Abstract-The Decisive Role of Hydrophobicity on the Effect of Alkylammonium Chlorides on Protein Stability: A Terahertz Spectroscopic Finding

Debasish Das Mahanta, Rajib Kumar Mitra, and Nirnay Samanta

http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.7b04088?journalCode=jpcbfk

Many biologically important processes involve a subtle interplay between Columbic and hydrophobic interactions among molecular groups with water. A comprehensive understanding of such processes, specially while occurring simultaneously in the same molecule is of practical importance. In this contribution we report the ultrafast (sub-ps to ps) collective hydrogen bond dynamics of water in the extended hydration layers in a series of alkylammonium chloride salts using THz time domain spectroscopic (TTDS) technique (0.3-1.6 THz (10-55 cm-1)). We found the THz absorption coefficient (α) of the salt solutions systematically vary with the salt type. We obtain the hydrogen bond relaxation dynamics by fitting the frequency dependent dielectric constants in a multiple Debye dielectric relaxation model. We found these salts to transform from being a water ‘structure breaker’ to ‘structure maker’ with increasing carbon content. We also investigate their effect on a model protein ‘bovine serum albumin’ and found systematic trend towards disrupting the protein secondary structure. The associated changes in the protein hydration in presence of these salts have also been investigated using TTDS.