FT-IR study of the nature of the proton and Li+ motions in gramicidin A and C

Gramicidin A and C as well as their proton and Li+ complexes were studied by FT-IR spectroscopy. With an increasing number of hydrated and protonated channels, infrared continuous absorption is observed. It proves large proton polarizability due to very fast collective fluctuations and shifts of the excess proton in hydrogen-bonded single file water chains in the gramicidin channels. The infrared spectrum of the hydrated Li+ cation in the channels shows a continuum in the far-infrared region. This continuum is explained in terms of Li+ ion fluctuations and shifts between the electron lone pairs of two water molecules and at least three carbonyl O atoms of the backbone. The continuum indicates large Li+ polarizability of these channels. If the Lif-bonded system is polarized by an external electrical field, the diffusion of the Li+-water complex in field direction is favored.