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Dirk E De Vos

Publications and source records attributed to Dirk E De Vos.

16 recordsLinked to original sources
PubMed2007

Imidazolium ionic liquids as solvents for cerium(IV)-mediated oxidation reactions.

Use of imidazolium ionic liquids as solvents for organic transformations with tetravalent cerium salts as oxidizing agents was evaluated. Good solubility was found for ammonium hexanitratocerate(IV) (ceric ammonium nitrate, CAN) and cerium(IV) triflate in 1-alkyl-3-methylimidazolium triflate ionic liquids. Oxidation of benzyl alcohol to benzaldehyde in 1-ethyl-3-methylimidazolium triflate was studied by in-situ FTIR spectroscopy and 13C NMR spectroscopy on carbon-13-labeled benzyl alcohol. Careful control of the reaction conditions is necessary because ammonium hexanitratocerate(IV) dissolved in an ionic liquid can transform benzyl alcohol not only into benzaldehyde but also into benzyl nitrate or benzoic acid. The selectivity of the reaction of cerium(IV) triflate with benzyl alcohol in dry ionic liquids depends on the degree of hydration of cerium(IV) triflate: anhydrous cerium(IV) triflate transforms benzyl alcohol into dibenzyl ether, whereas hydrated cerium(IV) triflate affords benzaldehyde as the main reaction product. Reactions of ammonium hexanitratocerate(IV) with organic substrates other than benzyl alcohol have been explored. 1,4-Hydroquinone is quantitatively transformed into 1,4-quinone. Anisole and naphthalene are nitrated. For the cerium-mediated oxidation reactions in ionic liquids, high reaction temperatures are an advantage because under these conditions smaller amounts of byproducts are formed.

Journal Article↗
PubMed2007

Palladium catalysts on alkaline-earth supports for racemization and dynamic kinetic resolution of benzylic amines.

Palladium catalysts on alkaline-earth supports were studied as new heterogeneous catalysts for racemization of chiral benzylic amines such as 1-phenylethylamine. Particularly 5 % Pd/BaSO(4) and 5 % Pd/CaCO(3) were able to selectively racemize amines, with minimal formation of secondary amines or hydrogenolysis to ethylbenzene. In contrast, these side reactions were pronounced on Pd/C. A reaction mechanism is proposed that is consistent with the reaction kinetics. The catalyst activity was found to depend on the number of available surface Pd atoms, determined by titration with CO. The selectivity crucially depends on the rate of condensation of the amine and the primary imine, which is highest on Pd/C. The racemization catalysts were combined in one pot with an immobilized lipase to perform dynamic kinetic resolution of chiral amines. High yields (up to 88 %) of essentially enantiopure amides were obtained in a single step. The chemo-enzymatic catalyst system proved to be stable and could be reused without losing the initial activity.

Amines↗
PubMed2006

Synthesis of N-acyl homoserine lactone analogues reveals strong activators of SdiA, the Salmonella enterica serovar Typhimurium LuxR homologue.

N-Acyl homoserine lactones (AHLs) are molecules that are synthesized and detected by many gram-negative bacteria to monitor the population density, a phenomenon known as quorum sensing. Salmonella enterica serovar Typhimurium is an exceptional species since it does not synthesize its own AHLs, while it does encode a LuxR homologue, SdiA, which enables this bacterium to detect AHLs that are produced by other species. To obtain more information about the specificity of the ligand binding by SdiA, we synthesized and screened a limited library of AHL analogues. We identified two classes of analogues that are strong activators of SdiA: the N-(3-oxo-acyl)-homocysteine thiolactones (3O-AHTLs) and the N-(3-oxo-acyl)-trans-2-aminocyclohexanols. To our knowledge, this is the first report of compounds (the 3O-AHTLs) that are able to activate a LuxR homologue at concentrations that are lower than the concentrations of the most active AHLs. SdiA responds with greatest sensitivity to AHTLs that have a keto modification at the third carbon atom and an acyl chain that is seven or eight carbon atoms long. The N-(3-oxo-acyl)-trans-2-aminocyclohexanols were found to be less sensitive to deactivation by lactonase and alkaline pH than the 3O-AHTLs and the AHLs are. We also examined the activity of our library with LuxR of Vibrio fischeri and identified three new inhibitors of LuxR. Finally, we performed preliminary binding experiments which suggested that SdiA binds its activators reversibly. These results increase our understanding of the specificity of the SdiA-ligand interaction, which could have uses in the development of anti-quorum-sensing-based antimicrobials.

