Predicting the reaction mechanism of nucleophilic substitutions at carbonyl and thiocarbonyl centres of esters and thioesters

In nucleophilic substitution reactions at carbonyl centres, there are two possible channels. The first one occurs when the attack of nucleophilic agents takes place simultaneously with the departure of the nucleofuge. This process is named as concerted. The second possibility is the formation of a reaction intermediate, typically a tetrahedral intermediate from which the nucleofuge departs after passing through a second transition state. This second mechanism is defined as stepwise. Whether a concerted or stepwise mechanism is to be expected for a given reaction depends on several factors. Among these determinants are the nucleophilicity of the attacking group, the leaving group ability of the nucleofuge, and the solvent, which affects both the stability of the intermediate or the transition states involved. The role of the electrophilic centre can however become an important factor that can determine the reaction mechanism. In this work we show that the group nucleophilic Fukui function model may be used to rationalize and to predict the reaction mechanism of the title compounds towards alicyclic amines. In general, when the electrophilic carbon centre is attached to the soft sulfur atom, the reaction mechanism is predicted to follow a stepwise route. When the electrophilic carbon atom is attached to a harder oxygen centre, the reaction mechanism is determined by chemical substitution at the nucleofuge moiety. Experimental verification for a set of four substrates is presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Substrate binding to mammalian 15-lipoxygenase

Lipoxygenases (LOs) are implicated in the regulation of metabolic processes and in several human diseases. Revealing their exact role is hindered by an incomplete understanding of their activity, including substrate specificity and substrate alignment in the active site. Recently, it has been proposed that the change in substrate specificity for arachidonic acid (AA) or linoleic acid (LA) could be part of an auto-regulatory mechanism related to cancer grow. Kinetic differences between reactions of 15-hLO with AA and LA have also led to the suggestion that the two substrates could present mechanistic differences. In the absence of a crystal structure for the substrate:15-LO complex, here we present an atomic-level study of catalytically competent binding modes for LA to rabbit 15-LO (15-rLO-1) and compare the results to our previous work on AA. Docking calculations, molecular dynamics simulations, re-docking and cross-docking calculations are all used to analyze the differences and similarities between the binding modes of the two substrates. Interestingly, LA seems to adapt more easily to the enzyme structure and differs from AA on some dynamical aspects that could introduce kinetic differences, as observed experimentally. Still, our study concludes that, despite the different chain lengths and number of insaturations between these two physiological substrates of 15-rLO-1, the enzyme seems to catalyze their hydroperoxidation by binding them with a common binding mode that leads to similar catalytically competent complexes.     

CO2 laser photoacoustic detection of ammonia emitted by ceramic industries

A homemade photoacoustic spectrometer has been constructed for monitoring gas emission from several sources. Numerous air pollutant gases are emitted exhaust of industries, vehicles and power plants. The photoacoustic technique is extremely sensitive and selective in detecting various gases. This work focuses on the gas emitted by the ceramic industry in northern Rio de Janeiro State in Brazil, the ceramic industry plays a remarkable role in the economy activity of this region, in recent years, this region developed into a significant red ceramic complex. The potential impact on the atmospheric environment of the region due to gaseous pollutant emissions from these anthropogenic sources needs to be evaluated. In this work we identified NH3 present in the samples collected in the kiln of a ceramic plant, in the concentration range of 33-52 ppmV. The ammonia gas present in our collected samples might come from the excess nitrogen in the manure soil from where the ceramic material was extracted. This soil was used for the sugarcane culture which is another important economic activity of this region.     

Determination of metal ions extracted by DTPA in a soil treated with effluent using an Hg-electroplated-Pt microelectrode

Determinations of Cu, Fe, Mn and Zn were performed in an oxisol soil treated with effluent originated from a biodigester septic cesspool. The extracts were obtained from a DTPA/TEA (pH 7.3) solution and analysed by electroanalytical methodologies using square wave anodic stripping voltammetry (SWASV) for the analysis of Cu, Mn and Zn and square wave voltammetry (SWV) for Fe analysis, both of them with a thin mercury film microelectrode (ME-Hg). DTPA (diethylenetriaminepentaacetic acid) complexes with Cu, Mn and Fe show electroactivity in the potentials range close to the ones for the free ions in solution, but the Zn complex did not show any electroactivity at the potential range analysed. SWASV/ME-Hg and SWV/ME-Hg results were in good agreement (r² = 0.996) with the flame atomic absorption spectroscopy (FAAS) analyses, demonstrating that electroanalytical methodologies can be used for micronutrient determinations in soil extracts without serious interferences of the matrix components. Some characteristics of the complex formed between DTPA-metals are explored by EPR experiments.     

