Wide-angle X-ray scattering and molecular dynamics simulations of supercooled protein hydration water (original) (raw)
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Dynamical Transition of Protein-Hydration Water
Physical Review Letters, 2010
Thin layers of water on biomolecular and other nanostructured surfaces can be supercooled to temperatures not accessible with bulk water. Chen et al. [Proc. Natl. Acad. Sci. U.S.A. 103, 9012 (2006)] suggested that anomalies near 220 K observed by quasielastic neutron scattering can be explained by a hidden critical point of bulk water. Based on more sensitive measurements of water on perdeuterated phycocyanin, using the new neutron backscattering spectrometer SPHERES, and an improved data analysis, we present results that show no sign of such a fragile-to-strong transition. The inflection of the elastic intensity at 220 K has a dynamic origin that is compatible with a calorimetric glass transition at 170 K. The temperature dependence of the relaxation times is highly sensitive to data evaluation; it can be brought into perfect agreement with the results of other techniques, without any anomaly.
Spectroscopy, 2010
This review article describes our neutron scattering experiments made in the past four years for the understanding of the single-particle (hydrogen atom) dynamics of a protein and its hydration water and the strong coupling between them. We found that the key to this strong coupling is the existence of a fragile-to-strong dynamic crossover (FSC) phenomenon occurring at around T L = 225 ± 5 K in the hydration water. On lowering of the temperature toward FSC, the structure of hydration water makes a transition from predominantly the high density form (HDL), a more fluid state, to predominantly the low density form (LDL), a less fluid state, derived from the existence of a liquid-liquid critical point at an elevated pressure. We show experimentally that this sudden switch in the mobility of hydration water on Lysozyme, B-DNA and RNA triggers the dynamic transition, at a temperature T D = 220 K, for these biopolymers. In the glassy state, below T D , the biopolymers lose their vital conformational flexibility resulting in a substantial diminishing of their biological functions. We also performed molecular dynamics (MD) simulations on a realistic model of hydrated lysozyme powder, which confirms the existence of the FSC and the hydration level dependence of the FSC temperature. Furthermore, we show a striking feature in the short time relaxation (β-relaxation) of protein dynamics, which is the logarithmic decay spanning 3 decades (from ps to ns). The long time α-relaxation shows instead a diffusive behavior, which supports the liquid-like motions of protein constituents. We then discuss our recent high-resolution X-ray inelastic scattering studies of globular proteins, Lysozyme and Bovine Serum Albumin. We were able to measure the dispersion relations of collective, intra-protein phonon-like excitations in these proteins for the first time. We found that the phonon energies show a marked softening and at the same time their population increases substantially in a certain wave vector range when temperature crosses over the T D . Thus the increase of biological activities above T D has positive correlation with activation of slower and large amplitude collective motions of a protein.
The Journal of …, 2008
A super-Arrhenius-to-Arrhenius dynamic crossover phenomenon has been observed in the translational R-relaxation time and in the inverse of the self-diffusion constant both experimentally and by simulations for lysozyme hydration water in the temperature range of T L ) 223 ( 2 K. MD simulations are based on a realistic hydrated powder model, which uses the TIP4P-Ew rigid molecular model for the hydration water. The convergence of neutron scattering, nuclear magnetic resonance and molecular dynamics simulations supports the interpretation that this crossover is a result of the gradual evolution of the structure of hydration water from a high-density liquid to a low-density liquid form upon crossing of the Widom line above the possible liquid-liquid critical point of water.
Modeling the Hydration Layer around Proteins: Applications to Small- and Wide-Angle X-Ray Scattering
Biophysical Journal, 2011
Small-/wide-angle x-ray scattering (SWAXS) experiments can aid in determining the structures of proteins and protein complexes, but success requires accurate computational treatment of solvation. We compare two methods by which to calculate SWAXS patterns. The first approach uses all-atom explicit-solvent molecular dynamics (MD) simulations. The second, far less computationally expensive method involves prediction of the hydration density around a protein using our new HyPred solvation model, which is applied without the need for additional MD simulations. The SWAXS patterns obtained from the HyPred model compare well to both experimental data and the patterns predicted by the MD simulations. Both approaches exhibit advantages over existing methods for analyzing SWAXS data. The close correspondence between calculated and observed SWAXS patterns provides strong experimental support for the description of hydration implicit in the HyPred model.
