Chemiluminescence (CL) of acridinium esters (AE) has found widespread use in analytical chemistry. Using the mechanism of the reaction of H2O2 with 10-methyl-9-(p-formylphenyl)acridinium carboxylate trifluoromethanesulfonate and a modified flow injection system, the reaction rates of each step in the mechanism were evaluated and used in a kinetic model to optimize the analysis of H2O2. Operational parameters for a flow injection analysis system (reagent pH, flow rate, sample volume, PMT settings) were optimized using the kinetic model. The system is most sensitive to reaction pH due to competition between AE hydrolysis and CL. The optimized system was used to determine H2O2 concentrations in natural waters, including rain, freshwater, and seawater. The lower limit of detection varied in natural waters, from 352 pM in open ocean seawater (mean, 779 pM +/- 15.0%, RSD) to 58.1 nM in rain (mean, 6,340 nM +/- 0.92%, RSD). The analysis is specific for H2O2 and is therefore of potential interest for atmospheric chemistry applications where organoperoxides have been reported in the presence of H2O2.
[reaction: see text]. Microwave heating is an efficient method for the acceleration of ring-closing metathesis reactions using ruthenium-based catalysts. The reaction can be rapidly conducted in either ionic liquids, such as 1-butyl-3-methylimidazolium tetrafluoroborate (bmim), or in a microwave transparent solvent (MTS) such as dichloromethane.
Absolute rate constants and degradation efficiencies for hydroxyl radical reactions with seven low-molecular-weight nitrosamines in water have been evaluated using a combination of electron-pulse radiolysis/absorption spectroscopy and steady-state radiolysis/GCMS measurements. The hydroxyl radical oxidation rate constants were found to depend upon nitrosamine size and to have a very good linear correlation with the number of methylene groups in these compounds. This correlation, given by In(k x OH) = (19.72 +/- 0.14) + (0.424 +/- 0.033) (#CH2), suggests that hydroxyl radical oxidation predominantly occurs by hydrogen atom abstraction from constituent methylene groups in each of these nitrosamines. In contrast, the hydrated electron reduction rate constants measured for these compounds were remarkably consistent, with an average value of (1.67 +/- 0.22) x 10(10) M(-1) s(-1). These reduction kinetic data are consistent with this predominantly diffusion-controlled reaction occurring at the N-NO moiety in these carcinogens. From steady-state radiolysis measurements under aerated conditions, specific hydroxyl radical degradation efficiencies for each nitrosamine were evaluated. For larger nitrosamines, the efficiency was constant at 100%; however, for the smaller alkyl substituted species, the efficiency was significantly lower, with a minimum value of only 80% determined for N-nitrosodimethylamine. The reduced efficiency is attributed to radical repair reactions competing with the slow peroxyl radical formation.
This paper examines the functional mechanism of thioxolone, a compound recently identified as a weak inhibitor of human carbonic anhydrase II by Iyer et al. (2006) J. Biomol. Screening 11, 782-791. Thioxolone lacks sulfonamide, sulfamate, or hydroxamate functional groups that are typically found in therapeutic carbonic anhydrase (CA) inhibitors, such as acetazolamide. Analytical chemistry and biochemical methods were used to investigate the fate of thioxolone upon binding to CA II, including Michaelis-Menten kinetics of 4-nitrophenyl acetate esterase cleavage, liquid chromatography-mass spectrometry (LC-MS), oxygen-18 isotope exchange studies, and X-ray crystallography. Thioxolone is proposed to be a prodrug inhibitor that is cleaved via a CA II zinc-hydroxide mechanism known to catalyze the hydrolysis of esters. When thioxolone binds in the active site of CA II, it is cleaved and forms 4-mercaptobenzene-1,3-diol via the intermediate S- (2,4-thiophenyl)hydrogen thiocarbonate. The esterase cleavage product binds to the zinc active site via the thiol group and is therefore the active CA inhibitor, while the intermediate is located at the rim of the active-site cavity. The time-dependence of this inhibition reaction was investigated in detail. Because this type of prodrug inhibitor mechanism depends on cleavage of ester bonds, this class of inhibitors may have advantages over sulfonamides in determining isozyme specificity. A preliminary structure-activity relationship study with a series of structural analogues of thioxolone yielded similar estimates of inhibition constants for most compounds, although two compounds with bromine groups at the C1 carbon of thioxolone were not inhibitory, suggesting a possible steric effect.
Absolute rate constants for hydroxyl radical, *OH, and hydrated electron, e(aq)(-), reactions with low-molecular-weight nitrosamines and nitramines in water at room temperature were measured using the techniques of electron pulse radiolysis and transient absorption spectroscopy. The bimolecular rate constants obtained, k (M(-1) s(-1)), for e(aq)(-) and *OH reactions, respectively, were as follows: methylethylnitrosamine, (1.67 +/- 0.06) x 10(10) and (4.95 +/- 0.21) x 10(8); diethylnitrosamine, (1.61 +/- 0.06) x 10(10) and (6.99 +/- 0.28) x 10(8); dimethylnitramine, (1.91 +/- 0.07) x 10(10) and (5.44 +/- 0.20) x 10(8); methylethylnitramine, (1.83 +/- 0.15) x 10(10) and (7.60 +/- 0.43) x 10(8); and diethylnitramine, (1.76 +/- 0.07) x 10(10) and (8.67 +/- 0.48) x 10(8), respectively. MNP/DMPO spin-trapping experiments demonstrated that hydroxyl radical reaction with these compounds occurs by hydrogen atom abstraction from an alkyl group, while the reaction of the hydrated electron was to form a transient radical anion. The latter adduct formation implies that the excess electron could subsequently be transferred to regenerate the parent chemical, which would significantly reduce the effectiveness of any free-radical-based remediation effort on nitrosamine/nitramine-contaminated waters.
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