Quantum phase transition in quasi-one-dimensional BaRu6O12

We report the first systematic study of the electrical transport and magnetic properties of BaRu6O12, which has a quasi-one-dimensional (quasi-1D) hollandite structure. We show that BaRu6O12 is quasi-1D electronically as well. Its physical properties were found to be extremely sensitive to disorder. Furthermore, a transition from being metallic with a resistance drop around 2 K to being weakly insulating as the applied magnetic field was increased was also found. We propose that these two features are related to the possible presence of a quantum phase transition in this material system.     

Displacement affinity chromatography of protein phosphatase one (PP1) complexes

Background  

Protein phosphatase one (PP1) is a ubiquitously expressed, highly conserved protein phosphatase that dephosphorylates target protein serine and threonine residues. PP1 is localized to its site of action by interacting with targeting or regulatory proteins, a majority of which contains a primary docking site referred to as the RVXF/W motif.     

High-throughput, low-volume, multianalyte quantification of plasma metabolites related to one-carbon metabolism using HPLC-MS/MS

Risk of chronic diseases, like cardiovascular disease and cancer, has been associated with biomarkers related to one-carbon metabolism, which comprises a metabolic network of cross-talking pathways. To address this complexity in epidemiological studies, we have established an isotope dilution HPLC-MS/MS method for quantification of 12 biomarkers and metabolites. All sample handling is performed by a robotic workstation. The assay uses 45 μL of plasma, and sample treatment consists of protein precipitation by trichloroacetic acid. The analytes were separated on a Fortis Phenyl column using an isocratic mobile phase that contained water, methanol and acetic acid. Methionine, methionine sulfoxide, choline, betaine, dimethylglycine, arginine, asymmetric dimethylarginine, symmetric dimethylarginine, homoarginine, creatinine, cystathionine and trimethyllysine all showed limits of detection well below the 5th percentile of plasma distributions in healthy humans, coefficients of variation were in the range 2.2–12.3 %, and recoveries were 80–131 %. Simple sample processing, low-volume consumption, multiplexing and high capacity/short run time of this method make it suitable for large-scale metabolic profiling of precious biobank samples.     

Chemo- and regioselective cyclohydrocarbonylation of alpha-keto alkynes catalyzed by a zwitterionic rhodium complex and triphenyl phosphite

alpha-Keto alkynes react with CO and H(2) in the presence of catalytic quantities of the zwitterionic rhodium complex (eta(6)-C(6)H(5)BPh(3))(-)Rh(+)(1,5-COD) and triphenyl phosphite affording either the 2-, 2(3H)-, or 2(5H)-furanones in 61-93% yields. The cyclohydrocarbonylation is readily accomplished using substrates containing alkyl, aryl, vinyl, and alkoxy groups at the acetylenic terminal, as well as a variety of primary, secondary, and tertiary alkyl, aryl, and heteroaryl groups connected to the ketone functionality. Structural and electronic properties present in the starting materials mediate the chemo- and regioselectivity of the reaction.     

Determination of vitamins A, D and E in a small volume of human plasma by a high-throughput method based on liquid chromatography/tandem mass spectrometry

We have developed an automated high-throughput assay for the determination of vitamin A (retinol), ergocalciferol (25-OH D2), cholecalciferol (25-OH D3) and vitamin E (α-tocopherol) in a small volume of human plasma. Sample preparation involved mixing 50 μL of plasma with 100 μL of ethanol containing isotope-labelled internal standards, followed by mixing with isooctane/chloroform (3:1, 300 μL). The organic phase was evaporated, and the sample reconstituted in 50 μL methanol. The analysis was performed using reversed-phase liquid chromatography with a gradient mobile phase containing water, methanol and ammonium formate. Chromatographic run-time was 5 min, and positive mode electrospray tandem mass spectrometry (MS/MS) was used for detection. The limits of detection were 0.10 μM for all-trans retinol and 3.3 nM for 25-OH D2 and 25-OH D3. Recoveries were 91.9-105.0%, and within- and between-day coefficients of variance (CVs) 2.4-5.3 and 3.1-8.2, respectively. The assay is presently being used in large-scale studies.     

Quantum-classical reentrant relaxation crossover in Dy2Ti2O7 spin ice

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.     

