Kinetics and mechanistic studies of the atmospheric oxidation of alkynes

Kinetics studies of the OH-initiated oxidation of 2-butyne, propyne, and acetylene were conducted at 100 Torr and 298 K using turbulent flow chemical ionization mass spectrometry. The major oxidation products were identified, and with the aid of supporting electronic structure thermodynamics calculations, a general OH-initiated oxidation mechanism for the alkynes is proposed. The major product branching ratio and the product-forming rate constants for the 2-butyne-OH adduct + O(2) reaction were experimentally determined as well. The atmospheric implications of the chemical oxidation mechanism and kinetics results are discussed.     

Chemometric approach to evaluate the parameters affecting electrospray: application of a statistical design of experiments for the study of arginine ionization

The effects of different experimental parameters on arginine electrospray ionization have been investigated with response surface modelling design. This chemometric technique allows a study of the effects of selected experimental variables and their interactions on the response of an experiment by performing a limited number of analyses. Six variables were studied: methanol content in the liquid phase, formic acid concentration, electrospray voltage, orifice voltage, mobile phase flow rate, and sheath gas flow rate. Signal abundance and signal-to-noise ratio of the protonated molecule and the protonated dimer were measured from the electrospray mass spectra and these four responses were tested by the design. The factor that exhibits the greatest influence on MH+ abundance is shown to be the liquid flow rate whereas the formation of protonated dimer is mainly controlled by the percentage of methanol in the mobile phase. A strong synergic effect of methanol content and formic acid concentration in the liquid has also been demonstrated in the study of noise level. Moreover, the capabilities of the multicriteria optimization method have been demonstrated through a successful prediction of a set of optimal experimental conditions.     

Predicted rate enhancements of dephosphorylation in functional oximate comicelles

The rate enhancements of dephosphorylation of p-nitrophenyldiphenyl phosphate by comicelles of cetyltrimethylammonium bromide with a functional cosurfactant are due wholly to increased concentration of the nucleophile in the micelle.     

Ugi–Smiles couplings: new entries to N-aryl carboxamide derivatives

The scope of the Ugi reaction has been extended by the use of phenols as carboxylic acid surrogates to afford N-aryl carboxamides. A Smiles rearrangement occurs as the last step of the mechanism instead of the classical final Mumm process. Various parameters concerning the nature of these inputs have been studied: the use of heteroaromatic derivatives, the substitution of the hydroxyl moiety by a thio entity (to afford functionalized thioamides), as well as the influence of the nature and position of substituents on the phenol. A three-component version (Passerini–Smiles) of this coupling has been developed as well. Following these couplings, various post-condensation transformations have been performed to reach more complex heteroaromatic fused systems. The easy functionalizations of phenols offer many opportunities for cyclization strategies: the reduction of the nitro group allows the formation of o-phenylenediamine derivatives, which, in turn, can be transformed into quinoxalines, benzotriazoles, and benzimidazoles. Various organometallic reactions of the Ugi–Smiles adducts have been successfully carried out, either from iodophenols (Heck couplings to give indoles, Ullmann reaction to form quinoxalines), or from allyl pyrimidines (azepine formation by RCM strategy) as starting phenol inputs. Finally, a new palladium-mediated oxidative cyclization led to the formation of tricyclic systems.     

Chlorofluoroiodomethane as a potential candidate for parity violation measurements

CHFClI is among the more favorable molecules for parity violation (PV) measurements in molecules. Despite the fact that calculated PV effects are two orders of magnitude smaller than in some organometallic compounds, CHFClI displays interesting features which could make possible a new experimental PV test on this molecule. Indeed, ultrahigh resolution spectroscopy using an ultrastable CO(2) laser is favored by several intrinsic properties of this molecule. For example, the high vapor pressure of CHFClI allows investigation by supersonic beam spectroscopy. Indeed, the spectroscopic constants have been accurately determined by microwave and millimetre wave spectroscopy. This is important for the subsequent selection of an appropriate absorption band of CHFClI that could be brought to co?ncide with the absorption of CO(2). Partially resolved (+)- and (-)-CHFClI enantiomers with respectively 63.3 and 20.5% ee's have been recently prepared and analyzed by molecular recognition using chiral hosts called cryptophanes. Finally, the S-(+)/R-(-) absolute configuration was ascertained by vibrational circular dichro?sm (VCD) in the gas phase.     

Crystal structures and catalytic mechanism of the Arabidopsis cinnamyl alcohol dehydrogenases AtCAD5 and AtCAD4

The cinnamyl alcohol dehydrogenase (CAD) multigene family in planta encodes proteins catalyzing the reductions of various phenylpropenyl aldehyde derivatives in a substrate versatile manner, and whose metabolic products are the precursors of structural lignins, health-related lignans, and various other metabolites. In Arabidopsis thaliana, the two isoforms, AtCAD5 and AtCAD4, are the catalytically most active being viewed as mainly involved in the formation of guaiacyl/syringyl lignins. In this study, we determined the crystal structures of AtCAD5 in the apo-form and as a binary complex with NADP+, respectively, and modeled that of AtCAD4. Both AtCAD5 and AtCAD4 are dimers with two zinc ions per subunit and belong to the Zn-dependent medium chain dehydrogenase/reductase (MDR) superfamily, on the basis of their overall 2-domain structures and distribution of secondary structural elements. The catalytic Zn2+ ions in both enzymes are tetrahedrally coordinated, but differ from those in horse liver alcohol dehydrogenase since the carboxyl side-chain of Glu70 is ligated to Zn2+ instead of water. Using AtCAD5, site-directed mutagenesis of Glu70 to alanine resulted in loss of catalytic activity, thereby indicating that perturbation of the Zn2+ coordination was sufficient to abolish catalytic activity. The substrate-binding pockets of both AtCAD5 and AtCAD4 were also examined, and found to be significantly different and smaller compared to that of a putative aspen sinapyl alcohol dehydrogenase (SAD) and a putative yeast CAD. While the physiological roles of the aspen SAD and the yeast CAD are uncertain, they nevertheless have a high similarity in the overall 3D structures to AtCAD5 and 4. With the bona fide CAD's from various species, nine out of the twelve residues which constitute the proposed substrate-binding pocket were, however, conserved. This is provisionally considered as indicative of a characteristic fingerprint for the CAD family.