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991.
We study the statistics of the optimal path in both random and scale-free networks, where weights are taken from a general distribution P(w). We find that different types of disorder lead to the same universal behavior. Specifically, we find that a single parameter (S defined as AL(-1/v) for d-dimensional lattices, and S defined as AN(-1/3) for random networks) determines the distributions of the optimal path length, including both strong and weak disorder regimes. Here v is the percolation connectivity exponent, and A depends on the percolation threshold and P(w). We show that for a uniform P(w), Poisson or Gaussian, the crossover from weak to strong does not occur, and only weak disorder exists. 相似文献
992.
Laye RH Wei Q Mason PV Shanmugam M Teat SJ Brechin EK Collison D McInnes EJ 《Journal of the American Chemical Society》2006,128(28):9020-9021
The synthesis, crystal structure and preliminary magnetic studies are reported for a very highly reduced, VIV8VIII2, polyoxo(alkoxo)vanadate with a unique square prismatic topology. 相似文献
993.
Quality-control materials in the USDA National Food and Nutrient Analysis Program (NFNAP) 总被引:1,自引:0,他引:1
Phillips KM Patterson KY Rasor AS Exler J Haytowitz DB Holden JM Pehrsson PR 《Analytical and bioanalytical chemistry》2006,384(6):1341-1355
The US Department of Agriculture (USDA) Nutrient Data Laboratory (NDL) develops and maintains the USDA National Nutrient Databank
System (NDBS). Data are released from the NDBS for scientific and public use through the USDA National Nutrient Database for
Standard Reference (SR) (). In 1997 the NDL initiated the National Food and Nutrient Analysis Program (NFNAP) to update and expand its food-composition
data. The program included: 1) nationwide probability-based sampling of foods; 2) central processing and archiving of food
samples; 3) analysis of food components at commercial, government, and university laboratories; 4) incorporation of new analytical
data into the NDBS; and 5) dissemination of these data to the scientific community. A key feature and strength of the NFNAP
was a rigorous quality-control program that enabled independent verification of the accuracy and precision of analytical results.
Custom-made food-control composites and/or commercially available certified reference materials were sent to the laboratories,
blinded, with the samples. Data for these materials were essential to ongoing monitoring of analytical work, to identify and
resolve suspected analytical problems, to ensure the accuracy and precision of results for the NFNAP food samples. 相似文献
994.
995.
The crystal structures of tetraphenylphosphonium squarate, bianthrone, and bis(benzophenone)azine are shown to contain a variety of C-H(delta+)...(delta+)H-C interactions, as well as a variety of C-H...O and C-H...C(pi) interactions. Each of these molecules possesses interactions that can possibly be characterized as either H-H bonds or weak hydrogen bonds based on the first four criteria proposed by Koch and Popelier. These interactions have been completely characterized topologically after the multipole refinement of the structures. It appears that weak interactions of the form C-H(delta+)...(delta+)H-C possess certain correlations between the various properties of the electron density at the bond critical points. The coexistence of the three types of interactions makes it possible to establish similarities and differences in the correlations of these weak interactions. This all leads to a better understanding of H-H interactions and how they fit into the hierarchy of weak interactions. 相似文献
996.
Topol IA Nemukhin AV Salnikow K Cachau RE Abashkin YG Kasprzak KS Burt SK 《The journal of physical chemistry. A》2006,110(12):4223-4228
Enzymatic hydroxylation reactions carried out by 2-oxoglutarate (2OG) dependent iron-containing oxygenases were recently implicated in oxygen sensing. In addition to oxygen depletion, two metals, cobalt and nickel, are capable of inducing hypoxic stress in cells by inhibiting oxygenase activity. Two possible scenarios have been proposed for the explanation of the hypoxic effects of cobalt and nickel: oxidation of enzyme-bound iron following cobalt or nickel exposure, and substitution of iron by cobalt or nickel. Here, by using density functional theory calculations, we modeled the reaction route from the reaction components to the high-spin metal-oxide intermediate in the activation of oxygen molecule by 2OG-dependent enzymes for three metal ions Fe(II), Ni(II), and Co(II) in the active site. An initial molecular model was constructed based on the crystal structure of iron-containing asparaginyl hydroxylase (FIH-1). Nickel- and cobalt-containing enzymes were modeled by a consequent replacement of the iron in the active center. The energy profiles connecting stationary points on the potential surfaces were computed by using the intrinsic reaction coordinate (IRC) technique from the located transition states. The results of calculations show that the substitution of iron by nickel or cobalt modifies the reaction energy profile; however, qualitatively, the reaction mechanism remains essentially the same. Thus, we would postulate that if the iron ion in the active site were substitutable by nickel and/or cobalt ions enzyme activity would be considerably altered due to high activation barriers. 相似文献
997.
