Raman imaging to quantify the thermal transformation degree of Pompeian yellow ochre caused by the 79 AD Mount Vesuvius eruption

Most of the wall paintings from Pompeii are decorated with red and yellow colors but the thermal impact of 79 AD Mount Vesuvius eruption promoted the partial transformation of some yellow-painted areas into red. The aim of this research is to develop a quantitative Raman imaging methodology to relate the transformation percentage of yellow ochre (goethite, α-FeOOH) into red color (hematite, α-Fe₂O₃) depending on the temperature, in order to apply it and estimate the temperature at which the pyroclastic flow impacted the walls of Pompeii. To model the thermal impact that took place in the year 79 AD, nine wall painting fragments recovered in the archeological site of Pompeii and which include yellow ochre pigment were subjected to thermal ageing experiments (exposition to temperatures from 200 to 400 °C every 25 °C). Before the experiments, elemental information of the fragments was obtained by micro-energy dispersive X-ray fluorescence (μ-ED-XRF). The fragments were characterized before and after the exposition using Raman microscopy to monitor the transformation degree from yellow to red. The quantitative Raman imaging methodology was developed and validated using synthetic pellets of goethite and hematite standards. The results showed almost no transformation (0.5% ± 0.4) at 200 °C. However, at 225 °C, some color transformation (26.9% ± 2.8) was observed. The most remarkable color change was detected at temperatures between 250 °C (transformation of 46.7% ± 1.7) and 275 °C (transformation of 101.1% ± 1.2). At this last temperature, the transformation is totally completed since from 275 to 400 °C the transformation percentage remained constant.

     

A Masked Form of an O-Borylated Breslow Intermediate for the Diastereoselective FLP-Type Activation of Aldehydes

Breslow intermediates are very often elusive species whose application in frustrated Lewis pair (FLP) chemistry is unprecedented. Described herein is the use of a masked form of an O-borylated Breslow (OBB) intermediate that performs FLP-type activation of the carbonyl function of five different benzaldehyde derivatives with complete diastereoselectivity. The resulting compounds are characterised in solution by NMR spectroscopy (compounds 4 – 8 ) and in solid state by X-ray diffraction analysis (compounds 4 – 6 ). A combined kinetic and theoretical investigation reveals the associative nature of the rate determining step and suggests that the OBB intermediate part is never released during the whole process.     

Characterisation of nanoparticle size and state prior to nanotoxicological studies

Before commencing any nanotoxicological study, it is imperative to know the state of the nanoparticles to be used and in particular their size and size distribution in the appropriate test media is particularly important. Particles satisfying standards can be commercially purchased; however, these invariably cannot be used directly and need to be dispersed into the relevant biological media. Often such changes in the environment or ionic strength, or a change in the particle concentration, results in some aggregation or a shift in the particle size distribution. Such unexpected aggregation, dissolution or plating out, if unaccounted for, can have a significant effect on the available nanoparticle dose and on interpretation of any results obtained thereafter. Here, we demonstrate the application of characterisation instrumentation that sizes nanoparticles based on their Brownian motion in suspension. Unlike classical light-scattering techniques, the nanoparticle tracking and analysis (NTA) technique allows nanoparticles to be sized in suspension on a particle-by-particle basis allowing higher resolution and therefore better understanding of aggregation than ensemble methods (such as dynamic light scattering (DLS) and differential centrifugation sedimentation (DCS)). Results will be presented from gold (standard) nanoparticles in biologically relevant media that emphasise the importance of characterisation of the nanoparticle dispersion. It will be shown how the NTA technique can be extended to multi-parameter analysis, allowing for characterization of particle size and light scattering intensity on an individual basis. This multi-parameter measurement capability allows sub-populations of nanoparticles with varying characteristics to be resolved in a complex mixture. Changes in one or more of such properties can be followed both in real time and in situ.     

Heteronuclear NMR Spectroscopy as Surface‐Selective Technique: A Unique Look at the Hydroxyl Groups of γ‐Alumina

Invited for the cover of this issue are the groups of Régis M. Gauvin at the Université Lille Nord de France, Laurent Maron from the Université de Toulouse, and Mostafa Taoufik from the Université Lyon 1. The image depicts data on the topology of the surface hydroxyl groups of γ‐alumina as a postcard. Read the full text of the article at 10.1002/chem.2013004883.     

