Transferts d'hydrogènes dans les cations radicaux ethylène-acetals dérivés de cyclopentanones et cyclohexanones

Hydrogen Transfer during Decompositions of Molecular Ions of Acetals Derived from Substituted Cyclopentanones and Cyclohexanones The isomerisation process of ethylene-acetal radical ions of low internal energy has been reinvestigated in more detail. The ring contraction of cyclohexane derivatives into methylcyclopentanes is quite general for these ions. The relative rates of [1, n] H transfers to the C free radical resulting from C? C cleavage α to the acetal group plays the most important part in the mechanism.     

An alternating direction scheme on a nonuniform mesh for reaction-diffusion parabolic problems

In this paper we develop a numerical method for two-dimensionaltime-dependent reaction-diffusion problems. This method, whichcan immediately be generalized to higher dimensions, is shownto be uniformly convergent with respect to the diffusion problems.This method, which can immediately be generalized to higherdimensions, is shown to be uniformly convergent with respectto the diffusion parameter.     

Charge dependent behavior of PNA/DNA/PNA triplexes in the gas phase

Intact noncovalent complexes were studied in the gas phase using negative ion nano-ESI mass spectrometry. Among various noncovalent systems studied in the gas phase, the interaction of DNA strands with peptide nucleic acids (PNAs) presents a strong interest as biologically relevant systems. PNAs originally described by Nielsen are used as DNA mimics as possible medical agents by imprisoning DNA single strands into stable noncovalent complexes. Two types of PNAs were investigated in the PNA/DNA multiplex: the original Nielsen's PNA and a modified backbone PNA by the introduction of syn- and anti-(aminoethyl)thiazolidine rings. We first investigated the stoichiometry of PNA/DNA multiplexes formed in solution and observed them in the gas phase via qualitative kinetics of complementary strand associations. It resulted in observing PNA2/DNA triplexes (ts) as the multiply deprotonated species, most stable in both the solution and gas phase. Second, charge-dependant decompositions of these species were undertaken under low-energy collision conditions. It appears that covalent bond cleavages (base releasing or skeleton cleavage) occur from lower ts charge states rather than ts unzipping, which takes place from higher charge states. This behavior can be explained by considering the presence of zwitterions depending on the charge state. They result in strong salt-bridge interactions between the positively charged PNA side chain and the negatively charged DNA backbone. We propose a general model to clearly display the involved patterns in the noncovalent triplex decompositions. Third, the relative stability of three PNA2/DNA complexes was scrutinized in the gas phase by acquiring the breakdown curves of their ts(6-) form, corresponding to the ts unzipping. The chemical structures of the studied PNAs were chosen in order to evidence the possible influence of backbone stereochemistry on the rigidity of PNA2/DNA complexes. It provided significantly different stabilities via V(m) measurements. The relative gas-phase stability order obtained was compared to that found in solution by Chassaing et al., and shows qualitative agreement.     

Quantitative determination of five ergot alkaloids in rye flour by liquid chromatography-electrospray ionisation tandem mass spectrometry

A confirmatory method for detecting five ergot alkaloids, ergocristine, ergotamine, ergonovine, ergocornine and alpha-ergokryptine, in rye flour is described using high performance liquid chromatography coupled to tandem mass spectrometry detection by monitoring two transition reactions per analyte. The procedure entails a liquid-liquid extraction followed by a clean-up step using a C18 solid-phase extraction (SPE) cartridge. An analogue compound, methysergide hydrogen maleinate, was used to assess both repeatability sample preparation and potential MS response fluctuations. The method was fully validated according to the European Union (EU) criteria. Detection and quantification limits of all analytes were calculated ranging from 7 to 11 microg/kg and from 23 to 37 microg/kg, respectively. Fifteen rye flour samples were investigated with the newly developed method, and none of them were above the current Swiss limits of 200mg/kg for total ergot alkaloids.     

On early events in the Pauson-Khand reaction

[reaction: see text]. The Magnus Pauson-Khand (PK) mechanism has been studied for the first time by electrospray ionization coupled to tandem mass spectrometry. It has been found that loss of CO from the PK complex precedes olefin coordination and insertion.     

Proton affinity ladder for uridine and analogs: influence of the hydroxyl group on the sugar ring conformation

A ladder of relative proton affinities (PA) for a series of modified uridines (e.g. araU, ddU, 5BrU, 5BrdU and 5IU) was established from competitive dissociations of proton-bound heterodimers using Cooks and co-workers' kinetic method. The studied heterodimers are constituted of a modified nucleoside and either an amino acid or a nucleoside with known PA value. These non-covalent heterodimers were prepared under electrospray conditions to be selected and dissociated into the ion-trap analyzer. These results allowed our PA ladder of uridine and deoxyuridine analogs substituted at the C-5 position in the uracil ring to be extended. From this scale, it was showed that the substitution of hydrogen atom at the C-2' position in the sugar ring by a hydroxyl group involves a decrease of about 7 kJ mol(-1). The experimental values for U, 5MeU, dU, 5MedU, ddU and araU are consistent with those obtained by DFT calculations (B3P86/6-31+G//B3LYP/6-31G(.)). Several neutral and protonated conformations of these compounds were considered, in particular the ring conformation of furanose and the orientation of the base with respect to the sugar ring. These calculated results showed the influence of sugar substituent on the conformation of the neutral form of theses nucleosides. However, the most stable protonated structure is the same for all the studied nucleosides except for araU, where the position of the anti 2'-OH group imposes a specific conformation.