On the position of the highest occupied molecular orbital in aqueous solutions of simple ions.

The energies of the highest occupied molecular orbital (HOMO) of four simple microsolvated aqua ion clusters (Na+, Ag+, Cl-, CN-) are computed for varying numbers of water molecules. Extrapolating to infinite hydration numbers we find that these energies approach a value of -6 eV. This limiting one-electron energy is within a margin of +/-1 eV independent of the character of the ion and is 4 eV lower compared to the estimate obtained for the HOMO energy of the ions in aqueous solution under periodic boundary conditions. We argue that this discrepancy must the attributed to a shift in the reference of the one-electron potential of the periodic solvent model.     

Systematic control of photochemistry: the dynamics of photoisomerization of a model cyanine dye

On-the-fly CASSCF nonadiabatic dynamics have been used to model the trans-cis isomerization of a model cyanine dye. Our results show that the photochemical generation of the trans versus cis product is dynamically controlled by the presence of an extended cis-trans conical intersection seam that persists along all torsional angles. This in turn suggests that the photochemistry could be completely controlled by controlling the distribution of momentum components in a wave packet excited by laser photolysis in a coherent control experiment.     

Amino acid analysis by micellar electrokinetic chromatography with laser-induced fluorescence detection: application to nanolitre-volume biological samples from Arabidopsis thaliana and Myzus persicae

Amino acids were derivatised with 4-fluoro-7-nitrobenzo-2,1,3-oxadiazol (NBD-F), separated by micellar electrokinetic chromatography (MEKC), and detected by argon-ion (488 nm) laser-induced fluorescence. The optimised MEKC background electrolyte conditions were: 40 mM sodium cholate, 5 mM beta-cyclodextrin in 20 mM aqueous borate buffer, pH 9.1, with 7% v/v acetonitrile. Using these conditions, 19 amino acids were separated within 17 min. The limits of detection were in the range of 7.6-42.2 pmol/mL and limits of quantitation from 0.05-0.14 nmol/mL. The method was systematically validated for injection volume error, migration time variation, calibration linearity, accuracy, precision, and recovery. Nanolitre volume samples of phloem sap of individual sieve element cells from the plant Arabidopsis thaliana and honeydew from the aphid Myzus persicae were directly analysed with this method. Quantitative amino acid concentrations in these two biological matrices were profiled for the first time. This method is particularly important because it allows the complete profile of the amino acids obtained from individual phloem elements, allowing cell to cell and plant to plant variation to be quantified, which to date has not been possible with Arabidopsis thaliana.     

Electron transfer dissociation of peptide anions

Ion/ion reactions of multiply deprotonated peptide anions with xenon radical cations result in electron abstraction to generate charge-reduced peptide anions containing a free-radical site. Peptide backbone cleavage then occurs by hydrogen radical abstraction from a backbone amide N to facilitate cleavage of the adjacent C-C bond, thereby producing a- and x-type product ions. Introduction of free-radical sites to multiply charged peptides allows access to new fragmentation pathways that are otherwise too costly (e. g., lowers activation energies). Further, ion/ion chemistry, namely electron transfer reactions, presents a rapid and efficient means of generating odd-electron multiply charged peptides; these reactions can be used for studying gas-phase chemistries and for peptide sequence analysis.     

Enantiopure eta4-(1-sulfinyldiene)iron(0) tricarbonyl complexes as templates for carbocycle construction via ring-closing metathesis

[reaction: see text] Enantiomerically pure 1-(1Z,3E)-sulfinyl iron(0) dienols have been elaborated to bis-olefins capable of undergoing ring-closing metathesis chemistry. Using Grubbs' ruthenium carbene catalyst, a six-membered carbocycle (with one chiral center) and seven-, eight-, and nine-membered carbocycles (with two chiral centers) have been prepared. Novel transformations include the LiClO4-promoted allylation of an alkylidene malonate and a reductive sulfur-carbon bond cleavage to convert a vinyl sulfoxide into an alkene with SmI2.     

Linker influence on the stereochemical outcome of glycosylations utilizing solid support-bound glycosyl phosphates

[reaction: see text] Glycosyl phosphates can be readily accessed on a solid support via a three-step procedure from support-bound glycals. These resin-bound glycosyl phosphates were successfully used as glycosylating agents for coupling with a series of nucleophiles. The stereochemical outcome of disaccharide formation was dependent on the nature of the linker connecting the saccharide to the polymer. Interestingly, other glycosyl donors such as thioglycosides and trichloroacetimidates did not exhibit such a dependence, indicating a different reaction mechanism for glycosylation.     

PHOTOOXIDATION OF A REFINED PETROLEUM OIL: INHIBITION BY β-CAROTENE AND ROLE OF SINGLET OXYGEN

Abstract— β-Carotene, at concentrations 0.1 m M , inhibits the formation of hydroperoxides and other oxidation products in a refined petroleum oil exposed to Pyrex-filtered UV. The effect appears to be due to ¹O₂ quenching. A mechanism incorporating ¹O₂ and radical processes is proposed as a model for environmental photooxidation of petroleum.