FT-IR study of the nature of K+, Rb+ and Cs+ cation motions in gramicidin A

Hydrated complexes of gramicidin A (gA) with K+, Rb+ and Cs+ cations were s tudied by FT-IR spectroscopy, The water bands observed in the FT-IR spectru m are identical as those obtained for the hydrated gA-Li+ and gA-Na+ comple xes studied previously, In the far infrared spectrum of the gA-K+ and gA-Rb + complexes an intense continuum and a strongly broadened band were found i ndicating large K+ and Rb+ polarizability in the gA channels. These K+ and Rb+ polarizabilities appear, similarly as in the case of the Li+ and Na+ co mplexes, as a result of the fast fluctuation of the K+ and Rb+ ions between two water molecules and four C=O groups of the gA backbone. The much small er barriers in the six minima Na+ potential explain the much larger mobilit y of Na+ when compared with the Li+ and other cations. (C) 2001 Elsevier Sc ience B,V. All rights reserved.