Antonio Deriu

Dynamics of hydrogen atoms in superoxide dismutase by quasielastic neutron scattering

The low energy dynamic of the enzyme Cu,Zn superoxide dismutase have been investigated by means of quasielastic neutron scattering in the temperature range 4-320 K. Below 200 K the scattering is purely elastic, while above this temperature a pronounced decrease in the elastic intensity is observed, together with the onset of a small quasielastic component. This behavior is similar to that previously observed in other more flexible globular proteins, and can be attributed to transitions between slightly different conformational substates of the protein tertiary structure. The presence of only a small quasielastic component, whose intensity is < or = 25% of the total spectrum, is related to the high structural rigidity of this protein.

Proton dynamics in supercooled water by molecular dynamics simulations and quasielastic neutron scattering

A detailed study of the single‐particle dynamics of liquid water in normal and supercooled regime has been carried out by comparing molecular dynamics (MD) simulation results with now available high resolution quasielastic neutron scattering (QENS) data. Simulation runs have been performed at 264, 280, 292, and 305 K, using the extended simple point charge model, well suited for reproducing single‐particle properties of H2O. The microscopic dynamics has been probed over a wide range of times and distances. The MD results indicate that a substantial coupling between translational and rotational dynamics exists already at about 1 ps. The decay of the translational dynamic correlations has been phenomenologically analyzed in terms of three exponential components, and the agreement between the parameters thus obtained from experimental and simulation derived datasets is quite satisfactory. Both QENS and MD data can not be described with sufficient accuracy by simple diffusion models over the entire range of e...

IN13 Backscattering Spectrometer at ILL: Looking for Motions in Biological Macromolecules and Organisms

In 1998, three partner groups (the French institutions Institut de Biologie Structurale and the Léon Brillouin Laboratory and the Italian Istituto Nazionale per la Fisica della Materia, now merged with the Consiglio Nazionale delle Ricerche, INFM-CNR) applied to operate the thermal backscattering spectrometer IN13, at the Institut Laue Langevin, as a French-Italian Collaborative Research Group (CRG). The plan was to have access to a dedicated spectrometer in order to explore how far neutron scattering could contribute to the understanding of dynamics in biological macromolecules: how “flexible” must be a biological object to perform its function?

Water dynamics in charged and uncharged polysaccharide gels by quasi-elastic neutron scattering.

Using a microeV neutron spectrometer we have studied the mobility of water in gels formed by two polysaccharides: agarose and hyaluronic acid. Agarose is a nearly uncharged polysaccharide; its gels are fairly stiff, quasi-random networks of fibre bundles. Hyaluronic acid is a highly charged polysaccharide capable of retaining large amounts of water in entangled meshworks with unusual rheological properties. We have analysed sets of quasi-elastic lineshapes broadened by two proton populations with different degrees of freedom. The resulting microscopic mobility parameters and their temperature dependence reveal a complex behaviour. The overall effect of the biopolymer network is to increase translational as well as rotational relaxation times, but the changes observed are not dramatic and cannot fully account for the strikingly different macroscopic properties of these gels. Local electrostatic interactions (over 3 to 20 A) do not appear to influence significantly the rheological behaviour.

Structure and Dynamics of a Thermoresponsive Microgel around Its Volume Phase Transition Temperature

Sustained drug delivery requires the use of multifunctional devices with enhanced properties. These properties include responsiveness to external stimuli (such as temperature, pH, ionic strength), ability to deliver suitably designed ligands to specific receptors, enhanced bioadhesion to cells, and cytocompatibility. Microgels represent one of such multifunctional drug delivery devices. Recently, we described the fabrication of a stable colloidal aqueous suspension of cytocompatible microgel spheres based on a poly(vinyl alcohol)/poly(methacrylate-co-N-isopropylacrylamide) network ( Ghugare, S. Mozetic, P. Paradossi, G. Biomacromolecules 2009 , 10 , 1589 ). These microgel spheres undergo an entropy-driven volume phase transition around the physiological temperature, this phase transition being driven by the incorporation of NiPAAm residues in the network. In that study, the microgel was loaded with the anticancer drug doxorubicin. As the microgel shrank, a marked increase in the amount of doxorubicin released was noted. Indeed, dynamic light scattering measurements showed the diameter reduction to be about 50%. In the present paper, we focus on some fundamental issues regarding modifications of the hydrogel architecture at a nanoscopic level as well as of the diffusive behavior of water associated with the polymer network around the volume phase transition temperature (VPTT). Sieving and size exclusion effects were studied by laser scanning confocal microscopy with the microgel exposed to fluorescent probes with different molecular weights. Confocal microscopy observations at room temperature and at 40 degrees C (i.e., below and above the VPTT) provided an evaluation of the variation of the average pore size (from 5 nm to less than 3 nm). Using quasielastic neutron scattering (QENS) with the IRIS spectrometer at ISIS, UK, the diffusive behavior of water molecules closely associated to the polymer network around the VPTT was investigated. A clear change in the values of diffusion coefficient of bound water was observed at the transition temperature. In addition, the local dynamics of the polymer itself was probed using the QENS spectrometer SPHERES at FRM II, Germany. For this study, the microgel was swollen in D(2)O. An average characteristic distance of about 5 A for the localized chain motions was evaluated from the elastic incoherent structure factor (EISF) and from the Q-dependence of the Lorentzian width.

