This study reports the first direct observation of multiple occupancy of the gramicidin A channel by Tl+ ions. 205Tl NMR has been used to study the equilibrium binding of Tl+ by gramicidin A incorporated in sonicated dimyristoylphosphatidylcholine vesicles. It is shown that only multiple-channel occupancy can account for the 205Tl chemical shifts measured. The data are analyzed to yield the equilibrium association constants of 450-600 and 5-20 M-1 for the binding of the first and the second ions at 34 degrees C, respectively.
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