At low temperatures, proteins exist in a glassy state, a state that has no conformational flexibility and shows no biological functions. In a hydrated protein, at temperatures տ220 K, this flexibility is restored, and the protein is able to sample more conformational substates, thus becoming biologically functional. This ''dynamical'' transition of protein is believed to be triggered by its strong coupling with the hydration water, which also shows a similar dynamic transition. Here we demonstrate experimentally that this sudden switch in dynamic behavior of the hydration water on lysozyme occurs precisely at 220 K and can be described as a fragile-to-strong dynamic crossover. At the fragile-to-strong dynamic crossover, the structure of hydration water makes a transition from predominantly high-density (more fluid state) to lowdensity (less fluid state) forms derived from the existence of the second critical point at an elevated pressure.glass transition ͉ liquid-liquid transition ͉ protein dynamics ͉ quasi-elastic neutron scattering W ithout water, a biological system would not function.Dehydrated enzymes are not active, but a single layer of water surrounding them restores their activity. It has been shown that the enzymatic activity of proteins depends crucially on the presence of at least a minimum amount of solvent water (1, 2). It is believed that Ϸ0.3 g of water per g of protein is sufficient to cover most of the protein surface with one single layer of water molecules and to fully activate the protein functionality. Thus, biological functions (3) such as enzyme catalysis can only be understood with a precise knowledge of the behavior of this single layer of water and how that water affects conformation and dynamics of the protein. The knowledge of the structure and dynamics of water molecules in the so-called hydration layer surrounding proteins is therefore of utmost relevance to the understanding of the protein functionality. It is well documented that at low temperatures proteins exist in a glassy state (4, 5), which is a solid-like structure without conformational flexibility. As the temperature is increased, the atomic motional amplitude increases linearly initially, as in a harmonic solid. In hydrated proteins, at Ϸ220 K, the rate of the amplitude increase suddenly becomes enhanced, signaling the onset of additional anharmonic and liquid-like motion (6-9). This ''dynamical'' transition of proteins is believed to be triggered by their strong coupling with the hydration water through the hydrogen bonding. The reasoning is derived from the finding that the protein hydration water shows some kind of dynamic transition at a similar temperature (10, 11). Here we demonstrate, through a high-resolution quasielastic neutron scattering (QENS) experiment, that this dynamic transition of hydration water on lysozyme protein is in fact the fragile-to-strong dynamic crossover (FSC) at 220 K, similar to that recently observed in confined water in cylindrical nanopores of silica materials (12, 13). Computer simulat...

The formation of equilibrium clusters has been studied in both a prototypical colloidal system and protein solutions. The appearance of a low-Q correlation peak in small angle scattering patterns of lysozyme solution was attributed to the cluster-cluster correlation. Consequently, the presence of long-lived clusters has been established. By quantitatively analyzing both the SANS (small angle neutron scattering) and NSE (neutron spin echo) data of lysozyme solution using statistical mechanics models, we conclusively show in this paper that the appearance of a low-Q peak is not a signature of the formation of clusters. Rather, it is due to the formation of an intermediate range order structure governed by a short-range attraction and a long-range repulsion. We have further studied dynamic features of a sample with high enough concentration at which clusters are formed in solution. From the estimation of the mean square displacement by using short-time and long-time diffusion coefficient measured by NSE and NMR, we find that these clusters are not permanent but have a finite lifetime longer than the time required to diffuse over a distance of a monomer diameter.

Neutron spin echo (NSE) and small angle neutron scattering (SANS) were used to investigate the correlation between structure and short-time dynamics of lysozyme solutions in the presence of protein clusters as previously reported. It was found that, upon increasing protein concentration, the selfdiffusion coefficient at the short time limit becomes much smaller than that of the corresponding hard-sphere and charged colloidal suspensions at the same volume fraction. Contrary to literature conclusions, we find that, at relatively low concentrations, the system consists mostly of monomers or dimers, while, at high concentrations, large dynamic clusters dominate. Our results will benefit the understanding of colloidal systems with both a short-range attraction and an electrostatic repulsion that are ubiquitous in many biologically relevant systems.

Water-based detergent systems offer several advantages, over organic solvents, for the cleaning of cultural heritage artifacts in terms of selectivity and gentle removal of grime materials or aged varnish, which are known to alter the readability of the painting. Unfortunately, easel paintings present specific characteristics that make the usage of water-based systems invasive. The interaction of water with wood or canvas support favors mechanical stresses between the substrate and the paint layers leading to the detachment of the pictorial layer. In order to avoid painting loss and to ensure a fine control (layer by layer) of grime removal, water-based cleaning systems have been confined into innovative chemical hydrogels, specifically designed for cleaning water-sensitive cultural heritage artifacts. The synthesized hydrogels are based on semi-interpenetrating chemical poly(2-hydroxyethyl methacrylate)/poly(vinylpyrrolidone) networks with suitable hydrophilicity, water retention properties, and required mechanical strength to avoid residues after the cleaning treatment. Three different compositions were selected. Water retention and release properties have been studied by quantifying the amount of free and bound water (from differential scanning calorimetry); mesoporosity was obtained from scanning electron microscopy; microstructure from small angle X-ray scattering. To demonstrate both the efficiency and versatility of the selected hydrogels in confining and modulating the properties of cleaning systems, a representative case study is presented.

