Correlation between macroscopic and microscopic relaxation dynamics of water: Evidence for two liquid forms.

Water is vital for life, and without it, biomolecules and cells cannot maintain their structures and functions. The remarkable properties of water originate from its ability to form hydrogen-bonding networks and dynamics, which the connectivity constantly alters because of the orientation rotation of individual water molecules. Experimental investigation of the dynamics of water, however, has proven challenging due to the strong absorption of water at terahertz frequencies. In response, by employing a high-precision terahertz spectrometer, we have measured and characterized the terahertz dielectric response of water from supercooled liquid to near the boiling point to explore the motions. The response reveals dynamic relaxation processes corresponding to the collective orientation, single-molecule rotation, and structural rearrangements resulting from breaking and reforming hydrogen bonds in water. We have observed the direct relationship between the macroscopic and microscopic relaxation dynamics of water, and the results have provided evidence of two liquid forms in water with different transition temperatures and thermal activation energies. The results reported here thus provide an unprecedented opportunity to directly test microscopic computational models of water dynamics.

Interfacial layers between ion and water detected by terahertz spectroscopy.

Dynamic fluctuations in the hydrogen-bond network of water occur from femto- to nanosecond timescales and provide insight into the structural/dynamical aspects of water at ion-water interfaces. Employing terahertz spectroscopy assisted with molecular dynamics simulations, we study aqueous chloride solutions of five monovalent cations, namely, Li, Na, K, Rb, and Cs. We show that ions modify the behavior of the surrounding water molecules and form interfacial layers of water around them with physical properties distinct from those of bulk water. Small cations with high charge densities influence the kinetics of water well beyond the first solvation shell. At terahertz frequencies, we observe an emergence of fast relaxation processes of water with their magnitude following the ionic order Cs > Rb > K > Na > Li, revealing an enhanced population density of weakly coordinated water at the ion-water interface. The results shed light on the structure breaking tendency of monovalent cations and provide insight into the properties of ionic solutions at the molecular level.

Probing Adaptation of Hydration and Protein Dynamics to Temperature.

Protein dynamics is strongly influenced by the surrounding environment and physiological conditions. Here we employ broadband megahertz-to-terahertz spectroscopy to explore the dynamics of water and myoglobin protein on an extended time scale from femto- to nanosecond. The dielectric spectra reveal several relaxations corresponding to the orientational polarization mechanism, including the dynamics of loosely bound, tightly bound, and bulk water, as well as collective vibrational modes of protein in an aqueous environment. The dynamics of loosely bound and bulk water follow non-Arrhenius behavior; however, the dynamics of water molecules in the tightly bound layer obeys the Arrhenius-type relation. Combining molecular simulations and effective-medium approximation, we have determined the number of water molecules in the tightly bound hydration layer and studied the dynamics of protein as a function of temperature. The results provide the important impact of water on the biochemical functions of proteins.

The origin and impact of bound water around intrinsically disordered proteins.

Proteins and water couple dynamically over a wide range of time scales. Motivated by their central role in protein function, protein-water dynamics and thermodynamics have been extensively studied for structured proteins, where correspondence to structural features has been made. However, properties controlling intrinsically disordered protein (IDP)-water dynamics are not yet known. We report results of megahertz-to-terahertz dielectric spectroscopy and molecular dynamics simulations of a group of IDPs with varying charge content along with structured proteins of similar size. Hydration water around IDPs is found to exhibit more heterogeneous rotational and translational dynamics compared with water around structured proteins of similar size, yielding on average more restricted dynamics around individual residues of IDPs, charged or neutral, compared with structured proteins. The on-average slower water dynamics is found to arise from excess tightly bound water in the first hydration layer, which is related to greater exposure to charged groups. The more tightly bound water to IDPs correlates with the smaller hydration shell found experimentally, and affects entropy associated with protein-water interactions, the contribution of which we estimate based on the dielectric measurements and simulations. Water-IDP dynamic coupling at terahertz frequencies is characterized by the dielectric measurements and simulations.

Long-range DNA-water interactions.

DNA functions only in aqueous environments and adopts different conformations depending on the hydration level. The dynamics of hydration water and hydrated DNA leads to rotating and oscillating dipoles that, in turn, give rise to a strong megahertz to terahertz absorption. Investigating the impact of hydration on DNA dynamics and the spectral features of water molecules influenced by DNA, however, is extremely challenging because of the strong absorption of water in the megahertz to terahertz frequency range. In response, we have employed a high-precision megahertz to terahertz dielectric spectrometer, assisted by molecular dynamics simulations, to investigate the dynamics of water molecules within the hydration shells of DNA as well as the collective vibrational motions of hydrated DNA, which are vital to DNA conformation and functionality. Our results reveal that the dynamics of water molecules in a DNA solution is heterogeneous, exhibiting a hierarchy of four distinct relaxation times ranging from ∼8 ps to 1 ns, and the hydration structure of a DNA chain can extend to as far as ∼18 Å from its surface. The low-frequency collective vibrational modes of hydrated DNA have been identified and found to be sensitive to environmental conditions including temperature and hydration level. The results reveal critical information on hydrated DNA dynamics and DNA-water interfaces, which impact the biochemical functions and reactivity of DNA.

Enabling High-Performance Surfaces of Biodegradable Magnesium Alloys via Femtosecond Laser Shock Peening with Ultralow Pulse Energy.

