The effect of monovalent cations on the collective dynamics of water and on a model protein

Abstract

In this contribution we present experimental evidences of the ultrafast (sub-ps to ps) collective hydrogen bond dynamics of water in the extended hydration layers of alkali metal chlorides by using THz time domain spectroscopy (TTDS) in the terahertz frequency region (0.3–2.1THz; 10–70cm−1). The behaviour of the frequency dependent absorption coefficient (α), refractive index (n) and the dielectric relaxation response time (τ), obtained by fitting both real and imaginary parts of the permittivity (ε) using a triple Debye relaxation model are studied. The timescales obtained for bulk water are of the order of ~9ps, 200fs and 80fs. The relaxation time constant in the presence of the salt shows an accelerated reorientation dynamics of the water dipoles, which, in turn renders an indirect support for the ‘water structure breaking’ ability of monovalent ions, the effect being ion specific. We have also studied the effect of these ions on the secondary structure as well as hydration dynamics around a model protein bovine serum albumin (BSA). It is found that the structure and hydration of the protein as well as the salt hydration suffer negligible perturbation in the presence of the alkali metal chlorides.