Dynamics of hydration water in deuterated purple membranes explored by neutron scattering

The function and dynamics of proteins depend on their direct environment, and much evidence has pointed to a strong coupling between water and protein motions. Recently however, neutron scattering measurements on deuterated and natural-abundance purple membrane (PM), hydrated in H2O and D2O, respectively, revealed that membrane and water motions on the ns–ps time scale are not directly coupled below 260 K (Wood et al. in Proc Natl Acad Sci USA 104:18049–18054, 2007). In the initial study, samples with a high level of hydration were measured. Here, we have measured the dynamics of PM and water separately, at a low-hydration level corresponding to the first layer of hydration water only. As in the case of the higher hydration samples previously studied, the dynamics of PM and water display different temperature dependencies, with a transition in the hydration water at 200 K not triggering a transition in the membrane at the same temperature. Furthermore, neutron diffraction experiments were carried out to monitor the lamellar spacing of a flash-cooled deuterated PM stack hydrated in H2O as a function of temperature. At 200 K, a sudden decrease in lamellar spacing indicated the onset of long-range translational water diffusion in the second hydration layer as has already been observed on flash-cooled natural-abundance PM stacks hydrated in D2O (Weik et al. in J Mol Biol 275:632–634, 2005), excluding thus a notable isotope effect. Our results reinforce the notion that membrane-protein dynamics may be less strongly coupled to hydration water motions than the dynamics of soluble proteins

Evidence of coexistence of change of caged dynamics at Tg and the dynamic transition at Td in solvated proteins

Mossbauer spectroscopy and neutron scattering measurements on proteins embedded in solvents including water and aqueous mixtures have emphasized the observation of the distinctive temperature dependence of the atomic mean square displacements, , commonly referred to as the dynamic transition at some temperature Td. At low temperatures, increases slowly, but it assume stronger temperature dependence after crossing Td, which depends on the time/frequency resolution of the spectrometer. Various authors have made connection of the dynamics of solvated proteins including the dynamic transition to that of glass-forming substances. Notwithstanding, no connection is made to the similar change of temperature dependence of obtained by quasielastic neutron scattering when crossing the glass transition temperature Tg, generally observed in inorganic, organic and polymeric glass-formers. Evidences are presented to show that such change of the temperature dependence of from neutron scattering at Tg is present in hydrated or solvated proteins, as well as in the solvents used unsurprisingly since the latter is just another organic glass-formers. The obtained by neutron scattering at not so low temperatures has contributions from the dissipation of molecules while caged by the anharmonic intermolecular potential at times before dissolution of cages by the onset of the Johari-Goldstein beta-relaxation. The universal change of at Tg of glass-formers had been rationalized by sensitivity to change in volume and entropy of the beta-relaxation, which is passed onto the dissipation of the caged molecules and its contribution to . The same rationalization applies to hydrated and solvated proteins for the observed change of at Tg.Comment: 28 pages, 10 figures, 1 Tabl

Vibrational density of states measurements in disordered systems

We present a data treatment procedure, based on an iterative technique, properly developed to subtract the multi-phonon contributions from the dynamic structure factor in a self-consistent way. With this technique, we derive the one-phonon vibrational density of states from the dynamic structure factor of different disordered systems, in the framework of the incoherent scattering approximation. We present results on glassy glucose (C6H12O6), a nearly perfect incoherent scatterer, due to high hydrogen content. The data treatment procedure has been found to work well also for the more complex case of dry and hydrated DNA

