Vibrational origin of the fast relaxation processes in molecular glass-formers

We study the interaction of the relaxation processes with the density fluctuations by molecular dynamics simulation of a flexible molecule model for o-terphenyl (oTP) in the liquid and supercooled phases. We find evidence, besides the structural relaxation, of a secondary vibrational relaxation whose characteristic time, few ps, is slightly temperature dependent. This i) confirms the result by Monaco et al. [Phys. Rev, E 62, 7595 (2000)] of the vibrational nature of the fast relaxation observed in Brillouin Light Scattering (BLS) experiments in oTP; and ii) poses a caveat on the interpretation of the BLS spectra of molecular systems in terms of a purely center of mass dynamics.Comment: RevTeX, 5 pages, 4 eps figure

Dipole Interactions and Electrical Polarity in Nanosystems -- the Clausius-Mossotti and Related Models

Point polarizable molecules at fixed spatial positions have solvable electrostatic properties in classical approximation, the most familiar being the Clausius-Mossotti (CM) formula. This paper generalizes the model and imagines various applications to nanosystems. The behavior is worked out for a sequence of octahedral fragments of simple cubic crystals, and the crossover to the bulk CM law is found. Some relations to fixed moment systems are discussed and exploited. The one-dimensional dipole stack is introduced as an important model system. The energy of interaction of parallel stacks is worked out, and clarifies the diverse behavior found in different crystal structures. It also suggests patterns of self-organization which polar molecules in solution might adopt. A sum rule on the stack interaction is found and tested. Stability of polarized states under thermal fluctuations is discussed, using the one-dimensional domain wall as an example. Possible structures for polar hard ellipsoids are considered. An idea is formulated for enhancing polarity of nanosystems by intentionally adding metallic coatings.Comment: 18 pages (includes 6 embedded figures and 3 tables). New references, and other small improvements. Scheduled for publication by J. Chem. Phys., Jan. 200

DNP Enhanced Frequency-Selective TEDOR Experiments in Bacteriorhodopsin

We describe a new approach to multiple [superscript 13]C–[superscript 15]N distance measurements in uniformly labeled solids, frequency-selective (FS) TEDOR. The method shares features with FS-REDOR and ZF- and BASE-TEDOR, which also provide quantitative [superscript 15]N–[superscript 13]C spectral assignments and distance measurements in U-[[superscript 13]C,[superscript 15]N] samples. To demonstrate the validity of the FS-TEDOR sequence, we measured distances in [U-[superscript 13]C,15N]-asparagine which are in good agreement with other methods. In addition, we integrate high frequency dynamic nuclear polarization (DNP) into the experimental protocol and use FS-TEDOR to record a resolved correlation spectrum of the Arg-[superscript 13]Cγ–[superscript 15]Nε region in [U-[superscript 13]C,15N]-bacteriorhodopsin. We resolve six of the seven cross-peaks expected based on the primary sequence of this membrane protein.National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant Number EB-001960)National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant Number EB-002804)National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant Number EB-001035)National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant Number EB-002026

On the stability of hole crystals in layered cuprates

Recent STM measurements have revealed the existence of periodic charge modulations at the surface of certain cuprate superconductors. Here we show that the observed patterns are compatible with the formation of a three-dimensional crystal of doped holes, with space correlations extending between different Cu-O layers. This puts severe constraints on the dynamical stability of the crystallised hole structure, resulting in a close relationship between the periodicity of the electronic modulation and the interlayer distance.Comment: completed reference list, fig. 3 corrected; accepted for publication in Eur. Phys. J. B, Rapid Note

