E. Cornicchi

Molecular-beam study of the water-helium system : Features of the isotropic component of the intermolecular interaction and a critical test for the available potential-energy surfaces

We report molecular-beam measurements of the total integral cross sections for the scattering of water molecules by helium atoms. A combined analysis of the new experimental data together with available differential cross section results has allowed an accurate determination of the isotropic component of the interaction potential for this prototypical system. The potential well shows a depth of 0.265 +/- 0.010 kJ/mol at a distance between He and the center of mass of the water molecule of 0.345 +/- 0.02 nm. An effective isotropic long-range attraction constant C(LR) = (6.3+/-0.3) x 10(-4) kJ mol(-1) nm(-6), including both dispersion and induction contributions, has also been determined. The most recent and accurate ab initio potential-energy surfaces have been tested against these new experimental results.

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.

Temperature dependence of fast fluctuations in single- and double-stranded DNA molecules: a neutron scattering investigation

Using elastic neutron scattering measurements we have investigated the picosecond dynamics of dry and hydrated powders of DNA in the double-stranded (dsDNA) and single-stranded (ssDNA) state in the temperature range from 20 to 300 K. The extracted mean square displacements of DNA hydrogen atoms exhibit an onset of anharmonicity at around 100 K. The dynamics of the hydrated samples shows a further anharmonic contribution appearing at a temperature Td  = 230–240 K. Such dynamical behaviour is similar to the well-studied dynamical transition found in hydrated protein powders. The mean square displacements of dsDNA and ssDNA are practically superimposed in the whole temperature range for both dry and hydrated samples. This suggests that the DNA local mobility in the picosecond timescale does not depend on the single- or double-stranded conformation.

Collective density fluctuations of DNA hydration water in the time-window below 1 ps

The coherent density fluctuations propagating through DNA hydration water were studied by neutron scattering spectroscopy. Two collective modes were found to be sustained by the aqueous solvent: a propagating excitation, characterised by a speed of about 3500 m/s, and another one placed at about 6 meV. These results globally agree with those previously found for the coherent excitations in bulk water, although in DNA hydration water the speed of propagating modes is definitely higher than that of the pure solvent. The short-wavelength collective excitations of DNA hydration water are reminiscent of those observed in protein hydration water and in the amorphous forms of ice.

Coupled thermal fluctuations of proteins and protein hydration water on the picosecond timescale

The mean square displacements (MSD) of a model protein, the maltose binding protein, and its hydration water have been estimated from the elastic neutron scattering intensity measured on the IN5 time-of-flight spectrometer. The availability of the protein in both fully deuterated and hydrogenated form allowed reliable separation of the contribution of the solvent interacting with the biomolecule from that of the hydrated biomolecule. The thermal fluctuations of hydration water and protein activate in the same temperature range between 200–220 K. This result supports a picture where the dynamical coupling between the biomolecule and the solvent is already effective in the picosecond timescale. A quantitative agreement of the MSD, with values from molecular dynamics simulations, is found.

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.

Glassy Character of DNA Hydration Water

The coherent excitations of DNA hydration water at 100 K have been investigated by neutron scattering spectroscopy to extract the excess signal of D(2)O-hydrated DNA with respect to dry DNA samples. A structural characterization of the sample, through the analysis of the static structure factor, has suggested that DNA hydration water is largely in an amorphous state up to high hydration degree, with only a small contribution coming from slightly deformed crystalline ice. To describe the inelastic spectra of DNA hydration water, we exploited a phenomenological model already applied in similar disordered systems, such as bulk water (Sacchetti et al. Phys. Rev. E2004, 69, 061203; Petrillo et al. Phys. Rev. E2000, 62, 3611-3618; Sette et al. Phys. Rev. Lett.1996, 77, 83-86) and protein hydration water (Orecchini et al. J. Am. Chem. Soc.2009, 131, 4664-4669). Over the low-energy range, the coherent dynamics of DNA hydration water is characterized by a branch at about 7.5 meV, a value slightly larger than that of bulk water. An additional mode in the energy range 20-35 meV is found, with a wavevector dependence seemingly connected with the structural features of amorphous ice. The ensemble of the results supports the glassy nature of DNA hydration water.

Low-frequency dynamics of water absorbed in Nafion membranes as a function of temperature

We performed a neutron scattering study to investigate the dynamical behaviour of water absorbed in Nafion at low hydration levels (λ = 6, λ = moles of water/moles of sulphonic acid sites) as a function of temperature in the range 200–300 K. To single out the signal of the confined water, the measurements were performed on samples hydrated with both H2O and D2O in the same temperature range. 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 most of the spectra originates from absorbed water. The estimated dynamical susceptibility exhibits features that resemble those of bulk water. In particular, the spectra display a bump at around 1 meV, possibly related to the α relaxation, the intensity of which is markedly affected by the temperature change. Two features due to the phonon-like collective hydrogen bond network dynamics are visible at approximately 7 meV and 25 meV.

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.

Molecular Beam Scattering Experiments On Species Of Atmospheric Relevance: Potential Energy Surfaces For Clusters And Quantum Mechanical Prediction Of Spectral Features

Accurate intermolecular potential energy surfaces for the major compo- nents of the atmosphere, leading to the characterization of the O2-O2, N2-N2 and N2-O2 dimers, have been obtained from the analysis of scattering experiments from our laboratory, also exploiting where avail- able second virial coefficient data. A harmonic expansion functional form describes the geometries of the dimers and accounts for the rel- ative contributions to the intermolecular interaction from components of different nature. For O2-O2, singlet, triplet and quintet surfaces are obtained accounting for the role of spin-spin coupling. The new sur- faces allow the full characterization of structure and internal dynamics of the clusters, whose bound states and eigenfunctions are obtained by exact quantum mechanics. Besides the information on the nature of the bond, these results can be of use in modelling the role of dimers in air and the calculated rotovibrational levels provide a guidance for the analysis of spectra, thus establishing the ground for atmospheric monitoring. The same approach is currently being extended to simple hydrocarbons and water molecules interacting with rare gas atoms or simple molecules.