Ion-binding selectivity

Cu /Zn mixtures. This system constitutes one of the most stringent tests for ion-binding selectivity because both ions have identical charges, are of similar sizes, and exhibit high affinities toward amine ligands (see Chapter 6 for dative bonding interaction). 

The elucidation of the spatial structures of ionophores and their ion complexes is essential for understanding the detailed mechanisms of their biological action. From this point of view, the nature of the conformational rearrangements accompanying complex formation is of special interest, because these very conformational changes contribute considerably to ion binding selectivity. For that reason, this review will focus on comparisons between ligand conformations in both the com-plexed and the uncomplexed state whenever data on both are available. 

The sodium and ammonium salts of PVSA are soluble in water but insoluble in organic solvents (6). The calcium salt is insoluble. Potentiometric titration studies indicate that PVSA is a strong acid that ionizes completely in water. Ion binding selectivity with alkali metals has been observed in viscosity and phase separation studies. Mark-Houwink-Sakurada (MHS) parameters ofK = 2.22 and a = 0.65 have been obtained for sodium PVSA in 0.5 M NaCl at 25°C. 

In fact, self-assembly of 5 -GMP depends critically on the caption present Pinnavia et al. using nmr(36) and Laszlo et al. (34,37-39) using 23 a and nmr showed a potassium-selective interaction of alkali metal cations with the central cavity in the 5 -GMP tetramers. When sodium and potassium ions are present in conjunction, an organized structure [(G, K , G ), 4 Na ] forms(40). Potassium ions bind selectively to the inner (cavity) site, where they are sandwiched in between two GMP tetramers(40). These are maintained at a spacing close to 3.3 A which is characteristic of base stacking (41), while the oxygen distances... 

In these rather well-defined structures, sodium ions bind selectively to the outer sites, screening the electrostatic repulsion of the negatively charged phosphate groups(40). This results in the formation of a species with the (very likely) stoichiometry (G4,K, G ), 4 Na . They do so by true site-binding, whereas or Rb ions undergo only atmospheric condensation, on the outer sites. Li ions interact only very weakly(43), they are too small for the... 

Like the photosynthetic reaction center and bacteriorhodopsin, the bacterial ion channel also has tilted transmembrane helices, two in each of the subunits of the homotetrameric molecule that has fourfold symmetry. These transmembrane helices line the central and inner parts of the channel but do not contribute to the remarkable 10,000-fold selectivity for K+ ions over Na+ ions. This crucial property of the channel is achieved through the narrow selectivity filter that is formed by loop regions from thefour subunits and lined by main-chain carbonyl oxygen atoms, to which dehydrated K ions bind. 

To determine the selectivity scale of metal ions binding by pectins by means of pH measurements, we assumed that the exchange of the protons carried by carboxyl functions by metal ions involved a pH-decreasing which is more pronounced when the affinity of pectins for a given metal ion is higher. 

We now look at the values of the free M concentration and hence to the binding strength to selected A synthesised in the cell. The constants are closely common to all cells in their common compartment, their cytoplasm. The values, suited to metabolism, can be put in series in which Na+ and K+ bind poorly and only to a few of the weakest donors based on neutral O-donor centres while other metal ions bind more strongly to O, N and S donors of proteins or small organic molecules in a well-recognised order, i.e. in the Irving/Williams series (see Section 2.17) ... 

While ionophore-free membranes based on classical ion exchangers are still in use for the determination of lipophilic ions, such sensors often suffer from insufficient selectivity, as it is governed solely by the lipophilicity pattern of ions, also known for anions as the Hofmeister sequence. This pattern for cations is Cs+ > Ag+ >K+ > NH > Na+ > Li+ > Ca2+ > Mg2+ and for anions CIOT > SCN- > I > Sal- > N03- > Br > N02- > Cl- > OAc- HC03- > SO - > HPO4. While the ion exchanger fixes the concentration of hydrophilic analyte ions in the membrane on the basis of the electroneutrality condition within the membrane, the second key membrane component is the ionophore that selectively binds to the analyte ions. The selectivity of... 

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

Glutamate and sodium/lithium-induced conformational changes in the GLT-1 transporter have been detected by the altered accessibility of trypsin-sensitive sites to the protease (59). These experiments in GLT-1 shows that lithium can occupy at least one of the sodium ion binding sites, but lithium by itself cannot support coupled transport (59). Therefore, at least one of the sodium binding sites in GLT-1 discriminates between sodium and lithium. As described earlier, this contrasts with EAAC-1, where lithium is able to support uptake. It should therefore be possible to identify residues that are responsible for the sodium/lithium selectivity difference between EAAC-1 and GLT-1. 

Silver nitrate may be incorporated in the adsorbent slurry (25 g l-1) giving a final concentration of about 5% in the dry plate. The silver ions bind reversibly with the double bonds in the unsaturated compounds, resulting in selective retardation, and the lipids are separated according to the number and configuration (cis or trans) of their double bonds. This technique is extremely useful in fatty acids, mono-, di- and particularly triacylglycerol analyses when even positional isomers may be resolved. Borate ions may also be incorporated in the silica gel and these plates are used to separate compounds with adjacent free hydroxyl groups. 

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