FTIR study of the nature of Na+ cation motion in gramicidin A

The hydrated complex of gramicidin A with Na⁺ cations was studied by FTIR spectroscopy. The water bands observed in the FTIR spectrum are identical with those obtained for the hydrated gA–Li⁺ complexes. In the far infrared spectrum of the gA–Na⁺ complex an intense continuum was found indicating large Na⁺ polarizability of the gA channels. This Na⁺ polarizability arises, similarly as in the case of the Li⁺ complex, due to the fast fluctuation of the Na⁺ ions between two water molecules and four CO groups of the gA backbone. The much smaller barriers in the six minima Na⁺ potentials explain the much larger mobility of the Na⁺ when compared with the Li⁺ ions. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 284–288, 1999