Pathways in permanganate oxidation of mandelic acid: reactivity and selectivity of intermediate manganese species.

We report a comprehensive kinetic and product study of the oxidation of mandelic acid (MA) by permanganate in the pH range of 1-13, including a full account of total oxidizing equivalents (five and three-electron change in acidic and basic media, respectively). In the entire pH range, the reaction shows a primary kinetic deuterium isotope effect (kH/kD ≥8-9), indicating rate-limiting hydride transfer. The deuterium label in α-deutero-mandelic acid is retained in benzaldehyde. Benzaldehyde (BZ) is formed in post-rate limiting steps due to reactions involving manganese intermediates. In alkaline pH (≥13), in the presence of barium acetate, Mn(VI) is removed as insoluble blue barium manganate; the stoichiometry of the first step of reduction was found to be: MA + 2Mn(VII) → PGA + 2Mn(VI). Manganate, MnO42-, is directly reduced to MnO2 giving an additional mole of phenylglyoxylic acid (PGA). The experimentally observed ratio of benzaldehyde to phenylglyoxylic (BZ/PGA) provides a basis for discrimination between mechanistic choices that include direct reduction of Mn(V) to Mn(III) (in an acidic medium), disproportionation to Mn(IV) and Mn(VI) or oxidation to Mn(VI) by a second mole of permanganate. Interestingly, at pH 4, a stoichiometric, soluble Mn(IV) is observed for the first time for hydroxy-acid oxidation, reminiscent of the Guyard reaction. Because of the widespread use of permanganate as an environmentally green oxidant, results from mandelic acid oxidation have implications for the remediation of dissolved organic matter (DOM) including hydrocarbons and nitroaromatics in waste and groundwater.

Imaging Reactive Oxygen Radicals in Excised Mouse Lung Trapped by Reaction with Hydroxylamine Probes Using 1 GHz Rapid Scan Electron Paramagnetic Resonance.

PURPOSE: Oxidative stress is proposed to be critical in acute lung disease, but methods to monitor radicals in lungs are lacking. Our goal is to develop low-frequency electron paramagnetic resonance (EPR) methods to monitor radicals that contribute to the disease. PROCEDURES: Free radicals generated in a lipopolysaccharide-induced mouse model of acute respiratory distress syndrome reacted with cyclic hydroxylamines CPH (1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine hydrochloride) and DCP-AM-H (4-acetoxymethoxycarbonyl-1-hydroxy-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid), which were converted into the corresponding nitroxide radicals, CP• and DCP•. The EPR signals of the nitroxide radicals in excised lungs were imaged with a 1 GHz EPR spectrometer/imager that employs rapid scan technology. RESULTS: The small numbers of nitroxides formed by reaction of the hydroxylamine with superoxide result in low signal-to-noise in the spectra and images. However, since the spectral properties of the nitroxides are known, we can use prior knowledge of the line shape and hyperfine splitting to fit the noisy data, yielding well-defined spectra and images. Two-dimensional spectral-spatial images are shown for lung samples containing (4.5 ± 0.5) ×1014 CP• and (9.9 ± 1.0) ×1014 DCP• nitroxide spins. These results suggest that a probe that accumulates in cells gives a stronger nitroxide signal than a probe that is more easily washed out of cells. CONCLUSION: The nitroxide radicals in excised mouse lungs formed by reaction with hydroxylamine probes CPH and DCP-AM-H can be imaged at 1 GHz.

Electron paramagnetic resonance characterization and electron spin relaxation of manganate ion in glassy alkaline LiCl solution and doped into Cs2SO4.

Manganate ion, MnO42-, has important roles in catalysis and potential roles in water treatment. EPR spectra of MnO42- in a glassy alkaline solution of concentrated LiCl at X-band and Q-band at 80 K exhibit g1 = 1.9776 ± 0.001, g2 = 1.9677 ± 0.001, g3 = 1.9560 ± 0.001 and A1 = 182 ± 9, A2 = 275 ± 15, and A3 = 400 ± 15 MHz. In Cs2SO4 the spectra were simulated with 1.908 ± 0.001, g2 = 1.909 ± 0.001, g3 = 1.937 ± 0.001 and A1 = 90 ± 20, A2 = 100 ± 20, and A3 = 400 ± 15 MHz. Simulations required large distributions in A values which suggests that hyperfine splittings are sensitive to differences in geometry. Continuous wave spectra are observable at 80 K in glassy alkaline LiCl, but only up to about 20 K in Cs2SO4. In glassy alkaline LiCl electron spin relaxation was measured at X-band using spin echo and inversion recovery from 4.2 to 60 K. Tm is 4.6 µs at 4.2 K and decreases at higher temperatures as it becomes driven by T1. T1 decreases from ca. 34 ms at 4.2 K to ca. 240 ns at 60 K. Tm and T1 in Cs2SO4 are too short to measure by electron spin echo. The distorted tetrahedral geometry of MnO42- results in faster relaxation than for other 3d1 spin systems that have square pyramidal (C4v) or distorted octahedral geometries.