Ionophores metal complexes

It was the first naturally occurring crown ether, and the earliest for which the antibiotic activity could be traced to its ionophoric properties (4). The actins give one-to-one complexes with many alkali and alkaline earth metal ions, with nonactin showing selectivity in the order NH4+ > K+ = Rb+ > Cs+ > Na+ > Ba2+ (5). The first X-ray crystal structure of an ionophore-metal complex was obtained from the nonactin-potassium thiocyanate complex (6). The potassium is bound by coordination to all four tetrahydrofuran oxygens and to the four ester carbonyl oxygens, which creates overall a "tennis ball seam" conformation to the carbon framework as it wraps around the metal. 

Shen, j. Brodbelt, J. S. Characterization of ionophore-metal complexes by infirared multiphoton photodissociation and collision activated dissociation in a quadrupole ion trap mass spectrometer. Analyst 2000, 125, 641-650. 

Anthraquinone, 1-hydroxy-calcium aluminum chelate compound, 1,2 metal complexes dyes, 6,86 Antiarthritis drugs labelled gold compounds, 6, 969 metal complexes, 6,758 Antibiotic M139163,2, 974 Antibiotics ionophoric, 6, 553 metal complexes selective binding, 6, 552... 

Naturally, very long T,c values are expected for solids as viewed from the expected correlation time at the low temperature side of the T c minimum (0.1-0.2 s) as shown in Figure 1. Indeed, their values turns out to be the order of 10-30 s for carbon sites in the absence of internal fluctuations as in polysaccharides such as (1 — 3)-p-D-glucan and (1 —> 3)-p-D-xylan,46 8 fibrous proteins such as collagen49 and silk fibroin,50 free and metal-complexed ionophores,51 or in some instances up to 1000 s as in crystalline polyethylene.52... 

Some synthetic ionophores form complexes with metallic ions containing a varying number of ligands per metal ion, usually 1 or 2. It then holds for the overall concentration of ion J in the membrane that... 

Valinomycin (137) is a macrocyclic dodecadepsipeptide and has shown an unequalled K+/Na discrimination.5" As a consequence of this prime ionophorous behaviour many studies have been carried out on both the free ligand and on its metal complexes, and here discussion is restricted to the nature of the metal complexes. 

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). 

The types of alkali and alkaline earth metal complexes subjected to molecular mechanics modeling fall into four categories crown ethers1255-2 ], cryptands1262 263 , spherands[264 2651, and other biologically important ligands such as ionophores and cyclic antibiotics1266-2691. 

Until the late 1960s, whereas there had been considerable interest in the transition metal complexes of natural and synthetic macrocyclic ligands (1—4), relatively few reports described complexes of alkaline earth and more particularly alkali metal cations. Research in this area was stimulated by the recognition of the importance of the biological role of Na+, K, Ca2 , and Mg2 and also the discovery and characterization of the natural antibiotic ionophores (5, 6). These macrocyclic antibiotics, such as valinomycin and nonactin, were shown to complex alkali metal cations with remarkable selectivity (7-9). 

Fig. 1 Partition of metal ions and counterions at the polymer (PVC)-supported ionophore liquid-membrane/aqueous-solution interface relation with interfacial boundary potentials. C rz + and Cx- are concentrations of the ionophore-metal-ion complex and its counterions, respectively. These concentrations were measured with FT-IR-ATR to a depth of pm range in the organic phase near the interface. When CMn+>nCx, the observed membrane potential f exhibits a Nernstian response that meets Eq. 2. When the value of 11+ is small but still greater than nCx, the anion effect distorted the log C-AE relation observed for a non-Nemstian response. When...

Perhaps the most apparent similarities with transition metal complexes are the main group metal ions and their supramolecular complexes. Here, the progression from the chelate to macrocyclic to cryptate effect can be readily recognized and the thermodynamics are, for the most-part, established. The term ionophore is often used for this class of hosts, originating from its use in biology for lipid-soluble molecules that serve as vehicles for transporting ions across membranes. 

The mechanistic problems associated with the transport of cations (especially Na and K+) through membranes have continued to receive attention. The structures of macrocyclic alkali-metal complexes and the thermodynamics of their formation have been reviewed, as has the general question of the selectivity of the carriers towards the metals. The role of hydration energy has been considered and it has been suggested that, at least in the case of the enniatin ionophores, sandwich... 

Group 1. The complexation reactions between murexide and Li+, Na and K+ in DMSO-MeCN mixtures have been studied by Li NMR spectroscopy. C1 NMR spectroscopy has been used to study ion pairing between CL or [CIOJ and alkali metal complexes of ionophore antibiotics. ... 

Extraction data of alkali metals with the picrate method have shown that pyridinyl compounds are better ionophores than their A-oxide counterparts (deleterious role of hydrogen bonding between A-oxide functionalities and water molecules). The highest phase-transfer values are observed for cone conformers, where selectivity follows the order Na" " > K" " > Rb" " > Cs" " > Li. In aprotic solvents A-oxide ligands are much stronger complexers, and alkali and lanthanide metal complexes have been prepared and characterized. The Eu(III) and Tb(III) complexes of tetra-A-oxide cone conformer are fluorescent upon UV light excitation at 312 nm. Unfortunately, these complexes totally lose their luminescence upon addition of water, indicating their modest stability in aqueous medium [56,57]. 

Yuan and Shao investigated the kinetics of several alkali metal ion transfers facilitated by DB18C6 at the water/DCE nano-interfaces. Their measurements yielded the rate constant for potassium transfer similar to that reported in ref. 13. Well shaped steady-state voltammograms were also obtained for other alkali metal cations, but the kinetic parameters determined for Li+, Rb and Cs+ showed significant correlation with the pipet radius. A similar approach was used by the same group to measure the kinetics of alkali metal transfers across the water/DCE interface facilitated with A-(2-tosylamino)-isopentyl-monoaza-15-crown-5. The association constants were measured for alkali metal complexes in DCE, and the selectivity of this ionophore was shown to follow the sequence... 

Until the later 1960 s there has been considerable interest in the transition metal complexes of natural and synthetic ionophores (Lindoy, 1975). Reports on alkaU and alkaline-earth complexes began to appear in the mid-60 s and research in this area was stimulated by the synthesis of macrocychc polyethers and cryptands. An introductory overview is presented by Weber and Vogtle (1981) while analytical applications are described by Kolthoff (1979), Blasius and Janzen... 

Metal complexes with podands are usually less stable and more labile than the corresponding coronates and cryptates. This is also true for rare earth ions, although the macrocyclic and macrobicyclic effects tend to be smaller than those observed for M(I) and M(II) ions, which have a smaller cationic charge. Some rare earth podates and complexes with naturally occurring ionophores are listed in table 6. 

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