Sodium binding sites of gramicidin A: sodium-23 nuclear magnetic resonance study

Abstract

In ethanol-water mixtures (90:10), the gramicidin dimer binds Na+ cations at well-defined sites, with a binding constant K = 4 M-1. Partial desolvation of Na+ occurs upon binding, as judged from the magnitude of the quadrupolar coupling constant (1.7 MHz) for bound Na. The binding sites are identified with the outer sites flanking the channel entrances. The rate constants for binding and release are k+ .ltoreq. 2.2 .times. 109 M-1 s-1 and k- .ltoreq. 5.5 .times. 108 s-1, respectively.