Diffusive properties of water studied by incoherent quasi-elastic neutron scattering

Using a pulsed-source neutron spectrometer we have measured the quasi-elastic incoherent dynamic structure factor of pure water in the temperature range 256-293 K. The high energy resolution and the wide energy transfer window allowed us to separate the translational diffusive motions from the relational ones. The translational dynamics has been analysed on the basis of a random-jump model that describes quite well the average behaviour of the quasi-elastic broadenings as a function of momentum transfer.

Mössbauer investigation of W-type hexaferrite of composition BaZn2−xCoxFe16O27

AbstractPolycrystauine samples of composition BaZn2−xCox16O27 (O<x<2) have been examined by thermomagnetic analysis and Mössbauer spectroscopy. Cobalt ions enter preferentially the octahedral sites of the spinel block and give rise to strong perturbations of the magnetic order. The 12k sublattice splits into two magnetically inequivalent sublattices. A spin reorientation transition is observed at a temperature that increases with increasing cobalt content. Co2-W undergoes a transition from planar to conical order at 515±5 K.

Conformational and segmental dynamics in lipid-based vesicles†

We have performed a detailed analysis of the conformational and segmental dynamics in lipid-based vesicles using quasielastic neutron scattering. Our data evidence the presence of dynamical heterogeneities: the hydrogens in the headgroups and in the initial part of the hydrophobic chains perform slow diffusive motions in a confined volume; those belonging to the end part of the chains show a faster confined diffusivity together with torsional isomerization transitions. A model with three hydrogen populations has been proposed to describe the main features of this complex dynamics. The addition of a charged polysaccharide component to the vesicles mostly slows the dynamics of the headgroups and of the upper part of the acyl chains without significantly affecting the isomerization dynamics.

Flexibility and drug release features of lipid/saccharide nanoparticles

The effect of lipophilic additives (excipients and drugs) on the behavior of lipid/saccharide nanoparticles has been investigated by incoherent elastic neutron scattering. Temperature scans from 20 K to 350 K have been performed on lecithin/chitosan particles loaded with isopropyl myristate and cetyl-stearyl alcohol, two lipophilic molecules with different melting temperatures which are commonly added to improve drug loading efficiency. In a similar way the effect of tamoxifen citrate, a lipophilic drug frequently used in breast cancer therapy, has also been studied. The different melting points of the two excipients affect mostly the low-temperature behavior of the nanoparticles. At physiological temperatures they both improve the particle flexibility. On the other hand addition of tamoxifen leads to stiffer structures and to lower amounts of released drug. The macroscopic features of the drug release appear to be correlated to the microscopic flexibility determined by neutron scattering. The data confirm also the role of chitosan as a stiffening and stabilizing agent of the lipid particles.

Vibrational density of states of nanocrystalline iron and nickel

We present an experimental determination of the vibrational density of states in nanocrystalline Fe and Ni by inelastic neutron scattering. Nanocrystalline specimens with different grain size and microstrain have been prepared by ball milling and thermal annealing. The vibrational density of states has been extracted from neutron time-of-flight spectra obtained with the spectrometer IN6 at the Institute Laue–Langevin. In comparison with reference coarse-grained specimens measured in the same conditions the nanocrystalline specimens exhibit: (i) a modest increase in the population of low-frequency modes and (ii) a distinct broadening of the transverse and longitudinal phonon peaks. The former feature is related to the presence of interface modes and it is critically compared with other observations in pure nanocrystalline metals. The latter is discussed in terms of reduced phonon lifetime due to the nanometric size of the crystallites.