Modification of bovine beta-lactoglobulin by glycation in a powdered state or in aqueous solution: adsorption at the air-water interface

The adsorption at the air-water interface of native and various glycated forms of beta-Lactoglobulin B (beta-LG), prepared under two different experimental conditions, was investigated by ellipsometry, surface tension and shear elastic constant measurements. The measurements were performed in 0.1 M phosphate buffer, 0.1 M NaCl, pH 6.8. It was found that the interfacial properties of beta-LG were more affected when the glycation was performed in solution than in the dry-way system. Dry-way glycated beta-LG, despite a higher glycation extent, affected slightly its interfacial behaviour. Solution glycated beta-LG exhibited a higher adsorption and more rigid interface as expressed by shear elastic constant measurement at saturation (16.5 mN/m against 8.7 and 11.5 mN/m for native and control treated beta-LG, respectively). These results were attributed to the specific molecular species induced during glycation in solution, which includes monomers and unfolded covalent homodimers of beta-LG molecules with a high tendency to self-association via non-covalent interactions.     

Surface plasmon near-field imaging of very thin microstructured polymer layers

We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes.     

Macro and microchemistry of trace metals in vitrified domestic wastes by laser ablation ICP-MS and scanning electron microprobe X-ray energy dispersive spectroscopy

Management of domestic wastes often relies on incineration, a process that eliminates large amount of wastes but also produces toxic residues that concentrate heavy metals. Those hazardous secondary wastes require specific treatment. Vitrification is seen as a powerful way to stabilise them. However, concern exists about the long term behaviour of these glass wastes and the potential release of toxic species into the environment. The answers will come with further investigation into the physico-chemical evolution of the vitrified wastes and the mobility of hazardous elements within the matrix with appropriate analytical methods. Laser ablation coupled with inductively coupled mass spectrometry (LA-ICP-MS) is a challenging technique for the chemical analysis of trace elements in solid materials. This paper presents an evaluation of the potential of LA- ICP-MS for macro and microanalysis of trace metals in domestic vitrified wastes with regards to other physical analytical techniques of solids such as scanning electronprobe X-ray energy dispersive spectroscopy (SEM-EDXS). Two typical samples, vitreous and crystallised, are used to compare the analytical performances of the two techniques. SEM-EDXS was used for mineralogical characterisation and chemical analysis of the mineralogical phases. Relative micro-analysis and bulk quantitative analysis of 30 major, minor and trace elements was performed by LA-ICP-MS: precision was between 10 and 20% for most elements and quantitative analysis proved possible with an accuracy of 20% and relative detection limits of 0.1 mg kg(-1).     

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

We report the structural properties of ultra-small ThO₂ and UO₂ nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO₂ and UO₂ NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO₂ and UO₂ materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th–O and U–O bond distances despite retaining average bond lengths that are characteristic of bulk ThO₂ and UO₂. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

ThO₂ and UO₂ nanoparticles synthesized using a COF-5 template exhibit unpassivated surfaces and provide insight into nanoscale properties of actinides.     

Synthesis of macrocycles via cobalt-mediated [2 + 2 + 2] cycloadditions

We investigated the formation of macrocycles from alpha,omega-diynes in cobalt-mediated co-cyclotrimerization reactions. Long-chain alpha,omega-diynes underwent metal-mediated [2 + 2 + 2] cycloadditions with nitriles, cyanamides, or isocyanates in the presence of CpCo(CO)2 (Cp = cyclopentadienide) to yield pyridine-containing macrocycles, i.e., meta- and para-pyridinophanes, such as 5m/5p, 35m/35p, and 41m/41p. The regioselectivity of these reactions was affected by the length and type of linker unit between the alkyne groups, as well as by certain stereoelectronic factors. An analogous alpha,omega-cyano-alkyne, 28, combined with an alkyne to yield two isomeric meta-pyridinophanes, such as 5m and 29m, and an ortho cycloadduct (benzannulation product), such as 29o. We developed a reaction protocol for these cobalt-based [2 + 2 + 2] cycloadditions that involves markedly improved conditions such that this process offers a convenient, flexible synthetic approach to macrocyclic pyridine-containing compounds. For example, diyne 6 reacted with p-tolunitrile in 1,4-dioxane to give 7p and 7m (7:1 ratio) in 87% yield at a moderate temperature of ca. 100 degrees C in 24 h without photoirradiation or syringe-pump addition. Isocyanates were also effective reactants, as exemplified by the formation of 44p almost exclusively (44p:44m > 50:1) in 64% yield from diyne 8 and 2-phenylethylisocyanate. By using this improved protocol we were able to co-cyclotrimerize long-chain alpha,omega-diynes with alkynes in certain cases to demonstrate a successful macrocyclic variant of the Vollhardt reaction. For instance, diyne 6 reacted with dipropylacetylene to give paracyclophane 57p and benzannulene 57o (2:1 ratio) in 29% yield.     

