex structure must be relatively stable, which can be attained by strengthening with hydrogen bonds. It should not, however, be too rigid if ion exchange is to be sufficiently rapid [153, 193]. 

When the ion-pair partitioning is indicated in the quadrant diagram (below) it becomes obvious that a circle of equilibria is present. Knowing the octanol pKa, the log P and the aqueous pKa should allow one to calculate the partition coefficient of the ion pair. From these equilibria one can write that the difference in log P between the acid and its salt is the same as the difference between the pKa s (Equation 9). The closer the pKa s, the more lipid soluble the ion pair will be, relative to the acid. Internal hydrogen bonding or chelation that stabilizes an ion pair will affect the octanol stability more than the aqueous stability, where it is less needed, and so will decrease the delta pKa. Chelation should therefore favor biolipid solubility of ion pairs. Ultimate examples are available in some ionophores. This is one of the properties of some of the herbicides I pointed out earlier. 

Uncomplexed valinomycin has a more extended conformation than it does in the potassium complex.385,386 The conformational change results in the breaking of a pair of hydrogen bonds and formation of new hydrogen bonds as the molecule folds around the potassium ion. Valinomycin facilitates potassium transport in a passive manner. However, there are cyclic changes between two conformations as the carrier complexes with ions, diffuses across the membrane, and releases ions on the other side. Tire rate of transport is rapid, with each valinomycin molecule being able to carry 104 potassium ions per second across a membrane. Tlius, a very small amount of this ionophore is sufficient to alter the permeability and the conductance of a membrane. 

Ionophores, or polyether (PET) antibiotics, produced by various species of Streptomyces, possess broad spectrum anticoccidial activities. They are chemically characterized by several cyclic esters, a single terminal carboxylic acid group, and several hydroxyl groups. Representative members of this class include salinomycin (SAL), monensin (MON), lasalocid (LAS), narasin (NAR), maduramicin (MAD), and semduramicin (SEM). The main chemical properties of interest in the extraction methodology are their low polarities and instability under acidic conditions. They are able to form stable complexes with alkaline cations. All of these compounds, with the exception of LAS, bind monovalent cations (e.g., Na+ and K+). Lasalocid has a tendency to form dimers and can form complexes with divalent cations such as Mg2+ and Ca2+. The formation of metal complexes results in all of these compounds adopting a quasi-cyclic formation consequent to head-to-tail hydrogen bonding. No MRLs have yet been set by the EU for any of the carboxylic acid PETs (98). 

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