F. Sacchetti

Collective dynamics of protein hydration water by brillouin neutron spectroscopy.

By a detailed experimental study of THz dynamics in the ribonuclease protein, we could detect the propagation of coherent collective density fluctuations within the protein hydration shell. The emerging picture indicates the presence of both a dispersing mode, traveling with a speed greater than 3000 m/s, and a nondispersing one, characterized by an almost constant energy of 6-7 meV. In agreement with molecular dynamics simulations [Phys. Rev. Lett. 2002, 89, 275501], the features of the dispersion curves closely resemble those observed in pure liquid water [Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2004, 69, 061203]. On the contrary, the observed damping factors are much larger than in bulk water, with the dispersing mode becoming overdamped at Q = 0.6 A(-1) already. Such novel experimental findings are discussed as a dynamic signature of the disordering effect induced by the protein surface on the local structure of water.

The development of the PRISMA spectrometer at ISIS

Abstract Inelastic neutron scattering experiments on single crystal specimens at a pulsed neutron source require new instrumentation and measuring techniques. In this paper we describe the development and performance of the PRISMA spectrometer which has recently been installed at the ISIS spallation neutron source, Rutherford Appleton Laboratory, UK for the purpose of carrying out such measurements. This spectrometer is particularly suited to the performance of quick overview measurements of the dispersion curves in single crystals over a wide neutron energy transfer range. It is also very efficient at measuring changes in the dynamical scattering function introduced by external parameters such as temperature, pressure or magnetic field. Experimental results from the commissioning phase which demonstrate the potential of the PRISMA spectrometer are presented.

The structure of liquid chlorine

We report neutron diffraction measurements of liquid chlorine at 200 K and 290 K and orthobaric densities. The scattering functions, S(K), and the radial distribution functions, g(r), are discussed by comparison with those of other homonuclear diatomic liquids like N2, O2, Br2 and I2. Strong orientational correlations, found in both thermodynamic states, are suggested to be related mainly to the quadrupolar interaction. The shapes of the g(r) functions and the number of first neighbours strongly indicate that, at short distance in the liquid, we have correlations among first neighbouring molecules very similar to that we find in solid chlorine. Some implications for the σ value used in the 2LJC intermolecular potential are discussed, and consequences are also indicated for the MD experiments already done to simulate the liquid halogens.

Vibrational Collective Dynamics of Dry Proteins in the Terahertz Region

The coherent density fluctuations of a perdeuterated dry protein have been studied by Brillouin neutron spectroscopy. Besides a nearly wavevector-independent branch located around 5 meV, a propagating mode with a linear trend at low wavevector Q is revealed. The corresponding speed of 3780 ± 130 m/s is definitely higher than that of hydrated proteins. Above Q = 0.8 Å(-1), this mode becomes overdamped, with lifetimes shorter than 0.1 ps, in fashion similar to glassy materials. The present results indicate that dry proteins sustain coherent density fluctuations in the THz frequency regime. The trend of the longitudinal modulus indicates that in this frequency range dry biomolecules are more rigid than hydrated proteins.

Absolute measurements of the stretching mode density of states in polycrystalline ice Ih

An incoherent inelastic neutron scattering experiment on polycrystalline ice Ih at T=20 K is reported. The experiment was performed on the high‐resolution chopper spectrometer at the Intense Pulsed Neutron Source (IPNS) of the Argonne National Laboratory. Scattering functions in the angular range from 4° to 20° were converted, after analytical corrections, into density of states of the system in the stretching frequency region. Analysis of these results leads to the following conclusions: (i) the number of modes in the stretching region of ice Ih is 8.1/cell which is that expected in the case of free molecules; (ii) the general shape of the density of states is in qualitative agreement with Raman scattering results, however the high energy side of the neutron data suggests that a non‐Raman active band exists at about 3500 cm−1; (iii) the overall bandwidth observed in this experiment is in good agreement with what we estimated by a coherent inelastic neutron scattering study on heavy ice Ih.

A honeycomb collimator for the neutron Brillouin scattering spectrometer BRISP

A new collimator for thermal neutrons, based on the honeycomb design, has been produced in a few prototype versions. The method of construction is simple and the overall cost of the device is rather low. Development of this collimator has been stimulated by the technical specifications of the spectrometer BRISP for thermal neutron Brillouin scattering, presently under construction. The performances of the prototypes have been measured on the monochromatic beam of a steady source and exploiting the time-of-flight technique at a pulsed neutron source. The expected response of the device has been calculated by means of a Monte Carlo simulation of the experiments. This paper describes the method of construction and reports on the results of the neutron test measurements in comparison with the Monte Carlo simulation.

Hydration structure of ethanol water solution at low alcohol concentration

The analysis of the hydration structure of ethanol in water is performed employing both neutron and X-ray scattering data. An abrupt structural change as a function of the molar concentration is evidenced and connected with the water distribution around the ethanol. The change in structure appears to be related to the hydrogen bonds in the system and a small isotope effect may appear on substituting hydrogen with deuterium.

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.

Direct experimental evidence of free-fermion antibunching

Fermion antibunching was observed on a beam of free noninteracting neutrons. A monochromatic beam of thermal neutrons was first split by a graphite single crystal, then fed to two detectors, displaying a reduced coincidence rate. The result is a fermionic complement to the Hanbury Brown and Twiss effect for photons.

SPACE RESOLUTION OF A SI/GD MICROSTRIP AS LINEAR POSITION-SENSITIVE DETECTOR FOR THERMAL NEUTRONS

Abstract A Si microstrip detector coupled to a Gd converter has been designed for operation as linear position-sensitive detector of thermal neutrons. Test measurements have been carried out on a first prototype. The measured space resolution is reported and discussed in comparison with the results of a Monte Carlo simulation. The space resolution of the Si microstrip detector, given by the response of the sensor to an ideal narrow incoming beam, has been also modeled by the Monte Carlo simulation program.