Valinomycin complex

Stability constants, measured in methanol solution, for alkaline earth complexes of a number of ionophores are given in Table XVI (280,289,571-577).8 The values for the complexes of valinomycin and enniatin B lie between the values for the crown ethers 15C5 and 18C6 (cf. Section II.C.5 above), for the middle four entries the values are slightly higher. Stabilities of enniatin B complexes show a modest maximum for Ca2+, and of valinomycin complexes show stabilities increasing up to Ba2+ (281). LogAi values for the Ca2+ complexes of acetate, benzoate, and salicylate are between 4.5 and 4.7 in methanol (578) - the... 

Fig. 7.5. Dependence of the stability constants of valinomycin complexes of alkali metal ions on the cation radius. (After A. Hofmanovd et al. [79].)...

Fig. 5.2 Ionophores the Na+-monensin complex [2] left) and K+-valinomycin complex [3] right, top and side views)...

Figure 8.30. Potassium-valinomycin complex. The potassium ion is embedded in the oxygen-rich inner part of the valinomycin moiety, whereas the hydrophobic side chains stretch out into the lipophilic part of the membrane. Valinomycin is composed of three units of the sequence L-lactate, L-valine, o-hydroxyisovalerate, and D-valine.

Ion binding cyclopeptides have also been described which are structurally related to the depsipeptides, enniatin and valinomycin. Complex formation of the... 

The valinomycin study just described illustrates the value of close comparison of NMR and X-ray diffraction data. Correlation of NMR assignments with X-ray data for known structures may allow the subsequent elucidation of conformational features for related materials on the basis of NMR data alone. It is notoriously difficult to relate solid-state structures determined by X-ray diffraction to the solution conformation observed by conventional NMR spectroscopy, although data from the former technique are often the only type available for work on receptor recognition. However, solid-state NMR may provide a link between these two techniques by indicating whether molecular conformation changes do occur when the physical state is altered. In contrast to the valinomycin complexes discussed above, it has been found that significant differences in the solution and solid-state spectra of morphine... 

Figure 24. Stereodiagram of the K -valinomycin complex showing the circular bracelet conformation of the peptide backbone (Neupert-Laves and Dobler, 1975).

The strength of the ion-dipole interaction depends not only on the M+-C =O distance but also on the angle between these groups. In the enniatin, beauvericin, complexes the angles are < 180 (unfavorable), and the shifts are much smaller than in the valinomycin complex, despite the comparable M -C =0 distance. Replacement of K+ by the smaller ion Na+ in the beauvericin cavity allows an angle closer to 180 to be obtained and the shifts are larger on complexation (1.7 and 2.3 ppm downfield). 

The stability constant, Kgtab, of the potassium-valinomycin complex in ethanol is rather high (ca. 10 /M vs 10 VM with Na ), hence the conformation of the K complex in solution is rigid, therefore stable and well defined. The geometry of the complex has been determined by the combined use of spectroscopic methods in several laboratories of which here only the results of... 

FIGU RE 1.2 Ovchinnikov s (1979) estimated stability constants for valinomycin complexes (in methanol solvent) with a series of class (a) metals and one class (b) metal (Ag ) were plotted against the AN(AIP)" metric. The AN(AIP) metric combines the influences of ion size or inertia and atomic ionization potential (Newman et al. 1998). As is clear in this figure, valinomycin is very selective for K+ and bonding with the one class (b) metal ion is more stable than might have been expected from the class (a) metal ion pattern. 

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