Ion pair binding of Ca~(2+) and Cl~- ions in micellar-packaged gramicidin A

摘要

The two independent NMR experiments were performed to investigate the interaction between CaCl2 and the gramicidin A (GA) ion transport channel, using 13C-enriched GA and GA molecules incorporated into dodecylphosphocholine (DPC) micelles. The chemicals shifts of C-13 labeled carbonyl carbons vs. CaCl2 concentration demonstrate that Ca2+ and Cl? ions interact as an ion pair within the GA structure with the Cl? ion located near the position of the carbonyl group of the Trp11 residue some 5.5 ? from the mouth of the GA helix, and the Ca2+ ion bound at the position of the carbonyl group of the Trp15 residue some 2.5 ? from the entrance to the helical pore. The measurements of the 35Cl line-widths and transverse relaxation times illustrate that the interaction occurs between Cl? ions and GA in DPC when in CaCl2 solution, that no interaction is detected between Cl? ions and GA in DPC when in NaCl solution, and that the interaction between Cl? ions and GA in DPC when in MgCl2 solution is much weaker than in CaCl2 solution. In short, a Cl? ion can enter the GA when it is paired with a divalent Ca2+ ion; and Ca2+ and Cl? ions as a pair exchange rapidly with sites of the GA dimer.