Wolstenholme DJ Robertson KN Gonzalez EM Cameron TS 《The journal of physical chemistry. A》2006,110(46):12636-12643
The crystal structure of the nonlinear optical material, iodoform octasulfur (CHI3.(S8)3), in the polar space group R3m, has been shown to contain three unique S...I and several S...S close contacts (相似文献
998.
One challenge of biology, medicine, and economics is that the systems treated by these serious scientific disciplines have no perfect metronome in time and no perfect spatial architecture—crystalline or otherwise. Nonetheless, as if by magic, out of nothing but randomness one finds remarkably fine-tuned processes in time and remarkably fine-tuned structures in space. Further, many of these processes and structures have the remarkable feature of “switching” from one behavior to another as if by magic. The past century has, philosophically, been concerned with placing aside the human tendency to see the universe as a fine-tuned machine. Here we will address the challenge of uncovering how, through randomness (albeit, as we shall see, strongly correlated randomness), one can arrive at some of the many spatial and temporal patterns in biology, medicine, and economics and even begin to characterize the switching phenomena that enables a system to pass from one state to another. Inspired by principles developed by A. Nihat Berker and scores of other statistical physicists in recent years, we discuss some applications of correlated randomness to understand switching phenomena in various fields. Specifically, we present evidence from experiments and from computer simulations supporting the hypothesis that water’s anomalies are related to a switching point (which is not unlike the “tipping point” immortalized by Malcolm Gladwell), and that the bubbles in economic phenomena that occur on all scales are not “outliers” (another Gladwell immortalization). Though more speculative, we support the idea of disease as arising from some kind of yet-to-be-understood complex switching phenomenon, by discussing data on selected examples, including heart disease and Alzheimer disease. 相似文献
999.
John C. Duda Patrick E. Hopkins Thomas E. Beechem Justin L. Smoyer Pamela M. Norris 《Superlattices and Microstructures》2010
The presented model predicts thermal boundary conductance at interfaces where one material comprising the junction is characterized by high elastic anisotropy. In contrast to previous approaches, the current methodology accounts for contributions from inelastic scattering through consideration of multiple-phonon interactions. Inelastic contributions become significant as the temperature, as well as the degree of acoustic mismatch between the materials, increases. Inclusion of the inelastic interactions is necessary for a variety of interfacial systems including the metal–graphite boundary examined here. Improvement is shown over existing approaches that address only elastic scattering as both three- and four-phonon interactions significantly augment the transport. 相似文献
1000.
Chin H. Wu Bibhuti B. Das Stanley J. Opella 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2010,202(2):127-134
13C NMR of isotopically labeled methyl groups has the potential to combine spectroscopic simplicity with ease of labeling for protein NMR studies. However, in most high resolution separated local field experiments, such as polarization inversion spin exchange at the magic angle (PISEMA), that are used to measure 1H–13C hetero-nuclear dipolar couplings, the four-spin system of the methyl group presents complications. In this study, the properties of the 1H–13C hetero-nuclear dipolar interactions of 13C-labeled methyl groups are revealed through solid-state NMR experiments on a range of samples, including single crystals, stationary powders, and magic angle spinning of powders, of 13C3 labeled alanine alone and incorporated into a protein. The spectral simplifications resulting from proton detected local field (PDLF) experiments are shown to enhance resolution and simplify the interpretation of results on single crystals, magnetically aligned samples, and powders. The complementarity of stationary sample and magic angle spinning (MAS) measurements of dipolar couplings is demonstrated by applying polarization inversion spin exchange at the magic angle and magic angle spinning (PISEMAMAS) to unoriented samples. 相似文献