The Journal of Chemical Physics, 2013
In this work, we compare experimental data on myoglobin hydrated powders from elastic neutron scattering, broadband dielectric spectroscopy, and differential scanning calorimetry. Our aim is to obtain new insights on the connection between the protein dynamical transition, a fundamental phenomenon observed in proteins whose physical origin is highly debated, and the liquidliquid phase transition (LLPT) possibly occurring in protein hydration water and related to the existence of a low temperature critical point in supercooled water. Our results provide a consistent thermodynamic/dynamic description which gives experimental support to the LLPT hypothesis and further reveals how fundamental properties of water and proteins are tightly related.
Two dynamic crossovers in protein hydration water and their thermodynamic interpretation
2009
Studies of liquid water in its supercooled region have led to many insights into the structure and behavior of water. While bulk water freezes at its homogeneous nucleation temperature of approximately 235 K, for protein hydration water, the binding of water molecules to the protein avoids crystallization. Here we study the dynamics of the hydrogen bond (HB) network of a percolating layer of water molecules, comparing measurements of a hydrated globular protein with the results of a coarse-grained model that has been shown to successfully reproduce the properties of hydration water. With dielectric spectroscopy we measure the temperature dependence of the relaxation time of protons charge fluctuations. These fluctuations are associated to the dynamics of the HB network of water molecules adsorbed on the protein surface. With Monte Carlo (MC) simulations and mean-field (MF) calculations we study the dynamics and thermodynamics of the model. In both experimental and model analyses we find two dynamic crossovers: (i) one at about 252 K, and (ii) one at about 181 K. The agreement of the experiments with the model allows us
The Journal of Chemical Physics, 2009
The diffusive dynamics of hydration water in lysozyme is studied by high-resolution incoherent quasielastic neutron scattering spectroscopy and molecular dynamics ͑MD͒ simulations in a temperature range of 290 K Ͻ T Ͻ 380 K. The hydration level of the protein powder sample is kept at h = 0.35 gram of water per gram of dry protein to provide monolayer of water coverage on the protein surfaces. Two lysozyme samples, the H 2 O hydrated and the D 2 O hydrated, are measured in the experiments. The difference spectra of the two are used to extract the diffusive dynamics of the hydration water. The self-diffusion constant D of the hydration water is obtained from the analyses of the low-Q spectra. The Arrhenius plot of the inverse diffusion constant ͓i.e., log͑1 / D͒ versus 1 / T͔ shows a dynamic crossover from a super-Arrhenius behavior at low temperatures to an Arrhenius behavior at high temperatures bordered at T D = 345Ϯ 5 K. We also observe a pronounced increase in the migration distance d of the hydration water molecules above T D . We present evidence from the neutron scattering experiment that this dynamic crossover temperature in the hydration water coincides with that of the reversible denaturation of lysozyme determined by specific heat measurements. We further performed MD simulations of hydrated lysozyme powder to offer a plausible reason for this coincidence of the crossover phenomenon with the reversible denaturation of the protein.
Journal of Physics: Condensed Matter, 2008
Water's behavior differs from that of normal fluids, having more than sixty anomalies. Simulations and theories propose that many of these anomalies result from the coexistence of two liquid phases with different densities. Experiments in bulk water confirm the existence of two local arrangements of water molecules with different densities, but, because of inevitable freezing at low temperature T , cannot ascertain whether the two arrangements separate into two phases. To avoid the freezing, new experiments measure the dynamics of water at low T on the surface of proteins, finding a crossover from a non-Arrhenius regime at high T to a regime that is approximately Arrhenius at low T . Motivated by these experiments, Kumar et al (2008 Phys. Rev. Lett. 100, 105701) investigated, by Monte Carlo simulations and mean field calculations on a cell model for water in two dimensions (2D), the relation of the dynamic crossover with the coexistence of two liquid phases. They show that the crossover in the orientational correlation time τ is a consequence of the rearrangement of the hydrogen bonds at low T , and predict that: (i) the dynamic crossover is isochronic, i.e. the value of the crossover time τ L is approximately independent of pressure P; (ii) the Arrhenius activation energy E A (P) of the low-T regime decreases upon increasing P; (iii) the temperature T * (P) at which τ reaches a fixed macroscopic time τ * τ L decreases upon increasing P; in particular, this is true also for the crossover temperature T L (P) at which τ = τ L .