Detailed Kinetics and Mechanism of the Oxidation of Thiocyanate Ion (SCN^–) by Peroxomonosulfate Ion (HSO₅^–). Formation and Subsequent Oxidation of Hypothiocyanite Ion (OSCN^–)

The haloperoxidase-catalyzed in vivo oxidation of thiocyanate ion (SCN^–) by H₂O₂ is important for generation of the antimicrobial hypothiocyanite ion (OSCN^–), which is also susceptible to oxidation by strong in vivo oxidizing agents (i.e., H₂O₂, OCl^–, OBr^–). We report a detailed mechanistic investigation on the multistep oxidation of excess SCN^– with peroxomonosulfate ion (HSO₅^– in the form of Oxone) in the range from pH 6.5 to 13.5. OSCN^– was detected to be the intermediate of this reaction under the above conditions, and a kinetic model is proposed. Furthermore, by kinetic separation of the consecutive reaction steps, the rate constant of the direct oxidation of OSCN^– by HSO₅^– was determined: <i>k</i>₂ = (1.6 ± 0.1) × 10² M^–1 s^–1 at pH 13.5 and <i>k</i>₂^H = (3.3 ± 0.1) × 10³ M^–1 s^–1 at pH 6.89. A critical evaluation of the estimated activation parameters of the elementary steps revealed that the oxidations of SCN^– as well as the consecutive OSCN^– by HSO₅^– are more likely to proceed via 2e^–-transfer steps rather than 1e^– transfer

Mechanism of Decomposition of the Human Defense Factor Hypothiocyanite Near Physiological pH

Relatively little is known about the reaction chemistry of the human defense factor hypothiocyanite (OSCN^–) and its conjugate acid hypothiocyanous acid (HOSCN), in part because of their instability in aqueous solutions. Herein we report that HOSCN/OSCN^– can engage in a cascade of pH- and concentration-dependent comproportionation, disproportionation, and hydrolysis reactions that control its stability in water. On the basis of reaction kinetic, spectroscopic, and chromatographic methods, a detailed mechanism is proposed for the decomposition of HOSCN/OSCN^– in the range of pH 4–7 to eventually give simple inorganic anions including CN^–, OCN^–, SCN^–, SO₃^2–, and SO₄^2–. Thiocyanogen ((SCN)₂) is proposed to be a key intermediate in the hydrolysis; and the facile reaction of (SCN)₂ with OSCN^– to give NCS(O)SCN, a previously unknown reactive sulfur species, has been independently investigated. The mechanism of the aqueous decomposition of (SCN)₂ around pH 4 is also reported. The resulting mechanistic models for the decomposition of HOSCN and (SCN)₂ address previous empirical observations, including the facts that the presence of SCN^– and/or (SCN)₂ decreases the stability of HOSCN/OSCN^–, that radioisotopic labeling provided evidence that under physiological conditions decomposing OSCN^– is not in equilibrium with (SCN)₂ and SCN^–, and that the hydrolysis of (SCN)₂ near neutral pH does not produce OSCN^–. Accordingly, we demonstrate that, during the human peroxidase-catalyzed oxidation of SCN^–, (SCN)₂ cannot be the precursor of the OSCN^– that is produced

Kinetics of Formation of the Host–Guest Complex of a Viologen with Cucurbit[7]uril

Host–guest complexation between the dicationic viologen 1-tri(ethylene glycol)-1′-methyl-<i>m</i>-xylyl-4,4′-bipyridinium and cucurbit[7]uril (<b>CB7</b>) was studied at pH = 4.5 in water. The stability constants of the mono- and bis-<b>CB7</b> adducts were determined at 25 °C by UV–vis spectroscopy. Stopped-flow kinetic experiments were performed to measure the formation and dissociation rate constants of the monoadduct: <i>k</i>₁ = (6.01 ± 0.03) × 10⁶ M^–1s^–1 and <i>k</i>_–1 = 52.7 ± 0.4 s^–1, respectively. Possible mechanisms of complexation are discussed in view of the kinetic results