59 research outputs found
Crowding effect on helix-coil transition: beyond entropic stabilization
We report circular dichroism measurements on the helix-coil transition of
poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding
agent. Using small angle neutron scattering, PEG solutions have been
characterized and found to be well described by the picture of a transient
network of mesh size , usual for semi-diluted chains in good solvent. We
show that the increase of PEG concentration stabilizes the helices and
increases the transition temperature. But more unexpectedly we also notice that
the increase of crowding agent concentration reduces the mean helix extent at
the transition, or in other words reduces its cooperative feature. This result
cannot be accounted for by an entropic stabilization mechanism. Comparing the
mean length of helices at the transition and the mesh size of the PEG network,
our results strongly suggest two regimes: helices shorter or longer than the
mesh size
4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)
We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.
Vibrational density of states and elastic properties of cross-linked polymers: combining inelastic light and neutron scattering
The vibrational dynamics of a new class of cross-linked polymers made up of cyclodextrins is here investigated in the microscopic range by the joint use of light and inelastic neutron scattering experiments. The effect of increasing the connectivity of the polymeric network on the vibrational dynamics of the system is studied by exploiting the complementarity of these two different probes. The derived
densities of vibrational states of the polymers evidence the
presence of the characteristic anomalous excess of vibrational
modes with respect to the Debye level, already observed in the low-frequency Raman spectra and referred to as boson peak (BP). The overall analysis of the spectra suggests an emerging picture in which the motions of hydrogen atoms of the polymers are progressively hampered when the cross-linking degree of the covalent network increases. At the same time, the frequency and intensity of the BP are found to significantly change by increasing the cross-linking degree of the polymeric network, as clearly suggested by the existence of a scaling-law for the BP evolution. These findings support the conclusion that the growing of the covalent connectivity of the system induces a general modifications of the elastic properties of these cyclodextrin-based polymers, which are, once again, modulated by the cross-linking agent/cyclodextrin molar ratio
Water dynamics in Shewanella oneidensis at ambient and high pressure using quasi-elastic neutron scattering
Quasielastic neutron scattering (QENS) is an ideal technique for studying water transport and relaxation dynamics at pico- to nanosecond timescales and at length scales relevant to cellular dimensions. Studies of high pressure dynamic effects in live organisms are needed to understand Earth’s deep biosphere and biotechnology applications. Here we applied QENS to study water transport in Shewanella oneidensis at ambient (0.1 MPa) and high (200 MPa) pressure using H/D isotopic contrast experiments for normal and perdeuterated bacteria and buffer solutions to distinguish intracellular and transmembrane processes. The results indicate that intracellular water dynamics are comparable with bulk diffusion rates in aqueous fluids at ambient conditions but a significant reduction occurs in high pressure mobility. We interpret this as due to enhanced interactions with macromolecules in the nanoconfined environment. Overall diffusion rates across the cell envelope also occur at similar rates but unexpected narrowing of the QENS signal appears between momentum transfer values Q = 0.7–1.1 Å−1 corresponding to real space dimensions of 6–9 Å. The relaxation time increase can be explained by correlated dynamics of molecules passing through Aquaporin water transport complexes located within the inner or outer membrane structures
The instrument suite of the European Spallation Source
An overview is provided of the 15 neutron beam instruments making up the initial instrument suite of the
European Spallation Source (ESS), and being made available to the neutron user community. The ESS neutron
source consists of a high-power accelerator and target station, providing a unique long-pulse time structure
of slow neutrons. The design considerations behind the time structure, moderator geometry and instrument
layout are presented.
The 15-instrument suite consists of two small-angle instruments, two reflectometers, an imaging beamline,
two single-crystal diffractometers; one for macromolecular crystallography and one for magnetism, two powder
diffractometers, and an engineering diffractometer, as well as an array of five inelastic instruments comprising
two chopper spectrometers, an inverse-geometry single-crystal excitations spectrometer, an instrument for vibrational
spectroscopy and a high-resolution backscattering spectrometer. The conceptual design, performance
and scientific drivers of each of these instruments are described.
All of the instruments are designed to provide breakthrough new scientific capability, not currently
available at existing facilities, building on the inherent strengths of the ESS long-pulse neutron source of high
flux, flexible resolution and large bandwidth. Each of them is predicted to provide world-leading performance
at an accelerator power of 2 MW. This technical capability translates into a very broad range of scientific
capabilities. The composition of the instrument suite has been chosen to maximise the breadth and depth
of the scientific impact o
La spectroscopie neutronique à écho de spin à champ nul ou par résonance
Ce cours présente un nouveau type de spectromètre à écho de spin, baptisé écho de spin par résonance, directement inspiré de la résonance magnétique nucléaire. Dans une première partie, nous rappelons la notion d'écho de spin et son application à la spectrométrie de haute résolution. Puis nous présentons les principes théoriques du spectromètre à écho de spin par résonance. Le spectromètre G1bis au laboratoire Léon Brillouin sert d'exemple aux performances de cette technique. Celle-ci est ensuite illustrée par quelques exemples d'expériences
Compression of random coils due to macromolecular crowding: Scaling effects
The addition of a macromolecular crowding agent to a dilute solution of polymer exerts a compressive force that tends to reduce the size of the chain. We study here the effect of changing the size ratio between the random coil and the crowding agent. The compression occurs at lower crowding agent concentration, Phi when polymer molecular weight increases. The Flory exponent nu(Phi) decreases from nu(0)similar or equal to 0.48 in water down to 0.3 with macromolecular crowding. The effective polymer-polymer interactions change from repulsive to strongly attractive inducing aggregation of the chains. This effect changes drastically for larger polymer sizes, being much more pronounced at high molecular weights
Reply to Comment on Dynamics of water in a molecular sieve by quasielastic neutron scattering
- …