Temperature-dependent dynamics of water confined in nafion membranes

We performed a neutron scattering study to investigate the dynamical behavior of water absorbed in Nafion at low hydration level as a function of temperature in the range 200-300 K. To single out the spectral contribution of the confined water, the measurements were done on samples hydrated with both H2O and D2O. Due to the strong incoherent scattering cross section of hydrogen atoms with respect to deuterium, in the difference spectra, the contribution from the Nafion membrane is subtracted out and the signal originates essentially from protons in the liquid phase. The main quantities we extracted as a function of the momentum transfer are the elastic incoherent structure factor (EISF) and the line width of the quasielastic component. Their trend suggests that the motion of hydrogen atoms can be schematized as a random jumping inside a confining region, which can be related to the boundaries of the space where water molecules move in the cluster they form around the sulfonic acid site. Through the calculated EISF, we obtained information on the size of such a region, which increases up to 260 K and then attains a constant value. Above this temperature, the number of water protons that are dynamically activated in the accessible time window increases with a faster rate. The jump diffusion dynamics is characterized by a typical jumping time which is stable at 5.3 ps up to similar to 260 K and then gradually decreases. The ensemble of the findings indicates that, within the limits of the energy resolution of the present experiment, water absorbed in the Nafion membrane undergoes a dynamical transition at around 260 K. We discuss the possible relationship of this dynamical onset with the behavior of the electrical conductivity of the membrane as a function of the temperature

Dynamics of water confined in fuel cell Nafion membranes containing zirconium phosphate nanofiller

A quasielastic neutron scattering investigation, to study the single particle dynamics of water absorbed in a Nafion/zirconium phosphate composite membrane hydrated at a saturation value, is herewith presented. The measurements were done on samples hydrated with both H2O and D2O to properly select the spectral contribution of the confined water. Both the elastic incoherent structure factor (EISF) and the linewidth of the quasielastic component are evaluated as a function of the momentum transfer. Their trend suggests that the motion of the system hydrogen atoms can be schematized as a random jumping inside a confining spherical region, which can be related to the boundaries of the cluster that water molecules form around the sulfonic and phosphate acid sites. The size of such a region, the characteristic time necessary to explore the region and the number of mobile protons involved in this motion are similar to those estimated for water absorbed in a simple Nafion membrane at a saturation water content. Also the calculated jump diffusion coefficient resembles that of water confined in a simple Nafion membrane, and both are consistent with the value of bulk water. The results indicate that the dynamical behaviour of water in Nafion membranes is nearly unaffected by the presence of zirconium phosphate nanoparticles

Mean-squared atomic displacements in hydrated lysozyme, native and denatured

We use elastic neutron scattering to demonstrate that a sharp increase in the mean-squared atomic displacements, commonly observed in hydrated proteins above 200 K and often referred to as the dynamical transition, is present in the hydrated state of both native and denatured lysozyme. A direct comparison of the native and denatured protein thus confirms that the presence of the transition in the mean-squared atomic displacements is not specific to biologically functional molecules

Coupled relaxations at the protein–water interface in the picosecond time scale

The spectral behaviour of a protein and its hydration water has been investigated through neutron scattering. The availability of both hydrogenated and perdeuterated samples of maltose-binding protein (MBP) allowed us to directly measure with great accuracy the signal from the protein and the hydration water alone. Both the spectra of the MBP and its hydration water show two distinct relaxations, a behaviour that is reminiscent of glassy systems. The two components have been described using a phenomenological model that includes two Cole–Davidson functions. In MBP and its hydration water, the two relaxations take place with similar average characteristic times of approximately 10 and 0.2 ps. The common time scales of these relaxations suggest that they may be a preferential route to couple the dynamics of the water hydrogen-bond network around the protein surface with that of protein fluctuations

Controlling the Protein Dynamical Transition with Sugar-Based Bioprotectant Matrices: A Neutron Scattering Study

Through elastic neutron scattering we measured the mean-square displacements of the hydrogen atoms of lysozyme embedded in a glucose-water glassy matrix as a function of the temperature and at various water contents. The elastic intensity of all the samples has been interpreted in terms of the double-well model in the whole temperature range. The dry sample shows an onset of anharmonicity at ∼100 K, which can be attributed to the activation of methyl group reorientations. Such a protein intrinsic dynamics is decoupled from the external environment on the whole investigated temperature range. In the hydrated samples an additional and larger anharmonic contribution is provided by the protein dynamical transition, which appears at a higher temperature T(d). As hydration increases the coupling between the protein internal dynamics and the surrounding matrix relaxations becomes more effective. The behavior of T(d) that, as a function of the water content, diminishes by ∼60 K, supports the picture of the protein dynamics as driven by solvent relaxations. A possible connection between the protein dynamical response versus T and the thermal stability in glucose-water bioprotectant matrices is proposed