Water has a number of anomalous physical properties, and some of these become drastically enhanced on supercooling below the freezing point. Particular interest has focused on thermodynamic response functions that can be described using a normal component and an anomalous component that seems to diverge at about 228 kelvin (refs 1-3). This has prompted debate about conflicting theories that aim to explain many of the anomalous thermodynamic properties of water. One popular theory attributes the divergence to a phase transition between two forms of liquid water occurring in the 'no man's land' that lies below the homogeneous ice nucleation temperature (TH) at approximately 232 kelvin and above about 160 kelvin, and where rapid ice crystallization has prevented any measurements of the bulk liquid phase. In fact, the reliable determination of the structure of liquid water typically requires temperatures above about 250 kelvin. Water crystallization has been inhibited by using nanoconfinement, nanodroplets and association with biomolecules to give liquid samples at temperatures below TH, but such measurements rely on nanoscopic volumes of water where the interaction with the confining surfaces makes the relevance to bulk water unclear. Here we demonstrate that femtosecond X-ray laser pulses can be used to probe the structure of liquid water in micrometre-sized droplets that have been evaporatively cooled below TH. We find experimental evidence for the existence of metastable bulk liquid water down to temperatures of 227(-1)(+2) kelvin in the previously largely unexplored no man's land. We observe a continuous and accelerating increase in structural ordering on supercooling to approximately 229 kelvin, where the number of droplets containing ice crystals increases rapidly. But a few droplets remain liquid for about a millisecond even at this temperature. The hope now is that these observations and our detailed structural data will help identify those theories that best describe and explain the behaviour of water.

The dynamic properties of nanoparticles suspended in a supercooled glass forming liquid are studied by x-ray photon correlation spectroscopy. While at high temperatures the particles undergo Brownian motion the measurements closer to the glass transition indicate hyperdiffusive behavior. In this state the dynamics is independent of the local structural arrangement of nanoparticles, suggesting a cooperative behavior governed by the near-vitreous solvent.

Elastic incoherent neutron scattering has been used to study the temperature dependence of the mean-square displacements of nonexchangeable hydrogen atoms in powders of a series of homomeric polypeptides (polyglycine, polyalanine, polyphenylalanine and polyisoleucine) in comparison with myoglobin at the same hydration level (h = 0.2). The aim of the work was to measure the dynamic behavior of different amino acid residues separately and assess the contribution of each type of side chain to the anharmonic dynamics of proteins. The results provide direct experimental evidence that the first anharmonic activation, at approximately 150 K, is largely due to methyl group rotations entering the time window of the spectrometer used; however, contributions on the order of 10-20% from the motions of other groups (e.g., the phenolic ring and the methylene groups) are present. Our data also indicate that the dynamical transition occurring at approximately 230 K can be attributed, at least at the hydration level investigated, mainly to motions involving backbone fluctuations.

Two main onsets of anharmonicity are present in protein dynamics. Neutron scattering on protein hydrated powders revealed a first onset at about 150 K and a second one at about 230 K (the so called dynamical transition). In order to assess the molecular origin of protein anharmonicity, we study different homomeric polypeptides by incoherent elastic neutron scattering, thus disentangling the contribution of different molecular groups in proteins. We show that methyl group rotations are the main contributors to the low temperature onset. Concerning the dynamical transition, we show that it also occurs in absence of side chains; however, the presence and mobility of side chains substantially increases the fluctuations amplitude without influencing the transition temperature. We also investigate the role of hydration on the anharmonic contributions. Our study shows that methyl group rotations are unaffected by hydration and confirms that the dynamical transition is suppressed in dry samples. In hydrated samples, while the pure backbone contribution does not depend on the hydration h at h > or = 0.2, in the presence of side chains the anharmonic fluctuations involved in the dynamical transition are enhanced by increasing the water content.

The X-ray Correlation Spectroscopy instrument is dedicated to the study of dynamics in condensed matter systems using the unique coherence properties of free-electron lasers. It covers a photon energy range of 4-25 keV. The intrinsic temporal characteristics of the Linac Coherent Light Source, in particular the 120 Hz repetition rate, allow for the investigation of slow dynamics (milliseconds) by means of X-ray photon correlation spectroscopy. Double-pulse schemes could probe dynamics on the picosecond timescale. A description of the instrument capabilities and recent achievements is presented.

