Solvent-Induced Reversal of Activities between Two Closely Related Heterogeneous Catalysts in the Aldol Reaction

The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes

Application de la RMN à l’étude des réactions du formaldéhyde avec les fonctions aminées de l’alanine et de la lysine en fonction du pH du milieu

Les réactions du formaldéhyde avec les fonctions aminées de l’alanine et de la lysine ont été étudiées à l’aide de la RMN du carbone et du proton.La réaction d’addition initiale, qui conduit à des dérivés N-monohydroxyméthylés, se fait suivant un mécanisme équilibré dont le calcul des constantes montre la réactivité accrue de la forme non protonée des fonctions amines par rapport à la forme protonée. Cette addition est plus importante sur la fonction å aminée de la lysine que sur la fonction a aminée.Dans le cas des composés comportant une seule fonction réactive (alanine, Na-acétyllysine), on observe uniquement les dérivés monohydroxyméthylés. En milieu acide, la lysine donne principalement les dérivés N-monohydroxyméthylés, par contre pour des pH compris entre 5 et 10, ceux-ci sont susceptibles de se condenser entre eux pour former une liaison diméthylène éther.Dans tous les cas les hydroxyméthyles peuvent être réduits en monométhyles. De plus, la Ne-méthyllysine peut subir une nouvelle hydroxyméthylation dont la réduction conduit à la Ne-diméthyllysine

Decarbonation and preservation method for the analysis of organic C and N contents and stable isotope ratios of low-carbonated suspended particulate material

The aim of this study was to determine a simple routine procedure to preserve, decarbonate and analyse low-carbonated filters of suspended particulate organic matter (POM) for particulate organic carbon and nitrogen content, δ13C and δ15N. Our goal was to analyse these four parameters from a single and entire filter of POM without altering the organic material. First, freezing (−20 °C) versus oven-drying (60 °C) were compared as the initial preservation step. Afterwards, non-acidified samples were compared to acid-treated samples using 0.12N HCl (diluted HCl rinsing at the end of the filtration) or 12N HCl (filters exposed to HCl fumes for 4 h in a dessicator). Regarding the preservation methods, our results indicate that freezing increases the uncertainty of δ15N measurements and, in combination with concentrated HCl treatment, leads to a loss of particulate nitrogen and an alteration of the δ15N signature. Consequently, we recommend drying to preserve filter samples. Regarding acid treatments, we found that (i) diluted HCl would not be sufficient to fully remove the carbonate from our samples, (ii) in contrast, a 4 h exposure of the filters to the HCl fumes was enough to remove all the inorganic carbon, and (iii) the concentrated HCl treatment did not alter the nitrogen measurements (only when drying without freezing is used to preserve the filters). Consequently, we propose that low-carbonated POM filters are preserved by drying and carbonates are removed by exposing the filters to HCl fumes (4 h) for the analysis of particulate organic C and N content and isotope ratios

Dynamics of particulate organic matter d15N and d13C during spring phytoplankton blooms in a macrotidal ecosystem (Bay of Seine, France)

International audienceTwo cruises (April and June 1997) were carried out in the Bay of Seine, a nitrate- and ammonium-enriched ecosystem of Western Europe, to identify the major mechanisms that control δ15N and δ13C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) δ15N ranged between 0.8 and 5.2‰ in April and between 2.2 and 6.2‰ in June, while particulate organic carbon (POC) δ13C ranged between -24.3 and -19.7‰, and between -20.0 and -16.2‰ during the same periods. During spring 1997, POM was highly dominated by autochthonous phytoplankton. It is shown that the variation of PON δ15N is due to both nitrate mixing between river and marine waters and fractionation of N stable isotopes during nitrate utilization by phytoplankton. Therefore, similarly to what was previously shown for open ocean, δ15N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC δ13C in spring is controlled by POC concentration and C:N ratio (in addition to Œtemperature effects¹), which are considered here as indicators of primary production and phytoplankton degradation, respectively. The co-variation of δ13C and δ15N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low δ15N) and phytoplankton production is low (low δ13C); then primary production increases (δ13C becomes higher) and the nitrate pool diminishes (δ15N becomes higher); at a later stage, the nitrate pool is depleted (high δ15N), part of the phytoplankton becomes degraded and production is still high (high δ13C)

Dynamics of particulate organic matter d¹⁵N and d¹³C during spring phytoplankton blooms in a macrotidal ecosystem (Bay of Seine, France)

Two cruises (April and June 1997) were carried out in the Bay of Seine, a nitrate- and ammonium-enriched ecosystem of Western Europe, to identify the major mechanisms that control d15N and d13C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) d15N ranged between 0.8 and 5.2‰ in April and between 2.2 and 6.2‰ in June, while particulate organic carbon (POC) d13C ranged between –24.3 and –19.7‰, and between –20.0 and –16.2‰ during the same periods. During spring 1997, POM was highly dominated by autochthonous phytoplankton. It is shown that the variation of PON d15N is due to both nitrate mixing between river and marine waters and fractionation of N stable isotopes during nitrate utilization by phytoplankton. Therefore, similarly to what was previously shown for open ocean, d15N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC d13C in spring is controlled by POC concentration and C:N ratio (in addition to ‘temperature effects’), which are considered here as indicators of primary production and phytoplankton degradation, respectively. The co-variation of d13C and d15N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low d15N) and phytoplankton production is low (low d13C); then primary production increases (d13C becomes higher) and the nitrate pool diminishes (d15N becomes higher); at a later stage, the nitrate pool is depleted (high d15N), part of the phytoplankton becomes degraded and production is still high (high d13C)

Natural isotope tracers and water filiation in the vine ecosystem

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