Energy dissipation in water is very fast and more efficient than in many other liquids. This behavior is commonly attributed to the intermolecular interactions associated with hydrogen bonds. Here, we investigate the dynamic energy flow in the hydrogen-bond network of liquid water by a pump-probe experiment. We resonantly excite intermolecular degrees of freedom with ultrashort single-cycle terahertz pulses and monitor its Raman response. By using ultrathin sample-cell windows, a background-free bipolar signal whose tail relaxes mono-exponentially is obtained. The relaxation is attributed to the molecular translational motions, using complementary experiments and force-field and ab initio molecular dynamics simulations. They reveal an initial coupling of the terahertz electric field to the molecular rotational degrees of freedom whose energy is rapidly transferred, within the excitation pulse duration, to the restricted-translational motion of neighboring molecules. This rapid energy transfer may be rationalized by the strong anharmonicity of the intermolecular interactions.Water is a major substance on the earth surface. Its diverse anomalous properties make life on our planet viable. Notably, its large heat capacity turns oceans and seas into giant heat reservoirs for regulating the earth climate. In living organisms, the same property makes water a superb thermal buffer for the function of bio-chemical reactions 1,2,3 . These thermodynamic peculiarities are commonly attributed to water's ability to form an intermolecular complex network which is based on thermally fluctuating hydrogen (H) bonds. Interestingly, as each water molecule forms on average close to four H-bonds with ~1ps lifetime in an almost tetrahedral configuration, 4,5,6 the three-dimensional network of H-bonded water molecules encompasses complex collective/cooperative intermolecular degrees of freedom with a very diverse dynamics 7 .

Whereas there is increasing evidence for ion-induced protein destabilization through direct ion-protein interactions, the strength of the binding of anions to proteins relative to cation-protein binding has remained elusive. In this work, the rotational mobility of a model amide in aqueous solution was used as a reporter for the interactions of different anions with the amide group. Protein-stabilizing salts such as KCl and KNO3 do not affect the rotational mobility of the amide. Conversely, protein denaturants such as KSCN and KI markedly reduce the orientational freedom of the amide group. Thus these results provide evidence for a direct denaturation mechanism through ion-protein interactions. Comparing the present findings with results for cations shows that in contrast to common belief, anion-amide binding is weaker than cation-amide binding.

We study the interaction of the guanidinium cation, a widely used protein denaturant, with amide groups, the common structural motif of proteins. Our results provide evidence for direct contact between guanidinium and ∼2 amide groups, but the interaction is transient and weaker than for other cations with high charge-density.

Ion–protein interactions are important for protein function, yet challenging to rationalize owing to the multitude of possible ion–protein interactions. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy shows that salts affect the peptide's reorientational dynamics, with a more pronounced effect for denaturing cations (Li+, guanidinium (Gdm+)) and anions (I−, SCN−) than for weakly denaturing ones (K+, Cl−). The effects of Gdm+ and Li+ were found to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm+ and Li+ to triglycine, yet with a different binding geometry: While Li+ predominantly binds to the C‐terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm+. This bidentate binding markedly affects peptide conformation, and may help to explain the high denaturation activity of Gdm+ salts.