P rotein dynamics is characterized by molecular motions occurring on a very large time-scale ranging from femtoseconds (vibrations) to seconds (long-range molecular diffusion). Within this broad interval, motions occurring in the pico-to nanoseconds time scale are of particular interest and biological relevance since they cover the transition region from "discrete" local excitations of small molecular subunits to slower processes involving cooperative motions of larger parts of the macromolecular assembly.1 This time window is exactly covered by neutron scattering techniques, in view of the typical instrumental energy resolution of neutron spectrometers.Relevant information on protein dynamics has been obtained by investigating the temperature dependence of the mean square displacements (msd's) of relevant protein atoms. In fact, neutron scattering 2 and a variety of techniques, including M€ ossbauer spectroscopy, 3 optical spectroscopy 4 and molecular dynamics (MD) simulations, 5,6 evidenced a steep increase of atomic msd occurring in the temperature range of 180À220 K and marking a harmonic to anharmonic transition upon increasing temperature. It is now widely accepted that protein dynamics is actually characterized by two anharmonic onsets:The first one occurs in the 100À150 K region, does not depend on the hydration level of the protein, and is largely attributable to methyl group rotations entering the time scale accessible by the instrumental resolution. 7À9The second one occurs at ∼220 K and is observed only in samples hydrated above a critical threshold (typically ∼0.2 g of water/g of protein). This second onset is known as the "protein dynamical transition". It is strongly coupled with solvent dynamics since it is suppressed in dry proteins and enhanced as hydration increases; moreover it can be substantially reduced when the proteins are embedded in confining matrices. 10À12Although its occurrence is clearly established, the physical origin of the protein dynamical transition is still a matter of discussion: in particular, it is highly debated whether it corresponds to a kind of "glass transition" occurring in the system or to a resolution effect due to thermally activated motions entering the finite time window covered by the spectrometer. 13,14The second onset of anharmonic dynamics is deemed necessary for enzyme activity and protein function; 15 however, the above statement has been questioned, since counterexamples of residual enzymatic activity in the absence of dynamical transition have been reported; 16,17 moreover, the dynamical transition has also been observed in denatured protein samples and in short synthetic peptides with neutron scattering, 18,19 terahertz spectroscopy, 20 and NMR. 21 The presence of a dynamical transition occurring at ABSTRACT: We give experimental evidence that the main features of protein dynamics revealed by neutron scattering, i.e., the "protein dynamical transition" and the "boson peak", do not need the protein polypeptide chain. We show that a rapid increase of hyd...

By X-ray photon correlation spectroscopy we quantify the influence of elasticity and viscosity on the capillary wave (CW) surface dynamics of a supercooled liquid. To fit the data a novel model combining Maxwell-Debye and Voigt-Kelvin viscoelasticity is derived yielding a saturation of relaxation rates at high q as well as an offset in the CW dispersion relation. Diffuse X-ray scattering confirms the result and data taken on the surface of supercooled polypropylene glycol (PPG-4000) evidence a low-frequency elastic plateau of the storage modulus. A possible connection between the observed solid-like response and the supercooled state is discussed.

X-ray photon correlation spectroscopy (XPCS) was employed to measure the time-dependent intermediate scattering function in an organic molecular glass former. Slow translational dynamics were probed in the glassy state and the correlation functions were calculated from two-dimensional speckle patterns recorded by a CCD detector. The image frames were analysed using a droplet algorithm together with an event correlation scheme. This method provides results analogous to standard intensity correlation algorithms but is much faster, hence addressing the recurrent problem of insufficient computing power for online analysis in XPCS. The event correlator has a wide range of potential future applications at synchrotrons and free-electron laser sources. research papers J. Appl. Cryst. (2012). 45, 807-813 Y. Chushkin et al. Event correlation in XPCS 811 Figure 6(a) The correlation function g (2) of the 2800 images measured at 220 K. Squares show the standard scheme, circles represent the event correlator, and dashed lines are fits. (b) The event correlation function g (2) of the 2800 images (circles) and the time-binned image series (squares).