ION-PAIR BINDING OF CA2- IONS IN MICELLAR-PACKAGED GRAMICIDIN-A( AND CL)

The two independent NMR experiments were performed to investigate the interaction between CaCl2 and the gramicidin A (GA) ion transport chan nel, using C-13-enriched GA and GA molecules incorporated into dodecyl phosphocholine (DPC) micelles. The chemical shifts of C-13 labeled car bonyl carbons vs. CaCl2 concentration demonstrate that Ca2+ and Cl- io ns interact as an ion pair within the GA structure with the Cl- ion lo cated near the position of the carbonyl group of the Trp(11) residue s ome 5.5 Angstrom from the mouth of the GA helix, and the Ca2+ ion boun d at the position of the carbonyl group of the Trp(15) residue some 2. 5 Angstrom from the entrance to the helical pore. The measurements of the Cl-35 line-widths and transverse relaxation times illustrate that the interaction occurs between Cl- ions and GA in DPC when in CaCl2 so lution, 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 divale nt Ca2+ ion; and Ca2+ and Cl- ions as a pair exchange rapidly with sit es of the GA dimer.