No abstract

The vibrational dynamics of vitreous germanium diselenide ͑v-GeSe 2 ͒, an amorphous glass belonging to the family of continuous random network forming glasses, has been investigated by means of inelastic neutron scattering. We employed three different neutron spectrometers, each of them providing different energy resolutions and spanning different portions of the energy-wave-vector ͑ប , Q͒ plane, which partially overlap so that complementary information can be obtained. Two well-defined peaks are resolved in the dynamic structure factor in a broad Q region, below and above the position of the first sharp diffraction peak in the static structure factor. The low-lying excitation is nondispersive and located around the boson peak frequency. The highfrequency mode presents a pseudoperiodic behavior and it can be associated to the high-frequency counterpart of the longitudinal acoustic ͑LA͒ mode. In the low-Q region this mode shows a positive dispersion of its apparent sound velocity and an attenuation which follows a Q 2 law. The combined use of a set of appropriate instruments allows an accurate determination of the nonergodicity parameter f͑Q͒. The LA dispersion curve of v-GeSe 2 is similar to that measured in other glasses where a second excitation appears in the spectra. On the contrary other glass forming materials, where this second mode is not detected, present a sinusoidal-like dispersion curve with a deeper minimum in the second pseudo-Brillouin zone.

The vibrational dynamics of vitreous glucose has been investigated by means of inelastic neutron scattering experiments, exploiting the coherent scattering cross section of deuterium in a fully deuterated sample and the high incoherent scattering cross section of hydrogen in a hydrogenated sample. The first part of the experiment allowed a rather detailed investigation of the collective dynamics in the THz range. The second part of the experiment was used to derive some information on the vibrational density of states of the system. The experiment confirms the presence of a propagating vibrational mode which is the natural extension at THz frequencies of the lower frequencies longitudinal sound mode. In addition, a second mode is also observed at a lower and almost constant frequency, showing an increasing intensity on reducing the wavelength. By comparing the dispersion relations of these collective modes to the experimental density of states, a possible relation between the low frequency mode and the well known excess of low frequency modes, that is, the boson peak, is identified.

The key design parameters for the ESS provide new opportunities in neutron diffraction. The long pulse at 5 MW is most intense and about two orders of magnitude larger than the neutron flux of today's leading pulsed sources. The peak brightness clearly exceeds those of the existing short pulse spallation sources. Tailoring the pulse with fast choppers results in a very flexible time-resolution and yields a unique versatility for measuring either with highest resolution or highest intensity for tiny samples or real-time studies of chemical reactions. This versatility is a characteristic feature of the proposed DREAM powder diffractometer [1], which can ultimately offer a d-resolution of 0.00028 Å. Other interesting features with respect to efficiency are the simultaneous use of the thermal and cold ESS moderators, by use of a solid Si bender which will be reflecting the cold neutrons into the incident beam, while transmitting the thermal neutrons. The detectors are based on a new technology using B-10 coated cathodes in inclined geometry [2] covering a large solid angle with position sensitivity appropriate for powder and single crystal diffraction. The instrument has entered into construction in early 2017. The project scope of the instrument has been set with a budget to deliver a world leading neutron powder diffractometer already with the start of user operation at the ESS in 2023. The design has been driven by the broad science case received from the European user community. These cases emphasize the needs for neutron diffraction for small or complex samples, in-situ studies of batteries, metal-organic framework structures, and phase-studies with weak signals related to magnetism and superconductivity. Possible upgrade options cover high-pressure studies with diamond anvil cells and polarized neutrons distinguishing magnetic diffraction or removing the typical large background of hydrogenous materials. With an additional detector for small scattering angles, DREAM will probe multiple length scales within a Q-range from 0.01 to 25 1/Å, which is sufficient for PDF-studies as well as for small angle scattering of nanoparticles. Simulating and benchmarking the instrument to world leading instruments demonstrates a far superior performance. [1] Schweika, W. et al. (2016) J. Physics Conf. Ser. 746 012013. [2] Modzel, G. et al. (2014) NIM A 743 90-95.

A multi-technique approach, combining circular dichroism spectroscopy, ultraviolet resonance Raman spectroscopy and small angle scattering techniques, has been deployed to elucidate how the structural features of the human telomeric G-quadruplex d[A(GGGTTA)3GGG] (Tel22) change upon thermal unfolding. The system is studied both in the free form and when it is bound to Actinomycin D (ActD), an anticancer ligand with remarkable conformational flexibility. We find that at room temperature binding of Tel22 with ActD involves end-stacking upon the terminal G-tetrad. Structural evidence for drug-driven dimerization of a significant fraction of the G-quadruplexes is provided. When the temperature is raised, both free and bound Tel22 undergo melting through a multi-state process. We show that in the intermediate states of Tel22 the conformational equilibrium is shifted toward the (3+1) hybrid-type, while a parallel structure is promoted in the complex. The unfolded state of the free Tel22 is consistent with a self-avoiding random-coil conformation, whereas the high-temperature state of the complex is observed to assume a quite compact form. Such an unprecedented high-temperature arrangement is caused by the persistent interaction between Tel22 and ActD, which stabilizes compact conformations even in the presence of large thermal structural fluctuations.