Denis Lesage

Gold‐ and Platinum‐Catalyzed Cycloisomerization of Enynyl Esters versus Allenenyl Esters: An Experimental and Theoretical Study

Ester-way to heaven: Unexpected formation of bicyclo[3.1.0]hexene 4 was the main focus of combined experimental and theoretical studies on the Au-catalyzed cycloisomerization of branched dienyne 1 (see scheme), which provided better understanding of the mechanistic details governing the cyclization of enynes bearing a propargylic ester group.Experimental and theoretical studies on Au- and Pt-catalyzed cycloisomerization of a branched dienyne with an acetate group at the propargylic position are presented. The peculiar architecture of the dienyne precursor, which has both a 1,6- and a 1,5-enyne skeleton, leads, in the presence of alkynophilic gold catalysts, to mixtures of bicyclic compounds 3, 4, and 5. Formation of unprecedented bicyclo[3.1.0]hexene 5 is the main focus of this study. The effect of the ancillary ligand on the gold center was examined and found to be crucial for formation of 5. Further mechanistic studies, involving cyclization of an enantioenriched dienyne precursor, (18)O-labeling experiments, and DFT calculations, allowed an unprecedented reaction pathway to be proposed. We show that bicyclo[3.1.0]hexene 5 is likely formed by a 1,3-OAc shift/allene-ene cyclization/1,2-OAc shift sequence, as calculated by DFT and supported by Au-catalyzed cyclization of isolated allenenyl acetate 7, which leads to improved selectivity in the formation of 5. Additionally, the possibility of OAc migration from allenyl acetates was supported by a trapping experiment with styrene that afforded the corresponding cyclopropane derivative. This unprecedented generation of a vinyl metal carbene from an allenyl ester supports a facile enynyl ester/allenenyl ester equilibrium. Further examination of the difference in reactivity between enynyl acetates and their corresponding [3,3]-rearranged allenenyl acetates toward Au- and Pt-catalyzed cycloisomerization is also presented.

Direct introduction of biological samples into a LTQ-Orbitrap hybrid mass spectrometer as a tool for fast metabolome analysis.

We report the direct introduction of biological samples into a high-resolution mass spectrometer, the LTQ-Orbitrap, as a fast tool for metabolomic studies. A proof of concept study was performed on yeast cell extracts that were introduced into the mass spectrometer by using flow injection analysis, with an acquisition time of 3 min. Typical mass spectra contained a few thousand m/z signals, 400 of which were found to be analytically relevant (i.e., their intensity was 3-fold higher than that of the background noise and they occurred in at least 60% of the acquisition profiles under identical experimental conditions). The method was validated by studies of the matrix effect, linearity, and intra-assay precision. Accurate mass measurements in the Orbitrap discriminated between isobaric ions and also indicated the elemental composition of the ions of interest with mass errors below 5 ppm, for identification purposes. The proposed structures were then assessed by MSn experiments via the linear ion trap, together with accurate mass determination of the product ions in the Orbitrap analyzer. When applied to the study of cadmium toxicity, the method was as effective as that initially developed by using LC/ESI-MS/MS for a targeted approach. The same metabolic fingerprints were also subjected to multivariate statistical analyses. The results highlighted a reorganization of amino acid metabolism under cadmium conditions in order to increase the biosynthesis of glutathione.

Tracking gold acetylides in gold(I)-catalyzed cycloisomerization reactions of enynes

The intermediacy of gold acetylides in the gold(I)-catalyzed cycloisomerization of enynes was questioned. While dinuclear gold complexes are observed under electrospray ionization conditions, the solution reactivity of gold acetylides also leads to the conclusion of their high affinity for the second coordination of a gold moiety, leading to dinuclear gold complexes. However, the involvement of gold acetylides and the corresponding diaurated species in the elementary steps of cycloisomerization mechanisms of enynes appears unlikely.

Zinc binding properties of the amyloid fragment Aβ(1–16) studied by electrospray-ionization mass spectrometry

A major hallmark of Alzheimer’s disease (AD) is the strong accumulation in brain of senile plaques, mainly composed of the amyloid-β peptide (Aβ). Recent studies have suggested that the zinc cation would be a possible key mediating factor for the formation of amyloid extracellular deposits, by binding to Aβ and triggering the involved aggregation process. From a previous circular dichroism (CD) study, we have proposed the N-terminal 1–16 region of Aβ(1–16), as the minimal fragment able to specifically bind zinc. Here we investigate the Zn2+ binding properties of Aβ(1–16) by electrospray-ionization mass spectrometry (ESI-MS). The stoichiometry of Aβ(1–16)/Zn2+ association and the relative affinity of different cations towards Aβ(1–16) are investigated by analyzing the mass spectra of Aβ(1–16) in the presence of different cations, introduced alone or in competition. Zn2+ binding sites are determined from collision-induced dissociation (CID) experiments conducted on the Aβ(1–16) cationized species. From these data, Aβ(1–16) is shown to form a 1:1 complex with Zn2+ and to bind up to three cations upon increasing the Zn2+ concentration. Under CID, zinc binding induces specific cleavages after the three histidines of the Aβ(1–16) sequence (H6, H13 and H14), showing their simultaneous implication in the Zn2+ coordination sphere. The binding of Aβ(1–16) to several Zn2+ cations appears less specific, but still implicates the three histidines, each of them behaving thus as an autonomous binding site. A model is proposed to explain both the specific and the aspecific interactions of Zn2+ with Aβ(1–16) that is confirmed here to behave as the minimal zinc-binding region of Aβ.