Early stages hydration of high initial strength Portland cement

This work complements a quantitative thermogravimetric study of the first 24 h of hydration of a high initial strength and sulphate resistant Portland cement (HS SR PC) using non-conventional differential thermal analysis (NCDTA) and Vicat needle method. Different water/cement (W/C) ratios from 0.35 to 0.85 were used to evaluate the most indicated operating conditions to maximize calcium hydroxide production for further use in CO₂ capture. Thermogravimetric analysis data performed at 4 and 24 h of hydration were also compared to the NCDTA and Vicat data for each kind of paste, to analyze the influence of the W/C ratio on the simultaneous hydration and setting process. The increase of the W/C ratio increases the induction time retards the solidification and setting processes but increases the hydration degree as the W/C ratio is increased from 0.45. At 24 h, products prepared with 0.35 W/C ratio present a little higher hydration degree than those prepared with W/C = 0.45, because of the highest level of temperature in the reacting mixture in the former case, during the first 8 h. There is a practical limit of W/C = 0.66 to prepare the pastes, due to a limit of the miscibility between HS SR PC and water, above which, the excess of water forms a separated phase that does not interfere in the hydration process.     

Determination of polychlorinated biphenyls in biosolids using continuous ultrasound-assisted pressurized solvent extraction and gas chromatography-mass spectrometry

An efficient continuous pressurized solvent extraction (PSE) method assisted by ultrasound energy was developed for the extraction of polychlorinated biphenyls (PCBs) from biosolids. Analytes were determined in the extracts by gas chromatography-mass spectrometry in selected ion monitoring (SIM) mode. A screening-type experimental design pointing to dynamic extraction time as the only significant variable in the extraction process was carried out to optimize PCB extraction from the biosolids. Final selected conditions for PSE were extraction temperature, 50 degrees C; static extraction time, 0 min; and dynamic extraction time, 30 min. Recovery of the PSE method was 73%, which was significantly improved (103%) when PSE was assisted with 30-min ultrasound (US-PSE). Precision of the overall method, expressed as relative standard deviation, was 3.6% and the detection limit was 0.037 mg/kg. The method was applied to the determination of PCBs in biosolids from different water treatment plants from central Chile.     

Arylbutyltellurides as precursors of dilithium arylthienylcyanocuprates in a straightforward approach to phenethylamine derivatives

The ring opening reaction of N-tosyl aziridines with dilithium arylthienylcyanocuprates generated from arylbutyltellurides produced phenethylamine derivatives in good to excellent yields.     

<Superscript>99m</Superscript>Tc labeling of the scorpion (<Emphasis Type="Italic">Tityus serrulatus</Emphasis>) antivenom

F(ab’)₂ is the fragment involved in the immunotherapy for scorpion stings and it would be convenient to label it with ^99mTc for organ distribution and pharmacokinetics studies. The aim of the present study was to label scorpion antivenom F(ab’)₂ with ^99mTc keeping its biological activity, integrity and stability. High labeling yield was obtained using stannous chloride and sodium borohydride. Stability, immunoreactivity and integrity of ^99mTc-F(ab’)₂ was preserved. It was not observed any difference between potencies of unlabeled and labeled antivenom. ^99mTc-F(ab’)₂ can be a useful tool for use in biodistribution and pharmacokinetics studies on the evaluation of the efficacy of the antivenom against scorpion envenomation.     

Evaluation of magnesium sulphate attack in mortar-metakaolin system by thermal analysis

This paper reports an experimental study on the magnesium sulphate resistance of mortar specimens incorporating 0, 10 and 20% of metakaolin (MK). The evidence of the attack was evaluated through the content of calcium hydroxide (portlandite) and formation of magnesium hydroxide (brucite) by thermal analysis (thermogravimetric and derivative thermogravimetric analysis). The mechanical degradation of the mortar specimens was evaluated through splitting tensile tests after 200 days of exposition to the magnesium solution. The addition of metakaolin resulted in a reduction in the content of calcium hydroxide and in a smaller formation of brucite in comparison with reference mixture. A tensile strength loss of about 7% was observed for the metakaolin mortars submitted to the magnesium solution attack for 200 days.