4-Butyrolactone↗
PubMed2006

Probing the Lewis acidity and catalytic activity of the metal-organic framework [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate).

An optimized procedure was designed for the preparation of the microporous metal-organic framework (MOF) [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate). The crystalline material was characterized by X-ray diffraction, optical microscopy, SEM, X-ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of alpha-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2-bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re-usability, and heterogeneity are critically assessed.

Journal Article↗
PubMed2006

Quorum signal molecules as biosurfactants affecting swarming in Rhizobium etli.

Swarming motility is suggested to be a social phenomenon that enables groups of bacteria to coordinately and rapidly move atop solid surfaces. This multicellular behavior, during which the apparently organized bacterial populations are embedded in an extracellular slime layer, has previously been linked with biofilm formation and virulence. Many population density-controlled activities involve the activation of complex signaling pathways using small diffusible molecules, also known as autoinducers. In Gram-negative bacteria, quorum sensing (QS) is achieved primarily by means of N-acylhomoserine lactones (AHLs). Here, we report on a dual function of AHL molecules in controlling swarming behavior of Rhizobium etli, the bacterial symbiotic partner of the common bean plant. The major swarming regulator of R. etli is the cinIR QS system, which is specifically activated in swarming cells by its cognate AHL and other long-chain AHLs. This signaling role of long-chain AHLs is required for high-level expression of the cin and rai QS systems. Besides this signaling function, the long-chain AHLs also have a direct role in surface movement of swarmer cells as these molecules possess significant surface activity and induce liquid flows, known as Marangoni flows, as a result of gradients in surface tension at biologically relevant concentrations. These results point to an as-yet-undisclosed direct role of long-chain AHL molecules as biosurfactants.

4-Butyrolactone↗
PubMed2006

Level of M(IP)2C sphingolipid affects plant defensin sensitivity, oxidative stress resistance and chronological life-span in yeast.

The antifungal plant defensin DmAMP1 interacts with fungal sphingolipids of mannosyldiinositolphosphorylceramide (M(IP)2C) class. We screened a Saccharomyces cerevisiae transposon (Tn) mutant library against DmAMP1 and identified one DmAMP1-resistant mutant with the Tn inserted in the M(IP)2C biosynthesis gene IPT1 (DmTn11) and one DmAMP1-hypersensitive mutant with the Tn inserted in rDNA (HsTnII). However, tetrad analysis pointed to HsTnII as a spontaneous mutant. Apparently, membranes of DmTn11 lack M(IP)2C, whereas membranes of HsTnII have increased M(IP)2C levels. In addition, DmTn11 and HsTnII are characterized by increased and reduced oxidative stress resistance/chronological life-span (CL), respectively. A putative involvement of M(IP)2C in oxidative stress and CL in yeast is discussed.

Defensins↗
PubMed2006

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting.

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity--ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Journal Article↗
PubMed2005

Synergism between molybdenum and lanthanum in the disproportionation of hydrogen peroxide into singlet oxygen.

The catalytic disproportionation of hydrogen peroxide into singlet molecular oxygen was studied using the combined action of lanthanum(III) and molybdenum(VI). A synergistic effect was observed between both metals, resulting in a strong acceleration of the H2O2 disproportionation. An optimum in the catalytic activity was found at La/Mo and La/NaOH molar ratios of 4/1 and 1/3, respectively.

Journal Article↗
PubMed2005

In situ space- and time-resolved sorption kinetics of anionic dyes on individual LDH crystals.

