Judith Peters

Hydration dependent studies of highly aligned multilayer lipid membranes by neutron scattering

We investigated molecular motions on a picosecond timescale of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) model membranes as a function of hydration by using elastic and quasielastic neutron scattering. Two different hydrations corresponding to approximately nine and twelve water molecules per lipid were studied, the latter being the fully hydrated state. In our study, we focused on head group motions by using chain deuterated lipids. Information on in-plane and out-of-plane motions could be extracted by using solid supported DMPC multilayers. Our studies confirm and complete former investigations by König et al. [J. Phys. II (France) 2, 1589 (1992)] and Rheinstädter et al. [Phys. Rev. Lett. 101, 248106 (2008)] who described the dynamics of lipid membranes, but did not explore the influence of hydration on the head group dynamics as presented here. From the elastic data, a clear shift of the main phase transition from the P(β) ripple phase to the L(α) liquid phase was observed. Decreasing water content moves the transition temperature to higher temperatures. The quasielastic data permit a closer investigation of the different types of head group motion of the two samples. Two different models are needed to fit the elastic incoherent structure factor and corresponding radii were calculated. The presented data show the strong influence hydration has on the head group mobility of DMPC.

Reversible membrane reorganizations during photosynthesis in vivo: Revealed by small-angle neutron scattering

In the present study, we determined characteristic repeat distances of the photosynthetic membranes in living cyanobacterial and eukaryotic algal cells, and in intact thylakoid membranes isolated from higher plants with time-resolved small-angle neutron scattering. This non-invasive technique reveals light-induced reversible reorganizations in the seconds-to-minutes time scale, which appear to be associated with functional changes in vivo.

Dynamics of heparan sulfate explored by neutron scattering

The temperature dependence of atomic fluctuations in heparan sulfate was measured for different time-scales between the picosecond and the nanosecond. The data established the role of hydration for the emergence of high-amplitude motions at 200-240 K, and the higher resilience of the polysaccharide compared to proteins measured under similar conditions.

Influence of pressure and crowding on the sub-nanosecond dynamics of globular proteins.

The influence of high hydrostatic pressure on the internal sub-nanosecond dynamics of highly concentrated lysozyme in aqueous solutions was studied by elastic incoherent neutron scattering (EINS) up to pressures of 4 kbar. We have found, with increasing pressure, a reduction in the dynamics of H atoms of folded lysozyme, suggesting a loss in protein mobility that follows a change in the local energy landscape upon the increase in packing density. Moreover, the amplitude of the protein fluctuations depends drastically on the protein concentration, and protein structural and interaction parameters as well as the dynamical properties are affected by pressure in a nonlinear way. A significant reduction of the mean squared displacement of H atoms occurs already at rather low pressures of a few hundred bars for lysozyme in bulk water solution. This trend is lifted at ∼2 kbar, which is probably due to a solvent-mediated effect. Conversely, for high protein concentrations (e.g., 160 mg mL(-1)), that is, under strong self-crowding conditions, as they are also encountered in the biological cell, strong restriction of the dynamics of protein motions takes place, reducing the mean squared displacement of H atoms by 60% and rendering its pressure dependence almost negligible. These results are also important for understanding the pressure stability of highly concentrated protein solutions in organisms thriving under hydrostatic pressure conditions such as in the deep sea, where pressures up to the kbar level are reached.

Virtual experiments: the ultimate aim of neutron ray-tracing simulations

We define a virtual neutron experiment as a complete simulation of an experiment, from source over sample to detector. The virtual experiment (VE) will ideally interface with the instrument control software for the input and with standard data analysis packages for the virtual data output. Virtual experiments are beginning to make their way into neutron scattering science with applications as diverse as instrument design/upgrade, experiment planning, data analysis, test of analysis software, teaching, and outreach. In this paper, we summarize the recent developments in this field and make suggestions for future developments and use of VEs.

