Jeff W. Denault

The Arabidopsis thaliana REDUCED EPIDERMAL FLUORESCENCE1 gene encodes an aldehyde dehydrogenase involved in ferulic acid and sinapic acid biosynthesis.

Recent research has significantly advanced our understanding of the phenylpropanoid pathway but has left in doubt the pathway by which sinapic acid is synthesized in plants. The reduced epidermal fluorescence1 (ref1) mutant of Arabidopsis thaliana accumulates only 10 to 30% of the sinapate esters found in wild-type plants. Positional cloning of the REF1 gene revealed that it encodes an aldehyde dehydrogenase, a member of a large class of NADP+-dependent enzymes that catalyze the oxidation of aldehydes to their corresponding carboxylic acids. Consistent with this finding, extracts of ref1 leaves exhibit low sinapaldehyde dehydrogenase activity. These data indicate that REF1 encodes a sinapaldehyde dehydrogenase required for sinapic acid and sinapate ester biosynthesis. When expressed in Escherichia coli, REF1 was found to exhibit both sinapaldehyde and coniferaldehyde dehydrogenase activity, and further phenotypic analysis of ref1 mutant plants showed that they contain less cell wall–esterified ferulic acid. These findings suggest that both ferulic acid and sinapic acid are derived, at least in part, through oxidation of coniferaldehyde and sinapaldehyde. This route is directly opposite to the traditional representation of phenylpropanoid metabolism in which hydroxycinnamic acids are instead precursors of their corresponding aldehydes.

Proton affinity of deuterated acetonitrile estimated by the kinetic method with full entropy analysis

Abstract Branching ratios for the dissociation of proton-bound dimers of selected alkyl nitriles, [R1CN⋯H+⋯NCR2], have been measured as a function of collision energy, using a triple quadrupole mass spectrometer. The system shows a small collision energy dependence consistent with small differences in entropy requirements for the competing fragmentation channels. The extended form of the kinetic method provides a value for the proton affinity and an approximate value for the relative entropy difference between the two dissociation channels (which corresponds approximately to the relative entropy of protonation of the two bases). A comparison is made between these results and those from earlier standard kinetic method data treatments. The proton affinity of d3-acetonitrile, estimated by the extended kinetic method, is 186.3 ± 1.2 (±1.4) kcal mol−1, where the standard deviation is given, followed by the 90% confidence limits in parentheses. The proton affinity of acetonitrile, estimated using the extended kinetic method as 188.2 ± 1.2 (±1.4) kcal mol−1, is statistically the same as that obtained for d3-acetonitrile. The relative entropies of protonation, Δ(ΔS), for d3-acetonitrile and acetonitrile, referenced to a series of alkyl nitriles, are 1.8 ± 0.3 (±0.3) and −1.1 ± 0.3 (±0.3) cal mol−1 K−1, respectively. Direct comparison of the isotopomers of acetonitrile using the standard and extended kinetic methods was employed to arrive at a more accurate value for the proton affinity difference between d3-acetonitrile and acetonitrile, and this method yielded a difference of ∼0.2 kcal mol−1. The direct comparison was also used to show that the proton affinity difference is a result of isotopic substitution. Normal secondary kinetic isotope effects were observed for the dissociation of the proton-bound dimer, CH3CN⋯H+⋯CD3CN. The branching ratio, kH/kD, was constant at 1.2 over the laboratory collision energy range of 5–50 eV.

Alkali chloride cluster ion dissociation examined by the kinetic method: heterolytic bond dissociation energies, effective temperatures, and entropic effects.

Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, [Rb-35Cl-Mi]+, where Mi = Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 ± 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 ± 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single- and multiple-collision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T→V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T→V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using MassKinetics modeling (Drahos and Vékey, J. Mass Spectrom. 2001, 36, 237).

Estimation of ionization energies of substituted anilines by the kinetic method

Abstract Ionization energy (IE) determinations by the kinetic method are extended to a functionalized class of compounds: substituted anilines. The radical-cation-bound dimers of substituted anilines were generated under self-CI conditions and the dissociation products examined using two collision energies, 2 and 10 eV, in a triple quadrupole tandem mass spectrometer. Using N , N -diethylaniline (IE = 6.98 ± 0.02 eV), 3,5-dimethylaniline (IE = 7.2 eV), N -methylaniline (IE = 7.33 ± 0.02 eV), 2-methylaniline (IE = 7.44 ± 0.02 eV), 3-methylaniline (IE = 7.50 ± 0.02 eV), aniline (IE = 7.720 ± 0.002 eV), and benzylamine (IE = 8.64 ± 0.05 eV) as reference compounds, a linear correlation is observed between the natural log of the ratio of the fragment ion abundances versus ionization energy for each collision energy. The effective temperature of the activated cluster ions rises from 1180 to 1470 K as the collision energy is raised from 2 to 10 eV. The average estimated IE values for 4-methoxyaniline, 2-methoxyaniline, 4-methylaniline, 3-chloroaniline and 3-fluoroaniline are determined to be 7.00 ± 0.15 eV, 7.19 ± 0.07 eV, 7.37 ± 0.07 eV, 8.27 ± 0.08 eV, 8.37 ± 0.07 eV respectively.

