L. C. Pardo

Molecular Mechanism of Long-Range Diffusion in Phospholipid Membranes Studied by Quasielastic Neutron Scattering

The motion of phospholipids has previously been studied on many time scales due to the significance for living cells and technological applications. The motions on a pico- to nanosecond time scale were determined by quasielastic neutron scattering (QENS) to be much faster than the ones on the microsecond scale covered by fluorescence recovery after photobleaching (FRAP). This was explained by assuming that the molecules rattle fast in a cage of neighbors (observed with QENS) from which they escape once in a while; this escape was then the primary step of the slower diffusion measured by FRAP. However, nanosecond MD simulation studies could not observe any escape events; recent findings even suggested that the long-range motion in phospholipid membranes on short time scales is not diffusive but has flow-like characteristics. To check this novel view, we have repeated the QENS experiments with todays significantly improved instrumentation. By using the advantage of QENS that allows tuning of the observation time in the pico- to nanosecond range, it was possible to study the evolution of motions in this time frame. Localized motions, e.g., of the head and tail groups, appear separated from the long-range motion and do not obfuscate the analysis as they do in a mean squared displacement plot. The results for the long-range motion are indeed compatible with flow patterns, whereas the localized motions can account for the fast motions interpreted as motions in a cage before. Hereby, we give experimental evidence for a completely different mechanism of long-range motion on short time scales in phospholipid membranes.

SPHERES, Jülich's high-flux neutron backscattering spectrometer at FRM II

SPHERES is a third-generation neutron backscattering spectrometer, located at the 20 MW German neutron source FRM II and operated by the Jülich Centre for Neutron Science. It offers an energy resolution (fwhm) better than 0.65 μeV, a dynamic range of ± 31 μeV, and a signal-to-noise ratio of up to 1750:1.

α and β relaxation dynamics of a fragile plastic crystal

We present a thorough dielectric investigation of the relaxation dynamics of plastic crystalline Freon112, which exhibits freezing of the orientational degrees of freedom into a glassy crystal below 90 K. Among other plastic crystals, Freon112 stands out by being relatively fragile within Angells [Relaxations in Complex Systems, edited by K. L. Ngai and G. B. Wright (NRL, Washington, DC, 1985), p. 3] classification scheme and by showing an unusually strong beta relaxation. Comparing the results to those on Freon112a, having only a single molecular conformation, points to the importance of the presence of two molecular conformations in Freon112 for the explanation of its unusual properties.

X-ray and molecular dynamics study of liquid structure in pure methylchloromethane compounds ((CH3)4−nCCln)

We have studied liquid structure for a whole family of methylchloromethane compounds ((CH3)4−nCCln), exploiting the interplay of x-ray diffraction measurements and molecular dynamics (MD) computations. To this end we report for the first time x-ray spectra for 1,1,1-trichloroethane (n=3), and 2,2-dichloropropane (n=2), together with a new determination for carbon tetrachloride (n=4). A consistent set of molecular models for MD simulation has also been developed for the full family, providing excellent accord with thermodynamic properties (vaporization enthalpy and density over the full liquid phase), and with diffraction data alike. The theoretical results have allowed the interpretation of the salient features in the experimental spectra and of the trends peculiar to this family of compounds, basically characterized by the suppression of one of the two main peaks in the spectrum as the number of chlorines is diminished. A numerical method that constructs radial correlation functions for ideal dimer geome...

Broadband dielectric spectroscopy on benzophenone: alpha relaxation, beta relaxation, and mode coupling theory.

We have performed a detailed dielectric investigation of the relaxational dynamics of glass-forming benzophenone. Our measurements cover a broad frequency range of 0.1 Hz to 120 GHz and temperatures from far below the glass temperature well up into the region of the small-viscosity liquid. With respect to the alpha relaxation this material can be characterized as a typical molecular glass former with rather high fragility. A good agreement of the alpha relaxation behavior with the predictions of the mode coupling theory of the glass transition is stated. In addition, at temperatures below and in the vicinity of T(g) we detect a well-pronounced beta relaxation of Johari-Goldstein type, which with increasing temperature develops into an excess wing. We compare our results to literature data from optical Kerr effect and depolarized light scattering experiments, where an excess-wing-like feature was observed in the 1-100 GHz region. We address the question if the Cole-Cole peak, which was invoked to describe the optical Kerr effect data within the framework of the mode coupling theory, has any relation to the canonical beta relaxation detected by dielectric spectroscopy.

