Gas-phase carbene radical anions: new mechanistic insights

The gas-phase reactivity of the CHCl*- anion has been investigated with a series of halomethanes (CCl4, CHCl3, CH2Cl2, and CH3Cl) using a FA-SIFT instrument. Results show that this anion primarily reacts via substitution and by proton transfer. In addition, the reactions of CHCl*- with CHCl3 and CH2Cl2 form minor amounts of Cl2*- and Cl-. The isotopic distribution of these two products is consistent with an insertion-elimination mechanism, where the anion inserts into a C-Cl bond to form an unstable intermediate, which eliminates either Cl2*- or Cl- and Cl*. Neutral and cationic carbenes are known to insert into single bonds; however, this is the first observation of such reactivity for carbene anions.     

Treasures from the Free Radical Renaissance Period--miscellaneous hexenyl radical kinetic data

Rate constant data and Arrhenius parameters have been determined for a series of substituted hexenyl radicals of differing electronic and steric demand. Electron-withdrawing groups (CF(3), CO(2)Et) directly attached to the radical centre slightly accelerate 5-exo ring-closure (k(cis) + k(trans) ～ 2.1 × 10(5) s(-1) at 25°) relative to donating groups (OMe; 1.6 × 10(5) s(-1) at 25°). Sterically demanding groups (tert-Bu), as expected, slow the cyclization process (1 × 10(5) s(-1)). These observations are consistent with subtle changes in activation energy for 5-exo ring-closure. Interestingly, the nature of the solvent would appear to have a significant influence on this chemistry with the cis/trans stereoselectivity sometimes improved as the solvent polarity is increased. Except for the system containing the CF(3) (electron-withdrawing) group which displays an increase in the cyclization/capture rate constant (k(c)/k(H)), a general decrease in the k(c)/k(H) ratio as solvent polarity is increased is noted; these changes have been speculated to arise mainly from changes in k(H) in the various solvents employed.     

Development of an enzyme-linked immunosorbent assay (ELISA)-like fluorescence assay to investigate the interactions of glycosaminoglycans to cells

Sulfated glycosaminoglycans were labeled with biotin to study their interaction with cells in culture. Thus, heparin, heparan sulfate, chondroitin 4-sulfate, chondroitin 6-sulfate and dermatan sulfate were labeled using biotin-hydrazide, under different conditions. The structural characteristics of the biotinylated products were determined by chemical (molar ratios of hexosamine, uronic acid, sulfate and biotin) and enzymatic methods (susceptibility to degradation by chondroitinases and heparitinases). The binding of biotinylated glycosaminoglycans was investigated both in endothelial and smooth muscle cells in culture, using a novel time resolved fluorometric method based on interaction of europium-labeled streptavidin with the biotin covalently linked to the compounds. The interactions of glycosaminoglycans were saturable and number of binding sites could be obtained for each individual compound. The apparent dissociation constant varied among the different glycosaminoglycans and between the two cell lines. The interactions of the biotinylated glycosaminoglycans with the cells were also evaluated using confocal microscopy. We propose a convenient and reliable method for the preparation of biotinylated glycosaminoglycans, as well as a sensitive non-competitive fluorescence-based assay for studies of the interactions and binding of these compounds to cells in culture.     

Two-coordinate d9 complexes. synthesis and oxidation of NHC nickel(I) amides

Reaction of the d9-d9 Ni(I) monochloride dimer, [(IPr)Ni(mu-Cl)]2 (1), with NaN(SiMe3)2 and LiNHAr (Ar = 2,6-diisopropylphenyl) gives the novel monomeric, 2-coordinate Ni(I) complexes (IPr)Ni{N(SiMe3)2} (2) and (IPr)Ni(NHAr) (3). Reaction of 2 with Cp2Fe+ results in its 1-e- oxidation followed by beta-Me elimination to give a base-stabilized iminosilane complex [(IPr)Ni(CH3){kappa1-N(SiMe3)=SiMe2.Et2O}][BArF4] (6). Oxidation of 3 gives [(IPr)Ni(eta3-NHAr)(THF)][BArF4] (4), which upon loss of THF affords dimeric [(IPr)Ni(N,eta3:NHC6iPr2H3)]2[BArF4]2 (5).     

Clustering of rare earths in GeAs sulfide glass

Clustering of rare-earth dopants in GeAs sulfide glasses was studied by fluorescence spectroscopy of Pr-doped glasses and by EPR measurements of Gd-doped samples. The linewidth of the g  2 resonance of Gd³⁺, as well as the relative intensity of emission from the ¹D₂ level of Pr³⁺, was used as a relative measure of rare-earth clustering. Rare earths were found to have low solubility in uncodoped GeAs sulfide glasses, which also displayed poor fluorescence efficiency due to severe clustering. Codoping such glasses with Ga greatly enhanced rare-earth solubility and dispersal, particularly for Ga:rare earth ratios ≥ 10:1, as evidenced by the narrower EPR resonances and more intense luminescence of Gd- and Pr-doped glasses, respectively. In, P and Sn were also observed to ‘decluster’ rare earths, although less efficiently than Ga, whereas codoping with I was found to have no effect on clustering. These phenomena are explained by a structural model in which (1) rare-earth dopants and codopants are spatially associated and (2) rare-earth dispersal is accomplished by a statistical distribution of codopants in tetrahedral network sites.     

An Inexpensive Laser Raman Spectrometer Based on CCD Detection

We describe an inexpensive modular Raman spectrometer system that can be assembled from commercial components for under $5000. Three typical applications are presented: a demonstration of a vibrational isotope effect, a Raman polarization experiment, and a resonance Raman experiment. This spectrometer system should make it easier to include the important topic of Raman spectroscopy in the undergraduate physical and analytical chemistry laboratory curricula.     

Analysis of carbenoxolone by ultra‐high‐performance liquid chromatography tandem mass spectrometry in mouse brain and blood after systemic administration

Carbenoxolone is a derivative of glycyrrhetinic acid found in the root of Glycyrrhiza glabra, colloquially known as licorice. It has been used as a treatment for peptic and oral ulcers. In recent years, carbenoxolone has been utilized in basic research for its ability to block gap junctional communication. Better understanding the distribution of carbenoxolone after systemic administration can lead to a better understanding of its potential sites of action. Presented is an ultra high‐performance liquid chromatography tandem mass spectrometer (UHPLC–MS/MS) method for the identification and quantification of carbenoxolone in mouse blood and brain tissue. Twenty mice were injected intraperitoneally with 25 mg/kg carbenoxolone and brain tissue and blood were collected for analysis. Blood concentrations (mean ± SD) at 15, 30, 60 and 120 min were determined to be (n = 5) 5394 ± 778, 2636 ± 836, 1564 ± 541 and 846 ± 252 ng/mL, respectively. Brain concentrations (mean ± SD) at 15, 30, 60 and 120 mins were determined to be (n = 5) 171 ± 62, 102 ± 35, 55 ± 10 and 27 ± 9 ng/g, respectively. The analysis of these specimens at the four different time points resulted in blood and brain half‐lives in mice of ~43 and 41 min, respectively. The UHPLC–MS/MS method was determined to be sensitive and robust for quantification of carbenoxolone.