The Role of Water in Platinum‐Catalyzed Cycloisomerization of 1,6‐Enynes: A Combined Experimental and Theoretical Gas Phase Study

PtII‐catalyzed cycloisomerization of a 1,6‐enyne proceeds quickly in CHCl3 at room temperature, and also in the gas phase. However, calculations predict a slow reaction, because of the formation of a very stable chelate that undergoes oxidative cyclization at a high energy cost. The electrophilic activation of the alkyne followed by nucleophilic attack of the double bond to generate a cyclopropyl carbene would lead to a faster reaction, but the decomplexation of the double bond to generate an unsaturated platinum species is also energetically disfavored. However, decomplexation can be envisaged if an adventitious molecule of water enters the coordination sphere of the metal. This crucial role of water in PtII‐catalyzed cycloisomerization of enynes, initially sensed some years ago (Echavarren et al., J. Am. Chem. Soc. 2001, 123, 10511) is now supported by mass spectrometry studies.

Formation and Characterization of Gaseous Adducts of Carbon Dioxide to Magnesium, (CO2)MgX− (X=OH, Cl, Br)

A good fix: the structure and chemical reactivity of a reduced form of CO(2) bonded to magnesium, XMg(η(2)-O(2)C)(-), is reported. Upon reaction with water it loses CO, while it adds CH(3) upon reaction with alkyl halides, thereby signifying nucleophilicity of the carbon atom in XMg(η(2)-O(2)C)(-) in S(N)2 reactions.

Evaluation and optimization of DNA delivery into gliosarcoma 9L cells by a cholesterol-based cationic liposome

This paper reports results concerning the transfection of gliosarcoma cells 9L using an original cholesterol-based cationic liposome as carrier. This cationic liposome was prepared from triethyl aminopropane carbamoyl cholesterol (TEAPC-Chol) and a helper lipid, dioleoyl phosphatidyl ethanolamine (DOPE). The used concentration of liposome was not cytotoxic as revealed by the MTT test. TEAPC-Chol/DOPE liposomes allowed the plasmids encoding reporter genes to enter the nucleus as observed both by electron microscopy and functionality tests using fluorescence detection of green fluorescent protein (GFP) and luminometric measurements of luciferase activity. By changing the cationic lipid/DNA molar charge ratio, optimal conditions were determined. Further, improvement of the transfection level has been obtained by either precondensing plasmid DNA with poly-L-lysine or by adding polyethylene glycol (PEG) in the transfection medium. The optimal conditions determined are different depending on whether the transfection is made with cells in culture or with tumors induced by subcutaneous (s.c.) injection of cells in Nude mice. For in vivo assays, a simple method to overcome the interference of haemoglobin with the chemiluminescence intensity of luciferase has been used. These results would be useful for gaining knowledge about the potential for the cationic liposome TEAPC-Chol/DOPE to transfect brain tumors efficiently.

Critical Evaluation of Kinetic Method Measurements: Possible Origins of Nonlinear Effects

AbstractThe kinetic method is a widely used approach for the determination of thermochemical data such as proton affinities (PA) and gas-phase acidities (ΔH°acid). These data are easily obtained from decompositions of noncovalent heterodimers if care is taken in the choice of the method, references used, and experimental conditions. Previously, several papers have focused on theoretical considerations concerning the nature of the references. Few investigations have been devoted to conditions required to validate the quality of the experimental results. In the present work, we are interested in rationalizing the origin of nonlinear effects that can be obtained with the kinetic method. It is shown that such deviations result from intrinsic properties of the systems investigated but can also be enhanced by artifacts resulting from experimental issues. Overall, it is shown that orthogonal distance regression (ODR) analysis of kinetic method data provides the optimum way of acquiring accurate thermodynamic information.

Comparison of the activation time effects and the internal energy distributions for the CID, PQD and HCD excitation modes.

Reproducibility among different types of excitation modes is a major bottleneck in the field of tandem mass spectrometry library development in metabolomics. In this study, we specifically evaluated the influence of collision voltage and activation time parameters on tandem mass spectrometry spectra for various excitation modes [collision-induced dissociation (CID), pulsed Q dissociation (PQD) and higher-energy collision dissociation (HCD)] of Orbitrap-based instruments. For this purpose, internal energy deposition was probed using an approach based on Rice-Rampserger-Kassel-Marcus modeling with three thermometer compounds of different degree of freedom (69, 228 and 420) and a thermal model. This model treats consecutively the activation and decomposition steps, and the survival precursor ion populations are characterized by truncated Maxwell-Boltzmann internal energy distributions. This study demonstrates that the activation time has a significant impact on MS/MS spectra using the CID and PQD modes. The proposed model seems suitable to describe the multiple collision regime in the PQD and HCD modes. Linear relationships between mean internal energy and collision voltage are shown for the latter modes and the three thermometer molecules. These results suggest that a calibration based on the collision voltage should provide reproducible for PQD, HCD to be compared with CID in tandem in space instruments. However, an important signal loss is observed in PQD excitation mode whatever the mass of the studied compounds, which may affect not only parent ions but also fragment ions depending on the fragmentation parameters. A calibration approach for the CID mode based on the variation of activation time parameter is more appropriate than one based on collision voltage. In fact, the activation time parameter in CID induces a modification of the collisional regime and thus helps control the orientation of the fragmentation pathways (competitive or consecutive dissociations).

The Pauson–Khand Mechanism Revisited: Origin of CO in the Final Product†

The mechanism of the Pauson-Khand reaction has been studied by mass spectrometry and it has been found, through ion-molecule reaction with (13) CO, that the carbon monoxide incorporated into the product cyclopentenone is one that has been retained within the complex. Theoretical and kinetic calculations support this finding, which provides a complementary explanation for the effect of Pauson-Khand promoters.