New aspects of the reactivity of tyrosinase

Tyrosinase was found to be active in the sulfoxidation of thioanisol, producing the (R)-sulfoxide with high enantiomeric excess. The activity of the enzyme with phenolic and diphenolic substrates in a mixed aqueous Hepes buffer pH 6.8-methanol-glycerol solvent was also investigated over a range of temperatures. These experiments enabled us to deduce the thermodynamic parameters associated with substrate binding to the enzyme and the activation parameters associated with the rate determining step of the enzymatic reaction.     

Effect of strain in the proximal ligand on the binding of nitric oxide and carbon monoxide to chelated protoheme complexes

The binding of NO and CO to chelated protoheme-l-histidine methyl ester (HM-H), protoheme-glycyl-l-histidine methyl ester (HM-GH), and free protoheme (HM) has been studied in methanol-DMSO solution. In all cases, the NO adducts are five-coordinated, indicating that binding of NO occurs with displacement of the axial base, and confirms the strong negative trans effect exerted by NO in heme complexes, though it is found that the presence of strain in the iron-histidine bond of HM-H has a positive influence on NO binding, making it thermodynamically more favorable than for HM-GH. The equilibrium constants thus decrease in the series: HM > HM-H > HM-GH. In contrast to NO, CO has a positive trans effect, and therefore, an opposite trend is observed in the binding of this ligand to the heme complexes.     

Pb促进的Pd/γ-Al2O3催化剂上H2O2氧化甘油反应

制备了Pd含量为1 wt%、不同Pb/Pd摩尔比的γ-Al2O3负载Pd-Pb双金属催化剂,并用于常压、45°C和pH =11条件下H2O2氧化甘油反应中.催化剂的形貌和分散度采用扫描电镜-X射线电子散射谱和透射电镜进行了表征;双金属催化剂中合金相用X射线光电子能谱进行了验证.单金属Pd催化剂上反应结束后甘油转化率为19%,但随着Pb的加入甘油转化率增大到约100%.所制四个不同Pb/Pd原子比的双金属催化剂PdPb0.25, PdPb0.50, PdPb1.00和PdPb1.60均可氧化甘油至二羟基丙酮(DIHA),反应结束后DIHA选择性分别可达59%,58%,34%和25%.     

X‐ray diffraction,differential scanning calorimetry and evolved gas analysis of aged plutonium tetrafluoride (PuF4)

Journal of Radioanalytical and Nuclear Chemistry - A 30 year-old PuF4 sample consisting of brown powder (PuF4-b) and pink granules (PuF4-p) was analyzed. X-ray diffraction shows the bulk...     

Determination of electroactive organic acids by anion-exchange chromatography using a copper modified electrode

An ion-chromatographic method combined with electrochemical detection at a copper-based chemically modified glassy carbon electrode (Cu-GC) has been shown to provide a simple analytical approach for the determination of some common organic acids in alkaline medium. Under the optimized isocratic chromatographic conditions (i.e. 0.1 M NaOH plus 80 mM CH3COONa), organic acids such as gallic, ascorbic, gluconic, lactobionic, galacturonic and glucuronic acid could be separated in less than 20 min. Under constant potential amperometric detection (i.e. 0.55 V vs. Ag-AgCl) the Cu-GC modified electrode allowed detection limits between 2 and 5 pmol for all investigated organic acids while the linear dynamic range spanned generally over three orders of magnitude. Examples of applications included the separation and quantitation of some common organic acids in vinegar, honey and tea samples, are given.     

Effect of particle size in the hydrogenation of crotonaldehyde on supported Pt and Pt-Sn catalysts

The present work reports the effect of metal particle size on the selectivity to crotyl alcohol (S_UOL) in the liquid phase hydrogenation of crotonaldehyde over SiO₂ and a-Al₂O₃-supported Pt and Pt-Sn catalysts. It was demonstrated that, for the monometallic catalysts, a higher particle size led to a higher S_UOL, while for the bimetallic catalysts, this effect was not so important.     

Post-Processing Accept-Reject Samples: Recycling and Rescaling

Abstract

This article proposes alternative methods for constructing estimators from accept-reject samples by incorporating the variables rejected by the algorithm. The resulting estimators are quick to compute, and turn out to be variations of importance sampling estimators, although their derivations are quite different. We show that these estimators are superior asymptotically to the classical accept-reject estimator, which ignores the rejected variables. In addition, we consider the issue of rescaling of estimators, a topic that has implications beyond accept-reject and importance sampling. We show how rescaling can improve an estimator and illustrate the domination of the standard importance sampling techniques in different setups.     

Evidence for the quantum birth of our universe

We present evidence for a nonsingular origin of the Universe with intial conditions determined by quantum physics and relativistic gravity. In particular, we establish that the present temperature of the microwave background and the present density of the Universe agree well with our predictions from these intial conditions, after evolution to the present age using the Einstein-Friedmann equation. Remarkably, the quantum origin for the Universe naturally allows its evolution at exactly the critical density. We also discuss the consequences of these results to some fundamental aspects of quantum physics in the early Universe.