Ion interactions in (1-13C)d-Val8 andd-Leu14 analogs of gramicidin A, the helix sense of the channel and location of ion binding sites

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Abstract
Ion-induced chemical shifts in the carbonyl carbon resonances of synthesized and verified (1-13C)d-Val8 gramicidin A and (1-13C)d-Leu14 gramicidin A are utilized in combination with the previously determined location of the ion binding sites of the gramicidin A channel (using the carbonyls ofl-residues) to determine that the helix sense of the gramicidin A channel is left-handed. Having resolved the handedness issue, the location of the ion binding sites (which are fundamental to understanding the mechanism of ion transport) are further delineated with the results indicating two sites separated by just over 20 Å. Furthermore, the demonstration that the divalent barium ion interacts at the binding site while not being transported through the channel is used to argue that the mechanism of monovalentvs. divalent cation selectivity is due to the positive image force contribution to the central barrier.