PTV-LV-GC/MS/MS as screening and complementary method to HRMS for the monitoring of dioxin levels in food and feed

Recent developments in trapping efficiency inside ion trap mass spectrometer permitted to lower instrument detection limit (IDL). An IDL of 200 fg μl⁻¹ injected with a signal-to-noise ratio of 5:1 for tetrachlorodibenzo-p-dioxin (TCDD) was obtained by gas chromatography coupled to a quadrupole ion storage mass spectrometer in tandem mode (GC/MS/MS). Coupling large volume programmable temperature vaporizer (PTV-LV) injection to GC/MS/MS provides an alternative and complementary method to classical splitless-GC injection connected to high-resolution mass spectrometry (splitless-GC/HRMS) method for dioxin monitoring in food and feed.

An injection volume of 10 μl was found to be the best compromise between the sensitivity requirements and the robustness required for a high throughput method. PTV-LV-GC/MS/MS and Splitless-GC/HRMS were compared by performing analysis on five different matrices such as beef fat, yolk eggs, milk powder, animal feed and serum samples covering a concentration range of two orders of magnitude (i.e. 0.2–25 ng WHO-TEQ kg⁻¹). An analysis of variance (ANOVA) was carried out. Fisher tests pointed out that the method effect for all the 2,3,7,8 congeners was not significant, indicating that the null hypothesis (H₀: μ₁=μ₂=…=μ_n) was not rejected. Moreover, the interaction effects between methods and matrices were not significant for most of the 2,3,7,8 congeners. However, three congeners (2,3,7,8-TCDF; 1,2,3,4,7,8-HxCDD and 1,2,3,4,6,7,8-HpCDD) were characterized by P-values lower than the significance level (=0.05). In toxic equivalence (TEQ), the study showed that no significant bias was observed between the two methods. Consequently, PTV-LV-GC/MS/MS is an attractive technique and can be used as a cost effective complementary method to HRMS for dioxin levels monitoring in food and feed.     

Fractionation of casein hydrolysates using polysulfone ultrafiltration hollow fiber membranes

The objective of this study was to characterize the fractionation profile of casein hydrolysates obtained with polysulfone hollow fiber ultrafiltration membranes. The two-step ultrafiltration process developed by Turgeon and Gauthier [J. Food Sci., 55 (1990) 106] was used: a caseinate solution was submitted to proteolysis with chymotrypsin or trypsin, and the reaction mixture (RM) was subsequently ultrafiltered using a 30 kDa (MWCO) hollow-fiber polysulfone membrane. The total hydrolysate permeating from this first step was further fractionated using a 1 kDa (MWCO) membrane, producing the mixture of polypeptides (retentate) and the amino acid fraction (permeate). The effect of enzyme specificity and of membrane retentivitiy on the total composition (total nitrogen, fat, lactose, minerals) and amino acid profile of the fractions was studied. The overall composition of the fractions was not significantly affected by the nature of the enzyme but the degree of hydrolysis and the molecular weight distribution profile analyses showed a marked effect of the enzyme specificity, with trypsin giving a larger proportion of small peptides (< 200 Da) in the mixture of polypeptides. Amino acid profile analyses provided useful information on the phenomena governing the fractionation of amino acids with a polysulfone membrane: (1) the target amino acids of the enzyme are concentrated in the permeate as a result of their presence in all peptides produced by hydrolysis, (2) polar amino acids are retained by the membrane, (3) non-polar amino acids are not selectively rejected by the membrane. Our results suggest that the charge/hydrophobicity balance of the peptides produced is the predominant factor determining the fractionation of casein hydrolysates.