Potassium valinomycin complex

The sensor layer consists of a selective ionophore (e.g. valinomycin for potassium), a lipophilic anionic site (borate) and the cationic PSD. Before interaction with potassium, a lipophilic ion pair between the cationic PSD and borate anion is formed in the polymer layer. When valinomycin (also contained in the layer) selectively extracts potassium into the layer, then the positively charged valinomycin-potassium complex forms an ion pair with... 

Fig. 11. The valinomycin potassium complex (after (69)). The broken lines represent hydrogen bonds between peptide carbonyl atoms and NH groups shaded, the ester carbonyl atoms are coordinated to potassium in an octahedron...

With respect to the carrier mechanism, the phenomenology of the carrier transport of ions is discussed in terms of the criteria and kinetic scheme for the carrier mechanism the molecular structure of the Valinomycin-potassium ion complex is considered in terms of the polar core wherein the ion resides and comparison is made to the Enniatin B complexation of ions it is seen again that anion vs cation selectivity is the result of chemical structure and conformation lipid proximity and polar component of the polar core are discussed relative to monovalent vs multivalent cation selectivity and the dramatic monovalent cation selectivity of Valinomycin is demonstrated to be the result of the conformational energetics of forming polar cores of sizes suitable for different sized monovalent cations. 

Fig. 19. Space filling models of the molecular structure of the Valinomycin-potassium ion complex as originally determined.

Fig. 7.4. Conformations of (a) free valinomycin and (b) of its potassium complex. The carbonyl oxygen atoms, P, P, M and M are in especially exposed positions, so that they can initiate complexation of potassium ion. During complexation, hydrogen bonds 1 and 2 are broken, so that oxygen atoms R and R can take part in the co-ordination of the cation. Further smaller conformation changes allow oxygen atoms Q and Q to partake in formation of new hydrogen bonds, the molecule thus attaining the final round shape (see [44a ]). (By permission of the American Association for Advancement of Science.)...

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. 

Ohnishi, M., and Urry, D. W. Solution conformation of valinomycin-potassium ion complex. Science 168, 1091-1092 (1970). 

Fig. 2. The CPK molecular model of the valinomycin-potassium ion complex. The bare potassium ion is in an octahedral field made up of the polar acyl oxygens of the ester moieties. The side chains give a nonpolar exterior.

Fig. 1 Chemical structure of valinomycin (left). D-Val and L-Val are the D- and L-enantiomers of valine. D-Hyi is D-hydroxyisovaleric acid, and L-Lac is L-lactic acid. A solid-state structure of the potassium complex is shown on the right. (View this art in color at...

Pinkerton, Steinrauf, and Dawkins have determined the structure of the potassium complex of valinomycin (3). They, also, found that the... 

Rose MC, Henkens RW (1974) Stability of sodium and potassium complexes of valinomycin. Biophys Acta 372 426-435... 

Only rubidium whose ionic radius is close to that of potassium is bound at least as effectively as the latter. An X-ray structural analysis of the valinomycin-RbAuQ complex revealed essentially the same structure as in the complex, the octahedral cage of the oxygen atoms being by only 0.04 A wider due to minor rotations of the ester bonds... 

Fig. 6. a The primary structure of valinomycin [39] b the observed structure of the potassium complex of valinomycin, O carbon (D oxygen nitrogen... 

Valinomycin (3) is held by hydrogen bonds, all six peptide N-H groups acting as donors. In the potassium complex the acceptors are the amide... 

Neutral carriers are organic complexing agents which are capable of sequestering and transporting ionic species in a hydrophobic organic phase. The antibiotics, valino-mycin and nonactin were the first neutral carriers to be incorporated in an ISE These macrocyclic neutral carriers contain a polar internal cavity and an outer hydro-phobic shell. The excellent selectivity exhibited by valinomycin for potassium ions is... 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

These substances include primarily depsipeptides (compounds whose structural units consist of alternating amino acid and ar-hydroxy acid units). Their best-known representative is the cyclic antibiotic, valinomycin, with a 36-membered ring [L-Lac-L-Val-D-Hy-i-Valac-D-Val]3, which was isolated from a culture of the microorganism, Streptomyces fulvissimus. Figure 6.13 depicts the structure of free valinomycin and its complex with a potassium ion, the most important of the coordination compounds of valinomycin. 

An extensive range of synthetic analogues of valinomycin and the enniatins has been prepared and investigated these studies have been fully described in ai monograph.532 Synthetic peptides have also been prepared and their complexing properties investigated for example [L-Val-D-Pro-D-Val-L-Pro]3 reacts with potassium picrate to give an isolable 1 1 complex which is expected to have a structure similar to that of the K+ complex of (137).557 More detailed studies have been made on the complexation properties of the small synthetic peptides [L-Pro-Gly]3 and [L-Pro-Gly]4. 

Because the stability constant of its complex with potassium is much greater than that with sodium, valinomycin is a relatively specific potassium ionophore. In contrast, the mushroom peptide antamanide has a binding cavity of a different geometry and shows a strong preference for sodium ions.388,390 The structure of the Na+-antamanide complex is also shown in Fig. 8-22B. The Streptomyces polyether antibiotic monensin (Fig. 8-22D),389,391 a popular additive to animal feeds, is also an ionophore. However, its mode of action, which involves disruption of Golgi functions, is uncertain 392... 

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