Influence of Wire Initial Composition on Anode Microstructure and on Metal Transfer Mode in GMAW: Noteworthy Role of Alkali Elements

Metal Active Gas (MAG) welding in presence of Argon and CO₂ mixture as shielding gas is a largely developed process allowing the transfer of the liquid metal from the consumable wire anode to the workpiece according to various modes (short-arc, globular, spray-arc). The CO₂ presence in the shielding gas leads to the formation of an oxide layer, or gangue, wrapping the droplet, limiting the access to the spray-mode transfer, taking into account the low conductivity and the high viscosity of this layer. Several electrodes of various compositions have been tested thanks to Flux Cored Arc Welding, to limit the gangue formation or its negative contribution, based on Ti, La, Zr and alkali metals addition or reduction in silicon content. The results are interpreted considering the metal transfer mode for a given current intensity (330 and 410 A), with various CO₂ concentrations in the shielding gas. Finally, the role of the gangue, compared to the other factors governing the droplet detachment, is discussed. A decrease in silicon content limits significantly the gangue formation and gives access to spray arc transfer up to 30 vol.% of CO₂ at 330 A. Titanium addition leads to the same results. The tests in presence of zirconium proved the conductivity improvement of the gangue. The addition of alkali allows to stabilize the spray arc up to the noteworthy value of 60 vol.% of CO₂ at 330 A, supporting the hypothesis of a strong influence of viscosity on droplets detachment in the process.     

Thermal photoionization in resonance ionization spectroscopy

In Resonance Ionization Spectroscopy, neutral atoms are usually produced by thermal effects, using an oven or a hot surface at high temperature. The radiation from these thermal sources is intense enough to effectively contribute to the photoionization of atoms excited by laser pumping. We show that, for the most common experimental conditions, the ratio of thermal to laser photoionization can be higher than one for excited levels which lie as far as a few thousand cm^–1 below the ionization threshold. This result is obtained with the use of the analytic expression for the photoionization cross-section of a hydrogen-like atom. We suggest two applications of this thermal photoionization. Namely, the study of highly excited states and Quasi Resonance Ionization Spectrometry, using only one laser of low radiance.Commissariat à l'Energie Atomique, Centre d'Etudes Nucléaires de Fontenay-aux-Roses DERDCA/DCAEA/SEA, BP no 6, F-92265 Fontenay-aux-Roses Cedex, France (Visiting scientist at Ecole Polytechnique from Commissariat à l'Energie Atomique)Commissariat à l'Energie Atomique, Centre d'Etudes Nucléaires de Saclay, IRF/DPHG/PAS. Bât 462, F-91191 Gif-sur-Yvette Cedex, France     

Multistep synthesis of 2,5-diketopiperazines on different solid supports monitored by high resolution magic angle spinning NMR spectroscopy

The solid-phase synthesis of 2,5-diketopiperazines containing the trans-4-hydroxy-L-proline amino acid residue (Hyp) was performed on Ellman polystyrene, polyoxyethylene-polyoxypropylene (POEPOP), polystyrene-polyoxyethylene NovaSyn, and Wang resins, respectively. The reaction pathway allowed the introduction of different functional groups around the bicyclic scaffold in a combinatorial approach, and it generated mixtures of isomers. A detailed characterization of the single reaction steps by high resolution magic angle spinning (HRMAS) NMR spectroscopy was performed. The NMR spectral resolution of the resin-bound intermediates and final products was greatly influenced by the polymer matrix. The POEPOP resin permitted to obtain HRMAS NMR spectra with a resolution comparable with that of the spectra of the molecules in solution. Moreover, configurational and conformational isomers formed during the solid-phase reaction steps could be detected and easily assigned. Therefore, the combination of the HRMAS NMR technique with the use of nonaromatic resins may become an extremely powerful tool in solid-phase organic synthesis. This approach will allow the monitoring of multistep reactions and the conception of on-bead structural studies either on small molecules or on natural and/or synthetic oligomers.     

Methyl 2-((succinimidooxy)carbonyl)benzoate (MSB): a new,efficient reagent for N-phthaloylation of amino acid and peptide derivatives

A new, efficient, and readily available reagent, methyl 2-((succinimidooxy)carbonyl)benzoate (MSB), for N-phthaloylation of amino acids and amino acid derivatives is described. The phthaloylation procedure is simple and racemization-free and gives excellent results with alpha-amino acids, alpha-amino alcohols, dipeptides, alpha-amino carboxamides, and alpha-amino esters.     

Solid-phase synthesis of fullerene-peptides

The solid-phase synthesis of peptides (SPPS) containing [60]fullerene-functionalized amino acids is reported. A new amino acid, fulleropyrrolidino-glutamic acid (Fgu), is used for the SPPS of a series of analogues of different length based on the natural Leu(5)-Enkephalin and on cationic antimicrobial peptides. These fullero-peptides were prepared on different solid supports to analyze the influence of the resin on the synthesis. Optimized protocols for the coupling and deprotection procedures were determined allowing the synthesis of highly pure peptides in sufficient quantities for evaluation of biological activities. In particular, to avoid side reactions of the fullerene moiety with bases and nucleophiles, the removal of the protecting groups was performed under inert conditions (nitrogen or argon in the dark). We have encountered serious problems with the recovery of the crude compounds, especially when Fgu was inserted in the proximity of the resin core as fullero-peptides tend to remain embedded inside the resin. Eventually, all of the fullero-peptides were easily purified, and the cationic peptides were tested for their antimicrobial activities. They displayed a specific activity against the Gram-positive bacterium S. aureus and also lysed erythrocytes. The availability of a fullero-amino acid easily useable in the SPPS of fullero-peptides may thus open the way to the synthesis of new types of biologically active oligomers.     

1,3,5-Triazepane-2,6-diones as structurally diverse and conformationally constrained dipeptide mimetics: identification of malaria liver stage inhibitors from a small pilot library

The development of the 1,3,5-triazepane-2,6-dione system as a novel, conformationally restricted, and readily accessible class of dipeptidomimetics is reported. The synthesis of the densely functionalized 1,3,5-triazepane-2,6-dione skeleton was achieved in only four steps from a variety of simple linear dipeptide precursors. To extend the practical value of 1,3,5-triazepane-2,6-diones, a general polymer-assisted solution-phase synthesis approach amenable to library production in a multiparallel format was developed. The conformational preferences of the 1,3,5-triazepane-2,6-dione skeleton were investigated in detail by NMR spectroscopy and X-ray diffraction. The ring exhibits a characteristic folded conformation which was compared to that of related dipeptide-derived scaffolds including the more planar 2,5-diketopiperazine (DKP). Molecular and structural diversity was increased further through post-cyclization appending operations at urea nitrogens. Preliminary biological screens of a small collection of 1,3,5-triazepane-2,6-diones revealed inhibitors of the underexplored malaria liver stage and suggest strong potential for this dipeptide-derived scaffold to interfere with and to modulate biological pathways.