Valinomycin potassium complex, conformation

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. 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). 

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... 

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