Valinomycin-K+ complex

Fig. 21. Crystal structure of the Valinomycin-K+ complex. Reproduced with permission from Ref.100). This crystal structure confirmed within tenths of an Angstrom the structure derived previously in solution 97 98) and by means of conformational energy calculations...

The rate constant of the dissociation reaction usually becomes smaller with decreasing polarity of the solvent. For example, MI-NMR spectra of the valinomycin/K+ complex (80) and 13C-NMR spectra of the monensin/Na+ complex show a superposition of the spectra of the complexed and uncomplexed ligand when using an excess of the antibiotic in chloroform at 25 °C (104). 

Valinomycin-K complex has six carbonyl oxygen atoms in octahedral coordination around K. [From L. Stryer. Biochemistry. 4th ed (New York W. H. Freeman. 1995), p. 273.]... 

Since valinomycin transports K+ specifically, it must bind the ion rather than provide a channel, as does gramicidin. Since transport can occur only when the membrane is fluid and since valinomycin is a small molecule, this suggests that the valinomycin-K+ complex must move through the membrane i.e., it behaves as a mobile carrier. The hydrocarbon side chains of the cyclic structure can be imagined on the outside of the structure, thus making the latter compatible with the hydrocarbon portion of the lipid bilayer. The inside contains the 12 carbonyl groups of the ester and amide bonds, six of which coordinate the K+ in the space at the center of the structure (Fig. 6-19). 

A group of antibiotics (e.g., valinomycin, nigericin, and gramicidin A) transport cations across the cell membrane. Such agents, known as ionophores, are widely used to probe membrane structure and function. Ionophores uncouple oxidative phosphorylation. Valinomycin, a cyclic peptide (Figure 14-17), forms a lipid-soluble complex with K+ that readily passes through the inner membrane, whereas K+ by itself does not. In the valinomycin-K complex, hydrophobic groups, present on the outside, facilitate transport of the complex in the lipid environment ... 

K+, located on the inside, interacts with the hydrophilie groups. The matrix side of the inner membrane has a negative potential, so the positively charged valinomycin-K+ complex is drawn inward. The complex uncouples oxidative phosphorylation by decreasing the membrane potential. Unlike valinomycin, it exchanges H" " for K+ across the membrane. Its... 

The conformation of the valinomycin-K+ complex in solution has been determined by the combined use of physico-chemical methods such as ORD, IR, and... 

Elucidation of the complex conformation of antamanide has also been a gradual process. From the first analysis on the basis of IR, ORD, and NMR spectra, a bracelet conformation similar to the valinomycin-K+ complex was proposed, possessing all-... 

Figure 6 Stereoplot of the antibiotic ionophore valinomycin-K" complex.

Numerous spectroscopic studies and a conformational energy calculation of the solution conformation of the valinomycin-K complex are in good agreement... 

Fig. 3. Space-filling models of the valinomycin-K complex a view taken from the top of the molecule to demonstrate the optimum fit between host and guest, b view taken from one side of the complex showing the lipophilic periphery. Metal cation is completely covered and therefore not seen...

There is a danger that the lipophilic valinomycin/K complex may precipitate lipophilic anions such as the oxonol dyes and it is advisable to check on the calibration described above with another ionophore. The uncoupling agent FCCP is useftil in this respect as it selectively increases the H permeability of membranes. Tlic experimental protocol for measuring the potential using FCCP... 

Onishi, M., and Urry, D. W, Spectroscopic determination of the Valinomycin-K complex structure. Science, 168, 1091, 1970. 

No such increse of the merocyanine response was induced by use of the valinomycin-K complex, gramicidin or CCCP (not shown) It is suggested that at low concentrations of TPB, movements of TPB within the membrane increase the field change on the outer surface of the membrane but decrease it in the center of the membrane where carotenoids sense the field change. Under these conditions, the time response of the merocyanine (half time of about 5 ms) was fast enough to follow the potential change under illumination. 

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