The adipokine sFRP4 induces insulin resistance and lipogenesis in the liver

Secreted frizzled-related protein (sFRP) 4 is an adipokine with increased expression in white adipose tissue from obese subjects with type 2 diabetes and non-alcoholic fatty liver disease (NAFLD). Yet, it is unknown whether sFRP4 action contributes to the development of these pathologies. Here, we determined whether sFRP4 expression in visceral fat associates with NAFLD and whether it directly interferes with insulin action and lipid and glucose metabolism in primary hepatocytes and myotubes. The association of sFRP4 with clinical measures was investigated in obese men with or without type 2 diabetes and with or without biopsy-proven NAFLD. To determine the impact of sFRP4 on metabolic parameters, primary human myotubes (hSkMC), or primary hepatocytes from metabolic healthy C57B16 and from systemic insulin-resistant mice, i.e. aP2-SREBP-1c, were used. Gene expression of sFRP4 in visceral fat from obese men associated with insulin sensitivity, triglycerides and NAFLD. In C57B16 hepatocytes, sFRP4 disturbed insulin action. Specifically, sFRP4 decreased the abundance of IRS1 and FoxO1 together with impaired insulin-mediated activation of Akt-signalling and glycogen synthesis and a reduced suppression of gluconeogenesis by insulin. Moreover, sFRP4 enhanced insulin-stimulated hepatic de novo lipogenesis (DNL). In hSkMC, sFRP4 induced glycolysis rather than inhibiting insulin signalling. Finally, in hepatocytes from aP2-SREBP-1c mice, sFRP4 potentiates existing insulin resistance. Collectively, we show that sFRP4 interferes with hepatocyte insulin action. Physiologically, sFRP4 promotes DNL in hepatocytes and glycolysis in myotubes. These sFRP4-mediated responses may result in a vicious cycle, in which enhanced rates of DNL and glycolysis aggravate hepatic lipid accumulation and insulin resistance

Metallic Xenon, Molecular Condensates, and Superconductivity

A possibility of explaining the light absorption observed to occur under pressure-induced xenon metallization as due to the transition to the superconducting state is analyzed. The mechanism of the van der Waals bonding is discussed.Comment: LaTeX 2.09 (RevTeX), 4 pages, 4 PostScript figures included in tex

Statistical-mechanical theory of ultrasonic absorption in molecular liquids

We present results of theoretical description of ultrasonic phenomena in molecular liquids. In particular, we are interested in the development of microscopical, i.e., statistical-mechanical framework capable to explain the long living puzzle of the excess ultrasonic absorption in liquids. Typically, ultrasonic wave in a liquid can be generated by applying the periodically alternating external pressure with the angular frequency that corresponds to the ultrasound. If the perturbation introduced by such process is weak - its statistical-mechanical treatment can be done with the use of the linear response theory. We treat the liquid as a system of interacting sites, so that all the response/aftereffect functions as well as the energy dissipation and generalized (wave-vector and frequency dependent) ultrasonic absorption coefficient are obtained in terms of familiar site-site static and time correlation functions such as static structure factors or intermediate scattering functions. To express the site-site intermediate scattering functions we refer to the site-site memory equations in the mode-coupling approximation for the first-order memory kernels, while equilibrium properties such as site-site static structure factors, direct and total correlation functions are deduced from the integral equation theory of molecular liquids known as RISM or one of its generalizations. All the formalism is phrased in a general manner, hence the obtained results are expected to work for arbitrary type of molecular liquid including simple, ionic, polar, and non-polar liquids.Comment: 14 pages, 1 eps-figure, RevTeX4-forma

Quasi Harmonic Lattice Dynamics and Molecular Dynamics calculations for the Lennard-Jones solids

We present Molecular Dynamics (MD), Quasi Harmonic Lattice Dynamics (QHLD) and Energy Minimization (EM) calculations for the crystal structure of Ne, Ar, Kr and Xe as a function of pressure and temperature. New Lennard-Jones (LJ) parameters are obtained for Ne, Kr and Xe to reproduce the experimental pressure dependence of the density. We employ a simple method which combines results of QHLD and MD calculations to achieve densities in good agreement with experiment from 0 K to melting. Melting is discussed in connection with intrinsic instability of the solid as given by the QHLD approximation. (See http://www.fci.unibo.it/~valle for related papers)Comment: 7 pages, 5 figures, REVte

Mediterranean conundrums : pluridisciplinary perspectives for research in the social sciences

This paper has two purposes. First, it summarises the various papers presented at a Pluridisciplinary Conference on the Mediterranean treating the region from a variety of perspectives, a selection of which are published in this issue of History and Anthropology. Second, it attempts to explore some of the tensions between historians and anthropologists, and political scientists and geographers, in the treatment of the region.peer-reviewe