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.

Despite the widespread use of aqueous electrolytes as conductors, the molecular mechanism of ionic conductivity at moderate to high electrolyte concentrations remains largely unresolved. Using a combination of dielectric spectroscopy and molecular dynamics simulations, we show that the absorption of electrolytes at~0.3 THz sensitively reports on the local environment of ions. The magnitude of these high-frequency ionic motions scales linearly with conductivity for a wide range of ions and concentrations. This scaling is rationalized within a harmonic oscillator model based on the potential of mean force extracted from simulations. Our results thus suggest that long-ranged ionic transport is intimately related to the local energy landscape and to the friction for short-ranged ion dynamics: a high macroscopic electrolyte conductivity is thereby shown to be related to large-amplitude motions at a molecular scale.

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Correction for 'Quantifying transient interactions between amide groups and the guanidinium cation' by V. Balos et al., Phys. Chem. Chem. Phys., 2015, 17, 28539-28543.

To understand specific ion effects on a molecular level we explore the effect of salts on the rotational mobility of a model amide using dielectric spectroscopy. Based on our previous studies on the effect of strong denaturing anions or cations, here we study the additivity of the anionic and cationic effect. Using salts consisting of denaturing spherical anions and spherical cations we find such salts to affect the amide according to what one expects based on the additive activity of the individual ions. The guanidinium (Gdm) cation appears to be a notable exception, as our results suggest that GdmI (and accordingly GdmSCN) is less efficient in hindering the rotation of the amide than KI or GdmCl.