High hydrostatic pressure equipment for neutron scattering studies of samples in solutions

The design of new high pressure equipment for structural and dynamical studies on samples in solution is described. We present two sample cells for applying pressures up to 150 and 700 MPa (i.e. 1.5 and 7 kbar), respectively. These cells are mounted on special sticks and inserted into the calorimeter of a cryostat to regulate the temperature. Different parts of the equipment – the pressure controller, the sticks and the cells – are described. In addition, radiography tests which were performed with neutrons in situ at the Institut Laue Langevin to verify the tightness of the cells and the hydrostatic transmission of the pressure to the sample are presented. First results on model lipids are in agreement with former results by R. Winter et al. [Towards an understanding of the temperature/pressure configurational and free-energy landscape of biomolecules, J. Non-Equilib. Thermodyn. 32 (2007), pp. 41–97].

High hydrostatic pressure effects investigated by neutron scattering on lipid multilamellar vesicles

The effects of high hydrostatic pressure on the structure and dynamics of model membrane systems were investigated using neutron scattering. Diffraction experiments show shifts of the pre- and main-phase transitions of multilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) to higher temperatures with increased pressure which are close to results observed previously by other techniques, namely (10.4 ± 1.0) K kbar(-1) and (20.0 ± 0.5) K kbar(-1) for the two transitions. Backscattering spectroscopy reveals that the mean square displacements in the liquid phase are about 10% smaller at 300 bar and about 20% smaller at 600 bar compared to atmospheric pressure, whereas in the gel phase below the main phase transition the mean square displacements show a smaller difference in the dynamics of the three pressure values within the studied pressure range.

Kinetics of structural reorganizations in multilamellar photosynthetic membranes monitored by small angle neutron scattering

We demonstrate the power of time-resolved small-angle neutron scattering experiments for the investigation of the structure and structural reorganizations of multilamellar photosynthetic membranes. In addition to briefly summarizing our results on thylakoid membranes isolated from higher plants and in unicellular organisms, we discuss the advantages and technical and methodological limitations of time-resolved SANS. We present a detailed and more systematical investigation of the kinetics of light-induced structural reorganizations in isolated spinach thylakoid membranes, which show how changes in the repeat distance and in the long-range order of the multilamellar membranes can be followed with a time resolution of seconds. We also present data from comparative measurements performed on thylakoid membranes isolated from tobacco.Graphical abstract

Activity and molecular dynamics relationship within the family of human cholinesterases.

The temperature dependence of the dynamics of recombinant human acetylcholinesterase (hAChE) and plasma human butyrylcholinesterase (hBChE) is examined using elastic incoherent neutron scattering. These two enzymes belong to the same family and present 50% amino acid sequence identity. However, significantly higher flexibility and catalytic activity of hAChE when compared to the ones of hBChE are measured. At the same time, the average height of the potential barrier to the motions is increased in the hBChE, e.g. more thermal energy is needed to cross it in the latter case, which might be the origin of the increase in activation energy and the reduction in the catalytic rate of hBChE observed experimentally. These results suggest that the motions on the picosecond timescale may act as a lubricant for those associated with activity occurring on a slower millisecond timescale.

Molecular adaptation and salt stress response of Halobacterium salinarum cells revealed by neutron spectroscopy

Halobacterium salinarum is an extreme halophile archaeon with an absolute requirement for a multimolar salt environment. It accumulates molar concentrations of KCl in the cytosol to counterbalance the external osmotic pressure imposed by the molar NaCl. As a consequence, cytosolic proteins are permanently exposed to low water activity and highly ionic conditions. In non-adapted systems, such conditions would promote protein aggregation, precipitation, and denaturation. In contrast, in vitro studies showed that proteins from extreme halophilic cells are themselves obligate halophiles. In this paper, adaptation via dynamics to low-salt stress in H. salinarum cells was measured by neutron scattering experiments coupled with microbiological characterization. The molecular dynamic properties of a proteome represent a good indicator for environmental adaptation and the neutron/microbiology approach has been shown to be well tailored to characterize these modifications. In their natural setting, halophilic organisms often have to face important variations in environmental salt concentration. The results showed deleterious effects already occur in the H. salinarum proteome, even when the external salt concentration is still relatively high, suggesting the onset of survival mechanisms quite early when the environmental salt concentration decreases.