Molecular Thorium Trihydrido Clusters Stabilized by Cyclopentadienyl Ligands

Hydrogenolysis of alkyl‐substituted cyclopentadienyl (Cp^R) ligated thorium tribenzyl complexes [(Cp^R)Th(p‐CH₂‐C₆H₄‐Me)₃] ( 1 – 6 ) afforded the first examples of molecular thorium trihydrido complexes [(Cp^R)Th(μ‐H)₃]_n (Cp^R=C₅H₂(^tBu)₃ or C₅H₂(SiMe₃)₃, n=5; C₅Me₄SiMe₃, n=6; C₅Me₅, n=7; C₅Me₄H, n=8; 7 – 10 and 12 ) and [(Cp^#)₁₂Th₁₃H₄₀] (Cp^#=C₅H₄SiMe₃; 13 ). The nuclearity of the metal hydride clusters depends on the steric profile of the cyclopentadienyl ligands. The hydrogenolysis intermediate, tetra‐nuclear octahydrido thorium dibenzylidene complex [(Cp^ttt)Th(μ‐H)₂]₄(μ‐p‐CH‐C₆H₄‐Me)₂ (Cp^ttt=C₅H₂(^tBu)₃) ( 11 ) was also isolated. All of the complexes were characterized by NMR spectroscopy and single‐crystal X‐ray analysis. Hydride positions in [(Cp^Me4)Th(μ‐H)₃]₈ (Cp^Me4=C₅Me₄H) were further precisely confirmed by single‐crystal neutron diffraction. DFT calculations strengthen the experimental assignment of the hydride positions in the complexes 7 to 12 .     

Amidoximes α-Hydroxylées Dérivées de Sucres

The two epimeric α-hydroxyamidoximes 5 and 6 - and some derivatives thereof, particularly oxadiazoles 16 and 17 - have been stereospecifically prepared from the keto sugar 1. Compound 5 was found to exist in two polymorphous crystalline forms (α and β) depending on the crystallization solvent. Both forms are orthorhombic, space group P2₁2₁2₁ (α-form : a = 10.408(3), b = 10.559(2), c = 14.144(2) Å; β-form : a = 7.890(1), b = 10.305(2), c = 19.064(4) Å). The calculation of pseudorotation parameters showed that each polymorph is associated with a slightly different conformation of the dioxolane rings. The Oacetyl derivative 11 of 6 adopts a different conformation of the furanose ring. In the three structures, a network of hydrogen bonds exists. The amidoxime 5 forms a cupric complex [Cu(5)₂] whose ESR spectrum proved its square structure.     

Measurement of 13C and 15N isotope labeling by gas chromatography/combustion/isotope ratio mass spectrometry to study amino acid fluxes in a plant-microbe symbiotic association

We have developed a method based on a double labeling with stable isotopes and gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses to study amino acid exchange in a symbiotic plant-microbe association. Isotopic precision was studied for 21 standards including 15 amino acid derivatives, three N-protected amino acid methyl esters, three amines and one international standard. High correlations were observed between the δ(13)C and δ(15)N values obtained by GC/C/IRMS and those obtained by an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (R(2) = 0.9868 and 0.9992, respectively). The mean precision measured was 0.04‰ for δ(13)C and 0.28‰ for δ(15)N (n = 15). This method was applied in vivo to the symbiotic relationship between alfalfa (Medicago sativa L.) and N(2)-fixing bacteria. Plants were simultaneously labeled over 10 days with (13)C-depleted CO(2) ((12)CO(2)), which was assimilated through photosynthesis by leaves, and (15)N(2) fixed via nodules. Subsequently, the C and N isotope compositions (i.e. δ(13)C and δ(15)N) of free amino acids were analyzed in leaves and nodules by GC/C/IRMS. The method revealed the pattern of C and N exchange between leaves and nodules, highlighting that γ-aminobutanoic acid and glycine may represent an important form of C transport from leaves to the nodules. The results confirmed the validity, reliability and accuracy of the method for assessing C and N fluxes between plants and symbiotic bacteria and support the use of this technique in a broad range of metabolic and fluxomic studies.     

Kinetic study of the oxidation of L-threonine by MnO4? ions

A self-catalytic effect attributed to Mn²⁺ ions was observed when studying the oxidation of L-threonine by permanganate ions. The process obeys the rate equation: