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Neutron intensity elastic scans on trehalose, maltose, and sucrose/H(2)O mixtures as a function of concentration, temperature, and exchanged wave vector are presented. The experimental findings show a crossover in molecular fluctuations between harmonic and anharmonic dynamical regimes. A new operative definition for the degree of fragility of glass-forming systems is furnished by using explicitly the connection between viscosity and mean-square displacement. The procedure is tested for the investigated mixtures and for a set of glass-forming systems. In this frame, the stronger character of trehalose/H(2)O mixture indicates a better attitude in respect to maltose and sucrose/H(2)O mixtures to encapsulate biostructures in a more rigid matrix.

An analysis in terms of elastic scans of the neutron intensity of homologous disaccharide (trehalose, maltose, sucrose)/H[sub 2]O mixtures as a function of temperature and exchanged wave vector has been carried out. The experimental findings, showing a crossover in molecular fluctuations between harmonic and anharmonic dynamical regimes, allow us to characterize the system “flexibility.” A new operative definition for the “fragility” degree, by using elastic incoherent neutron scattering, is furnished. In this frame the lower flexibility and fragility character of trehalose/H[sub 2]O mixture with respect to maltose and sucrose/H[sub 2]O mixtures indicate a better attitude to encapsulate biostructures in more rigid and temperature insensitive structures in approaching the glass transition. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have pursued the neutron examination of SrCr[sub x] Ga[sub 12–x] O[sub 19] , with special emphasis on the role of magnetic dilution in the spin dynamics and freezing. We have examined samples with different levels of magnetic dilution, ranging from 7.0 < x < 8.8, characterized by neutron powder diffraction with Rietveld refinements and bulk susceptibility. The elastic magnetic response examined by high-resolution neutron scattering (µeV range) has confirmed the trend on increasing dilution: the decrease of the freezing temperature as well as of the frozen moment/magnetic site. However, the least diluted sample that we have produced with x = 8.8, deviates from this trend, with a smaller than expected freezing temperature and frozen moment. This suggests that the freezing might be suppressed at the clean limit. PACS Nos.: 75.10Nr, 75.25+z, 75.40Cx, 75.40GbNous poursuivons l'étude du composé SrCr[sub x] Ga[sub 12–x] O[sub 19] par diffusion des neutrons, en portant une attention particulière au rôle de la dilution magnétique sur la dynamique et le gel de spin. Des échantillons correspondant à différents niveaux de dilution (7,0 < x < 8,8) sont caractérisés par diffraction de poudre, par affinement par la méthode de Rietveld et par des mesures de susceptibilité magnétique. La diffusion élastique à haute résolution (gamme µeV) confirme les évolutions à dilution croissante : des diminutions de la température de gel et du moment gelé/site magnétique sont observées. Cependant l'échantillon le moins diluté (x = 8,8) s'écarte de cette tendance, la température de gel et le moment gelé étant plus faible qu'attendu. Ceci suggère que le gel pourrait ne pas exister pour un système non dilué (x = 9,0). [Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Neutron incoherent elastic scattering data have been collected on homologues disaccharides (trehalose, maltose, sucrose)/H2O mixtures as a function of temperature. The results show a dynamical transition at T∼238 K, T∼235 K and T∼233 K for trehalose, maltose and sucrose/H2O mixtures, respectively, marking a cross-over in molecular fluctuations between harmonic and non-harmonic dynamical regimes, proving a higher ‘rigidity’ of trehalose mixture with respect to maltose and sucrose mixtures. This finding accounts for the higher cryptobiotic effectiveness of trehalose, namely a capability to encapsulate biostructures in a more rigid environment. [Copyright &y& Elsevier]

Abstract: The aim of the present work is to investigate the chemical physics mechanisms of protein stabilization by homologous disaccharides (trehalose, maltose and sucrose). On this purpose the structural and dynamical properties of dUTPase-inhibitor candidate and dUTPase-inhibitor candidate/disaccharide mixtures have been investigated by elastic neutron scattering (ENS), quasi-elastic neutron scattering (QENS) and photon correlation spectroscopy (PCS). The decrease in the ENS intensity profiles vs temperature for the disaccharide–water mixtures is less marked in the case of trehalose/water mixture. This indicates that trehalose shows a larger structural resistance to temperature changes and a higher “rigidity” in comparison with maltose/H2O and sucrose/H2O mixtures. In addition the protein/hydrated–disaccharide mixtures show a linear dependence between the solvent viscosity and the local mean-square displacement of hydrated dUTpase/disaccharide systems. This result shows that the protein dynamics is coupled with that of the surrounding matrix. Furthermore, QENS results on the binary disaccharide–H2O/D2O mixtures indicate that the water dynamics is affected by all the disaccharides and particularly by trehalose. Finally, PCS findings indicate that the protein hydrodynamic radius in solution does not change at low disaccharide concentrations, while reveal, at high disaccharide concentration, a breakdown of the Stokes–Einstein law. The experimental findings are discussed and interpreted in the frame of the current theories. [Copyright &y& Elsevier]

Polarized neutron scattering has been used to obtain the magnetic moment at specific crystallographic sites of the austenitic and martensitic phases of two nonstoichiometric Ni-Mn-Ga single crystals with close composition. These alloys have been chosen because they exhibit different structures in the paramagnetic state and inverse positions of the respective martensitic transformation and Curie temperature. The diffraction analysis revealed a remarkable result: Despite the similar alloy composition, the magnetic moments of Mn are quite different for the two alloys at the same crystallographic position. Furthermore, such a difference enabled us to assess that the exchange coupling between Mn atoms switches from ferro- to antiferromagnetic at a distance between 2.92 and 3.32 Å in the martensite. These results are of great importance to guide first principles calculations that, up to now, have not been contrasted with experiments at the atomic level. [ABSTRACT FROM AUTHOR]