The fast degradation rate and poor wear resistance of magnesium (Mg) alloys in physiological environments have limited their potential usage as next-generation biodegradable orthopedic implant materials. In this work, femtosecond laser shock peening (fs-LSP) was successfully applied to simultaneously improve the surface mechanical, corrosion, and tribocorrosion properties of WE43 Mg alloys in blood bank buffered saline solution at body temperature. Specifically, the treated surfaces of WE43 Mg alloys via fs-LSP with ultralow pulse energy were investigated under different power densities, confining mediums, and absorbent materials. It was found that the combination of a black tape and a quartz layer gave the optimum peening effect under a power density of 28 GW/cm2, which simultaneously strengthened the surface and reduced the corrosion kinetics. In addition, a rapid self-repassivation was observed in fs-LSP-treated WE43 surfaces during tribocorrosion, promising sustained corrosion resistance under mechanical loading, critical to the reliability of load-bearing implants. Finally, the subsurface microstructural evolution and residual stress development in WE43 after fs-LSP were discussed based on the results from transmission electron microscopy analysis and finite element simulations.

Insights into Hydration Dynamics and Cooperative Interactions in Glycerol-Water Mixtures by Terahertz Dielectric Spectroscopy.

We report relaxation dynamics of glycerol-water mixtures as probed by megahertz-to-terahertz dielectric spectroscopy in a frequency range from 50 MHz to 0.5 THz at room temperature. The dielectric relaxation spectra reveal several polarization processes at the molecular level with different time constants and dielectric strengths, providing an understanding of the hydrogen-bonding network in glycerol-water mixtures. We have determined the structure of hydration shells around glycerol molecules and the dynamics of bound water as a function of glycerol concentration in solutions using the Debye relaxation model. The experimental results show the existence of a critical glycerol concentration of ∼7.5 mol %, which is related to the number of water molecules in the hydration layer around a glycerol molecule. At higher glycerol concentrations, water molecules dispersed in a glycerol network become abundant and eventually dominate, and four distinct relaxation processes emerge in the mixtures. The relaxation dynamics and hydration structure in glycerol-water mixtures are further probed with molecular dynamics simulations, which confirm the physical picture revealed by the dielectric spectroscopy.

High-Precision Megahertz-to-Terahertz Dielectric Spectroscopy of Protein Collective Motions and Hydration Dynamics.

The low-frequency collective vibrational modes in proteins as well as the protein-water interface have been suggested as dominant factors controlling the efficiency of biochemical reactions and biological energy transport. It is thus crucial to uncover the mystery of the hydration structure and dynamics as well as their coupling to collective motions of proteins in aqueous solutions. Here, we report dielectric properties of aqueous bovine serum albumin protein solutions as a model system using an extremely sensitive dielectric spectrometer with frequencies spanning from megahertz to terahertz. The dielectric relaxation spectra reveal several polarization mechanisms at the molecular level with different time constants and dielectric strengths, reflecting the complexity of protein-water interactions. Combining the effective-medium approximation and molecular dynamics simulations, we have determined collective vibrational modes at terahertz frequencies and the number of water molecules in the tightly bound and loosely bound hydration layers. High-precision measurements of the number of hydration water molecules indicate that the dynamical influence of proteins extends beyond the first solvation layer, to around 7 Å distance from the protein surface, with the largest slowdown arising from water molecules directly hydrogen-bonded to the protein. Our results reveal critical information of protein dynamics and protein-water interfaces, which determine biochemical functions and reactivity of proteins.

New Insights into the Dynamics of Zwitterionic Micelles and Their Hydration Waters by Gigahertz-to-Terahertz Dielectric Spectroscopy.

Gigahertz-to-terahertz spectroscopy of macromolecules in aqueous environments provides an important approach for identifying their global and transient molecular structures, as well as directly assessing hydrogen-bonding. We report dielectric properties of zwitterionic dodecylphosphocholine (DPC) micelles in aqueous solutions over a wide frequency range, from 50 MHz to 1.12 THz. The dielectric relaxation spectra reveal different polarization mechanisms at the molecular level, reflecting the complexity of DPC micelle-water interactions. We have made a deconvolution of the spectra into different components and combined them with the effective-medium approximation to separate delicate processes of micelles in water. Our measurements demonstrate reorientational motion of the DPC surfactant head groups within the micelles, and two levels of hydration water shells, including tightly and loosely bound hydration water layers. From the dielectric strength of bulk water in DPC solutions, we found that the number of waters in hydration shells is approximately constant at 950 ± 45 water molecules per micelle in DPC concentrations up to 400 mM, and it decreases after that. At terahertz frequencies, employing the effective-medium approximation, we estimate that each DPC micelle is surrounded by a tightly bound layer of 310 ± 45 water molecules that behave as if they are an integral part of the micelle. Combined with molecular dynamics simulations, we determine that tightly bound waters are directly hydrogen-bonded to oxygens of DPC, while loosely bound waters reside within 4 Å of micellar atoms. The dielectric response of DPC micelles at terahertz frequencies yields, for the first time, experimental information regarding the largest scale, lowest frequency collective motions in micelles. DPC micelles are a relatively simple biologically relevant system, and this work paves the way for more insight into future studies of hydration and dynamics of biomolecular systems with gigahertz-to-terahertz spectroscopy.