Amphipathic solvation of indole: implications for the role of tryptophan in membrane proteins.

The microscopic structure of the tryptophan side chain, indole, in an amphiphilic environment has been investigated using a combination of neutron diffraction measurements and simulations in solution. The results show that indole is preferentially solvated by hydrogen bonding interactions between water and alcohol -OH groups rather than the interaction being dominated by indole-methyl interactions. This has implications for understanding how tryptophan interacts with the amphipathic membrane environment to anchor proteins into membranes, where the results here suggest that the benzene ring of tryptophan interacts directly with the interfacial water at the membrane surface rather than being buried into the hydrophobic regions of the membrane bilayer.

Short-Range Interactions of Concentrated Proline in Aqueous Solution

Molecular interactions for proline in a highly concentrated aqueous solution (up to 1:5 proline:water molecular ratio) have been investigated using a variety of experimental and computational techniques. Rather than the solution containing either small crystallites or large aggregates of proline, three-dimensional structural analysis reveals the presence of proline-proline dimers. These dimers appear to be formed by cyclic electrostatic interactions between CO2(-) and NH2(+) groups on neighboring proline molecules, which causes the ring motifs of proline to be roughly parallel to one another. In addition, water appears to aggregate around the electrostatic groups of the proline-proline dimers where it may in fact bridge these groups on different molecules. The observed short-range interactions for proline in solution may explain its function as a hydrotrope in vivo in which this observed dimerization might allow proline molecules to generate small pockets of a hydrophobic environment that can associate with nonpolar motifs of other molecules in solution. The results presented here emphasize the need for careful three-dimensional analysis to assess the short-range order of highly concentrated solutions.

On the structure of water and chloride ion interactions with a peptide backbone in solution

The arrangement of water and chloride ions around a model peptide (glycyl-L-prolyl-glycine-NH2) was investigated using Molecular Dynamics (MD) simulations and complementary Empirical Potential Structure Refinement (EPSR) simulations which adapt the modelled structure to reproduce experimentally measured neutron diffraction data. The results are in good qualitative agreement and show a common picture for all hydrogen-containing amine and amide groups: namely that there are two common chloride interactions observed - a direct contact between Cl(-) and peptide backbone and a water-mediated interaction. The geometry of this mediation depends on the distance between chloride and nitrogen and hints towards two distinct modes of interaction between water and the ion, either along one of the O-H bonds or along the water dipole.

Dynamic heterogeneity in the glass-like monoclinic phases of CBrnCl4−n, n = 0,1,2

Glassy dynamics of rigid molecules is still a matter of controversy: the physics behind the relaxation process at time scales faster than that ruled by the viscosity, the so called Johari-Goldstein process, is not known. In this work we unravel the mechanism of such a process by using a simple molecular model in which the centers of mass of the molecules are forming an ordered lattice, and molecular reorientation is performed by jumps between equilibrium orientations. We have studied the dynamics of simple quasi-tetrahedral molecules CBr(n)Cl(4-n), n = 0, 1, 2, in their monoclinic phases by means of dielectric spectroscopy and nuclear quadrupole resonance: the first technique allows to measure in a broad time scale but it is insensitive to molecular particularities, while the second has a restricted time window but senses the movement of each chlorine atom separately. The dynamic picture emerging from these techniques is that the secondary relaxation process is related to the different molecular surroundings around each nonequivalent atom of the molecule. Dynamical heterogeneities thus seem to be the cause of the secondary relaxation in this simple model of glass.

Miscibility study in stable and metastable orientational disordered phases in a two-component system (CH3)CCl3+CCl4

The orientationally disordered stable and metastable mixed crystals of the two-component system methylchloroform ((CH3)CCl3)+carbon tetrachloride (CCl4) have been characterised from a crystallographic and thermodynamic point of view. The monotropic behaviour of the metastable phase in the pure components is maintained for the whole range of composition. The lattice symmetry of the stable orientationally disordered phase of methylchloroform has been found to be isostructural with that of the carbon tetrachloride compound. Continuous series of both stable and metastable mixed crystals give rise to a double isomorphism relationship, one for the stable state and another for the metastable stable of the pure components.