We used high-resolution quasielastic neutron scattering spectroscopy to study the single-particle dynamics of water molecules on the surface of hydrated DNA samples. Both H2O and D2O hydrated samples were measured. The contribution of scattering from DNA is subtracted out by taking the difference of the signals between the two samples. The measurement was made at a series of temperatures from 270 K down to 185 K. The Relaxing -Cage Model was used to analyze the quasielastic spectra. This allowed us to extract a Q-independent average translational relaxation time τT of water molecules as a function of temperature. We observe clear evidence of a fragileto-strong dynamic crossover (FSC) at TL = 222 ± 2 K by plotting log τT vs. T. The coincidence of the dynamic transition temperature Tc of DNA, signaling the onset of anharmonic molecular motion, and the FSC temperature TL of the hydration water suggests that the change of mobility of the hydration water molecules across TL drives the dynamic transition in DNA. . It was also found, from neutron and Xray scattering, or from Mössbauer spectroscopy, that the measured mean-squared atomic displacement x 2 of the bio-molecules exhibits a sharp rise in the same temperature range [1,2,3,4,5]. This sharp increase in x 2 was taken as a sign for a dynamic transition (or sometimes called glass-transition) in the bio-molecules occurring within this temperature range. In most of these papers, the authors suggest that the transition is due to a strong rise of anharmonicity of the molecular motions above this transition temperature [1]. Later on, it was demonstrated that the dynamic transition can be suppressed in dry bio-molecules [2], or in bio-molecules dissolved in trehalose [5]. Moreover, it can be shifted to a higher temperature for proteins dissolved in glycerol [4]. Thus the dynamic transition can be controlled by changing the surrounding solvent of the bio-molecules. On the other hand, it was found some time ago from Raman scattering that supercooled bulk water has a dynamic crossover transition at 220 K [6], similar to that predicted by Mode-Coupling theory [7]. Approximate coincidence of these two characteristic temperatures, one for the slowing down of bio-chemical activities and the sharp rise in x 2 in bio-molecules and the other for the dynamic crossover in water, suggests a relation between the dynamic transition of bio-molecules and that of their * Author to whom correspondence should be addressed. Electronic mail: sowhsin@mit.edu hydration water [8].Another striking experimental fact is that this dynamic transition temperature, as revealed by change of slope in x 2 vs. temperature plot, occurs at a universal temperature range from 250 to 200 K in all bio-molecules examined so far. This list includes globular proteins, DNAs, and t-RNAs. This feature points to the plausibility that the dynamical transitions are not the intrinsic properties of the bio-molecules themselves but are imposed by the hydration water on their surfaces.However, x 2 (mostly coming from hydroge...

We study the dynamics of hydration water in the protein lysozyme in the temperature range 180 K

No abstract

Small angle neutron scattering intensity distributions taken from cytochrome C and lysozyme protein solutions show a rising intensity at very small wave vector, Q, which can be interpreted in terms of the presence of a weak long-range attraction between protein molecules. This interaction has a range several times that of the diameter of the protein molecule, much greater than the range of the screened electrostatic repulsion. We show evidence that this long-range attraction is closely related to the type of anion present and ion concentration in the solution.PACS numbers: 87.14. Ee,61.12.Ex, 82.35.Rs The bottleneck of protein crystallography is the lack of systematic methods to obtain protein crystals. This is partly due to incomplete understanding of the physical chemistry conditions controlling the growth of protein crystals. A full comprehension of the effective protein interactions and phase behavior is therefore essential. It has been shown that the crystallization curves of some globular proteins appear to coincide with the phase diagrams of a hard sphere system interacting with a short range attraction [1,2,3]. Small angle neutron and X-ray scattering investigations of proteins suggest the presence of a short-range attractive interaction between protein molecules besides the electrostatic repulsion induced by the residual charges [4,5,6]. The DLVO potential has been successfully applied to many colloidal systems and protein solutions [3,4]. However, it does not seem to fully explain the rich behaviour of proteins [4,7,8,9], and due to the complexity of these systems (anisotropic property, irregular shape, distributed charge patches, etc.), a complete understanding of the properties of the effective interactions between protein molecules in solutions remains a challenge [8].Recent measurements of small angle neutron scattering (SANS) intensity distribution in protein solutions show interesting results [5,6,10]. Beside the normal first diffraction peak, it is present a peak (cluster peak) appearing at a much smaller scattering wave vector, Q, due to the formation of ordered clusters. The appearance of a cluster peak is explained as due to the competition of a short-range attraction and a long-range electrostatic repulsion [5,11,12]. Moreover, a rising intensity as Q approaches zero (zero-Q peak) is observed in both liquid-like and solid-like samples, which implies that the effective potential should have more features in addition to the well known short-range attraction and electrostatic repulsion. The existence of a long-range attraction between protein molecules has already been