Low dimensional ordering and fluctuations in methanol-β\beta-hydroquinone-clathrate studied by X-ray and neutron diffraction

Methanol-$\beta$-hydroquinone-clathrate has been established as a model system for dielectric ordering and fluctuations and is conceptually close to magnetic spin systems. In X-ray and neutron diffraction experiments, we investigated the ordered structure, the one-dimensional (1D) and the three-dimensional (3D) critical scattering in the paraelectric phase, and the temperature dependence of the lattice constants. Our results can be explained by microscopic models of the methanol pseudospin in the hydroquinone cage network, in consistency with previous dielectric investigations

Nanosecond molecular relaxations in lipid bilayers studied by high energy resolution neutron scattering and in-situ diffraction

We report a high energy-resolution neutron backscattering study to investigate slow motions on nanosecond time scales in highly oriented solid supported phospholipid bilayers of the model system DMPC -d54 (deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine), hydrated with heavy water. Wave vector resolved quasi-elastic neutron scattering (QENS) is used to determine relaxation times $\tau$, which can be associated with different molecular components, i.e., the lipid acyl chains and the interstitial water molecules in the different phases of the model membrane system. The inelastic data are complemented both by energy resolved and energy integrated in-situ diffraction. From a combined analysis of the inelastic data in the energy and time domain, the respective character of the relaxation, i.e., the exponent of the exponential decay is also determined. From this analysis we quantify two relaxation processes. We associate the fast relaxation with translational diffusion of lipid and water molecules while the slow process likely stems from collective dynamics

Short range ballistic motion in fluid lipid bilayers studied by quasi-elastic neutron scattering

Diffusion is the primary mechanism for the movement of lipids and proteins in a biological membrane. It is important in the formation of various macromolecular structures, such as lipid rafts. The commonly accepted theory for diffusion in membranes suggests that the molecules undergo continuous Brownian diffusion at long length scales, with a "rattling-in-the-cage" motion at short length scales, as shown in figure 1. However, this model has recently been challenged by experimental and simulation results. It has been observed that lipids move in loosely bound clusters rather than as individual molecules [1,2], and that there is a flow-like component to long range lipid diffusion [3]. Ballistic and sub-diffusive regimes have been observed in molecular dynamics simulations [4,5]. Diffusion is mainly studied by two experimental methods: fluorescence techniques and incoherent quasi-elastic neutron scattering. The two techniques access distinctly different length scales, resulting in a "blind spot" at mesoscopic distances. We note that the diffusion coefficients measured by these two techniques often differ by as much as orders of magnitude. The mechanism for diffusion, therefore, seems to depend on the length scale at which it is observed. The blind spot in the mesoscopic range will hopefully be closed in the future using high energy resolution lamor precession techniques performed with spin-echo spectrometers. To extend the window of length scales and investigate the motion of lipid molecules at very short distances, we used the unique capabilities of the IN13 thermal backscattering spectrometer. IN13 provides access to an exceptionally large Q range, covering length scales from 1.3 to 31 Å (0.2 Å -1 < Q < 5 Å -1 ). We used IN13 to study lipid diffusion at length scales smaller than a typical lipid-lipid distance in fluid bilayers. The aim of the experiment was to prove the validity of the Brownian diffusion model down to very small length scales. We chose a stacked model membrane system (DMPC) for this study and analysed the quasi-elastic neutron scattering response of the lipid molecules. Membranes were prepared as solid-supported, multi-lamellar membrane stacks on silicon wafers. Protonated lipids were hydrated by heavy water, so that the experiments were sensitive to the incoherent signal of the lipids. To increase the scattering signal, several wafers with thousands of highly oriented membranes were stacked. The membranes were studied in their physiologically relevant fluid state, at high temperature (T=30 °C) and full hydration. The width of the quasi-elastic energy response (full width at half maximum, FWHM) is shown in figure 2. If a particle diffuses via random Brownian motion, the time evolution of its displacement can be written as = 2Dt, and the quasi-elastic energy broadening has a Lorentzian shape, which demonstrate

Order and disorder - An integrative structure of the full-length human growth hormone receptor

Because of its small size (70 kilodalton) and large content of structural disorder (>50%), the human growth hormone receptor (hGHR) falls between the cracks of conventional high-resolution structural biology methods. Here, we study the structure of the full-length hGHR in nanodiscs with small-angle x-ray scattering (SAXS) as the foundation. We develop an approach that combines SAXS, x-ray diffraction, and NMR spectroscopy data obtained on individual domains and integrate these through molecular dynamics simulations to interpret SAXS data on the full-length hGHR in nanodiscs. The hGHR domains reorient freely, resulting in a broad structural ensemble, emphasizing the need to take an ensemble view on signaling of relevance to disease states. The structure provides the first experimental model of any full-length cytokine receptor in a lipid membrane and exemplifies how integrating experimental data from several techniques computationally may access structures of membrane proteins with long, disordered regions, a widespread phenomenon in biology

Aging and memory effects in beta-hydrochinone-clathrate

The out-of-equilibrium low-frequency complex susceptibility of the orientational glass methanol(73%)-beta-hydrochinone-clathrate is studied using temperature-stop protocols in aging experiments . Although the material does not have a sharp glass transition aging effects including rejuvenation and memory are found at low temperatures. However, they turn out to be much weaker, however, than in conventional magnetic spin glasses.Comment: 5 pages RevTeX, 6 eps-figures include

Neutron diffraction study of YVO3, NdVO3, and TbVO3

The structural and magnetic properties of YVO3, NdVO3 and TbVO3 were investigated by single-crystal and powder neutron diffraction. YVO3 shows a structural phase transition at 200 K from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group P21/b. But supplementary high-resolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium t2g orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below 77 K. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of NdVO3 and TbVO3 are closely similar to that of YVO3. However, only a single magnetic phase transition was found in the vanadium sublattice down to 9.5 K. Below 60 K the magnetic moments of the Nd- and Tb-ions are gradually polarized by the ordered vanadium moments. Below 11 K, we found a noncollinear order of the terbium moments

Ethanol enhances collective dynamics of lipid membranes

From inelastic neutron-scattering experiments and all atom molecular dynamics simulations we present evidence for a low-energy dynamical mode in the fluid phase of a 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayer immersed in a 5% water/ethanol solution. In addition to the well-known phonon that shows a liquidlike dispersion with energies up to 4.5 meV, we observe an additional mode at smaller energies of 0.8 meV, which shows little or no dispersion. Both modes show transverse properties and might be related to molecular motion perpendicular to the bilayer