Defect mode and crystal-electric-field effects on the thermal expansion and heat capacity of RB<Subscript>50</Subscript> boride

This study aimed to evaluate the suitability of using unfired and fired pumice as cement replacement materials as well as their effect on the thermal resistance of hardened ordinary Portland cement (OPC) pastes. Different OPC–pumice (unfired and fired) blends were prepared by partial replacement of OPC by 0, 10 and 20 of pumice (mass%). The effect of the addition of 1 and 5 % of active alumina on the mechanical properties and thermal resistance of different OPC–pumice (unfired) blends was investigated. The fire resistance test was done by exposing the hardened blended cement cubes to elevated temperatures of 200, 400, 600 and 800 °C for 3 h and allowed them to cool down to room temperature before testing for their mechanical properties. The phase composition and thermal analysis of some selected specimens were investigated by XRD, DSC and DTA/TG techniques. The obtained results indicated that replacing OPC by 10 and 20 % by pumice (unfired and fired) improved its thermal stability at different firing temperatures. The cement blend prepared by replacement of OPC with 10 % pumice showed the highest fire resistance. The addition of 1 and 5 % of alumina (A) to OPC–pumice blends causes a notable improve in their mechanical properties and thermal resistance.     

Slow spin relaxation in a highly polarized cooperative paramagnet

We report measurements of the ac susceptibility of the cooperative paramagnet Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation maximum in chi(T) and also an unexpected frequency dependence of this peak at low frequencies (<1 Hz), suggesting very slow spin relaxations are occurring. Measurements on samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated with dipolar spin correlations, representing unusual cooperative behavior in a paramagnetic system.     

Unconventional dynamics in triangular Heisenberg antiferromagnet NaCrO2

We report magnetization, specific heat, muon spin rotation, and Na NMR measurements on the S=3/2 rhombohedrally stacked Heisenberg antiferromagnet NaCrO2. This compound appears to be a good candidate for the study of isotropic triangular Heisenberg antiferromagnets with very weak interlayer coupling. While specific heat and magnetization measurements indicate the onset of a transition in the range Tc approximately 40-50 K, both muon spin rotation and NMR reveal a fluctuating crossover regime extending well below Tc, with a peak of relaxation rate T1(-1) around T approximately 25 K. This novel finding is discussed within the context of excitations in the triangular Heisenberg antiferromagnets.     

Quantitative profiling of biomarkers related to B‐vitamin status,tryptophan metabolism and inflammation in human plasma by liquid chromatography/tandem mass spectrometry

Vitamins B₂ and B₆ serve as cofactors in enzymatic reactions involved in tryptophan and homocysteine metabolism. Plasma concentrations of these vitamins and amino acids are related to smoking and inflammation, and correlate with other markers of immune activation. Large‐scale studies of these relations have been hampered by lack of suitable analytical methods. The assay described includes riboflavin, five vitamin B₆ forms (pyridoxal 5′‐phosphate, pyridoxal, 4‐pyridoxic acid, pyridoxine and pyridoxamine), tryptophan and six tryptophan metabolites (kynurenine, kynurenic acid, anthranilic acid, 3‐hydroxykynurenine, xanthurenic acid and 3‐hydroxyanthranilic acid), cystathionine, neopterin and cotinine. Trichloroacetic acid containing 13 isotope‐labelled internal standards was added to 60 µL of plasma, the mixture was centrifuged, and the resulting supernatant used for analysis. The analytes were separated within 5 min on a stable‐bond C8 column by a gradient‐type mobile phase containing acetonitrile, heptafluorobutyric acid and high concentration (650 mmol/L) of acetic acid, and detected using electrospray ionization tandem mass spectrometry (ESI‐MS/MS). The mobile phase ensured sufficient separation and high ionization efficiency of all analytes. Recoveries were 75–123% and within‐day and between‐day coefficients of variance (CVs) were 2.5–9.5% and 5.4–16.9%, respectively. Limits of detection ranged from 0.05 to 7 nmol/L. The method enables quantification of endogenous plasma concentrations of 16 analytes related to B‐vitamin status and inflammation, and may prove useful in large‐scale epidemiological studies. Copyright © 2009 John Wiley & Sons, Ltd.