Fluorescence spectroscopy is employed to follow the ion exchange of an emissive dye--a carboxylated perylene imide--on a layered double hydroxide. The exchange of the carboxylate ions starts at the edges of the layered double hydroxide crystals and is followed by diffusion to the basal plane. Such space-resolved observations provide a solid basis for modelling and studying the mechanisms of exchange.

Journal Article↗
PubMed2005

Porphyrin-functionalized dendrimers: synthesis and application as recyclable photocatalysts in a nanofiltration membrane reactor.

The convergent synthesis of a series of porphyrin-functionalized pyrimidine dendrimers has been accomplished by a procedure involving the nucleophilic aromatic substitution (NAS) as a key reaction step. The resulting dendritic porphyrin catalysts show high activity in the light-induced generation of singlet oxygen ((1)O2) from ground-state oxygen. These materials are synthetically useful photosensitizers for the oxidation of various olefinic compounds to the corresponding allylic hydroperoxides. Catalytic activities and regio- and stereoselectivities of the dendritic photosensitizers are comparable to those observed for mononuclear porphyrin catalysts. Recycling of the dendrimer-enlarged homogeneous photocatalysts was possible by solvent-resistant nanofiltration (SRNF) by using an oxidatively stable membrane consisting of a polysiloxane polymer and ultrastable Y zeolite as inorganic filler. Moreover, this membrane technology provides a safe way to isolate the hydroperoxide products under very mild conditions. The membrane showed high retention for the macromolecular catalysts, even in chlorinated solvents, but some oxidative degradation of the porphyrin units of the dendrimer was observed over multiple catalytic runs.

Catalysis↗
PubMed2003

Activation of hydrogen peroxide through hydrogen-bonding interaction with acidic alcohols: epoxidation of alkenes in phenol.

[reaction: see text] Electrophilic activation of hydrogen peroxide can be achieved in acidic alcohol solvents without the need for a metal catalyst. This concept is illustrated by the epoxidation of alkenes with H(2)O(2) employing phenol as a solvent. It is proposed that intermolecular hydrogen bonding between H(2)O(2) and phenol activates H(2)O(2) for oxygen-atom transfer. In this interaction, the role of phenol is purely catalytic.

Journal Article↗
PubMed2001

The cin quorum sensing locus of Rhizobium etli CNPAF512 affects growth and symbiotic nitrogen fixation.

Rhizobium etli CNPAF512 produces an autoinducer that inhibits growth of Rhizobium leguminosarum bv. viciae 248 and activates the Agrobacterium tumefaciens tra reporter system. Production of this compound in R. etli is dependent on two genes, named cinR and cinI, postulated to code for a transcriptional regulator and an autoinducer synthase, respectively. NMR analysis of the purified molecule indicates that the R. etli autoinducer produced by CinI is a saturated long chain 3-hydroxy-acyl-homoserine lactone, abbreviated as 3OH-(slc)-HSL. Using cin-gusA fusions, expression of cinI and cinR was shown to be growth phase-dependent. Deletion analysis of the cinI promoter region indicates that a regulatory element negatively controls cinI expression. Mutational analysis revealed that expression of the cinI gene is positively regulated by the CinR/3OH-(slc)-HSL complex. Besides 3OH-(slc)-HSL, R. etli produces at least six other autoinducer molecules, for which the structures have not yet been revealed, and of which the synthesis requires the previously identified raiI and raiR genes. At least three different autoinducers, including a compound co-migrating with 3OH-(slc)-HSL, are produced in R. etli bacteroids isolated from bean nodules. This is further substantiated by the observation that cinI and cinR are both expressed under symbiotic conditions. Acetylene reduction activity of nodules induced by the cin mutants was reduced with 60-70% compared with wild-type nodules, indicating that the R. etli 3OH-(slc)-HSL is involved in the symbiotic process. This was further confirmed by transmission electron microscopy of nodules induced by the wild type and the cinI mutant. Symbiosomes carrying cinI mutant bacteroids did not fully differentiate compared with wild-type symbiosomes. Finally, it was observed that the cinR gene and raiR control growth of R. etli.

4-Butyrolactone↗