Electron inity of 1,3,5,7-cyclooctatetraene determined by the kinetic method

The kinetic method is used to determine the electron inity (EA) of 1,3,5,7-cyclooctatetraene (COT), a compound that undergoes a significant structural change upon electron attachment. Collision-induced dissociation of anionic clusters of COT with a set of reference compounds (Ref), [COT·Ref]−·, at various collision energies, allowed deconvolution of the relative enthalpies and entropies of the competitive reactions. The adiabatic EA of COT is determined to be 0.58±0.10 eV, in good agreement with the value, 0.58±0.04 eV, of Wentworth and Ristau (J. Phys. Chem.1969,73, 2126) determined by thermal electron detachment as well as the more recent value, 0.55±0.02 eV, of Kato et al. (J. Am. Chem. Soc.1997,119, 7863) determined by equilibrium electron transfer with molecular oxygen. A large entropy difference, 25.6±10.0 e.u. (J mol−1 K−1), is observed between the two dissociation channels. This entropy difference corresponds to a negative 14.7±13.0 e.u. change for the dissociation of the dimer to give COT−· and the neutral reference compound and a positive 10.9±8.4 e.u. entropy change for the dissociation of the dimer to give Ref−· and neutral COT.

Fluorinated methyl cation (CF3+, CFH2+)–alkyl nitrile cluster ions

Ion-molecule reactions of CF(3)(+) or CFH(2)(+) with an acetonitrile-butyronitrile mixture yield product cluster ions in which the two neutral molecules are associated with the cation. The structures of the ion-molecule product ions were investigated by collision-induced dissociation in a multi-quadrupole mass spectrometer. The cluster ions fragment by loss of one neutral alkyl nitrile. The abundance of the fragment ions shows an unexpected inverse correlation with the proton affinity of the alkyl nitrile. This result is inconsistent with the formation of a simple cation-bound dimeric cluster containing two alkyl nitrile molecules and a fluorinated methyl cation; instead, a pair of non-symmetrical dimeric complexes, separated by a large internal barrier, is indicated. This result is analogous to that observed for the putative methyl cation-bound heterodimer of acetonitrile and butyronitrile. Collision energy dependence studies and ab initio calculations suggest that the dimeric complexes are formed in potential energy wells located in an approximately symmetrical potential energy surface. Quasi-equilibrium theory calculations were used in order to obtain additional insights into the experimental data.

Bi-radical–ion clusters: interrupted sigma bonds in the gas phase?

Novel cation-bound bi-radicals are generated in a chemical ionization source from either the stable free radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), or the phenoxyl radical and cations including carbonyl isocyanate, ([OCNCO+]), S+•, CS+• and, in one case, the proton. The dimeric cluster ions, mildly activated under collision-induced dissociation (CID) conditions, dissociate to yield the cationized monomer (e.g. the [OCNCO]+-bound TEMPO radical, [OCNCO]+-bound phenoxyl radical, S +•-bound TEMPO radical or CS +•-bound TEMPO radical). Such facile dissociation suggests a loosely bound structure comparable to that of a proton-bound dimer. Binding of this sort, radical–cation–radical, suggests the possible formation of an ‘interrupted sigma bond’ (electron–electron interactions mediated by the cation). The cluster ion comprised of Cl+ and two TEMPO radicals behaves differently and gives a complex set of lower abundance products under similar activation conditions. This indicates that in this case the dissociating cluster has a conventional covalently bound structure. Analogous behavior is observed in the cases of proton-bound bis-TEMPO clusters and the corresponding mixed phenoxyl–TEMPO species, the Cl+-bound bis-phenoxyl cluster and the Cl+-bound cluster with mixed TEMPO–phenoxyl cluster, as well as the S+•-bound bis-phenoxyl and CS+ •-bound bis-phenoxyl complexes. The CID of the putative proton-bound bis-phenoxyl cluster also suggests a conventional structure and its dissociation shows a strong dependence on collision energy, possibly because of facile rearrangement from the desired weakly bound bi-radical cluster ion. Copyright