2 research outputs found
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<sup>ā</sup>) and its conjugate acid hypothiocyanous acid (HOSCN), in part because of their instability in aqueous solutions. Herein we report that HOSCN/OSCN<sup>ā</sup> 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<sup>ā</sup> in the range of pH 4ā7 to eventually give simple inorganic anions including CN<sup>ā</sup>, OCN<sup>ā</sup>, SCN<sup>ā</sup>, SO<sub>3</sub><sup>2ā</sup>, and SO<sub>4</sub><sup>2ā</sup>. Thiocyanogen ((SCN)<sub>2</sub>) is proposed to be a key intermediate in the hydrolysis; and the facile reaction of (SCN)<sub>2</sub> with OSCN<sup>ā</sup> to give NCS(ī»O)SCN, a previously unknown reactive sulfur species, has been independently investigated. The mechanism of the aqueous decomposition of (SCN)<sub>2</sub> around pH 4 is also reported. The resulting mechanistic models for the decomposition of HOSCN and (SCN)<sub>2</sub> address previous empirical observations, including the facts that the presence of SCN<sup>ā</sup> and/or (SCN)<sub>2</sub> decreases the stability of HOSCN/OSCN<sup>ā</sup>, that radioisotopic labeling provided evidence that under physiological conditions decomposing OSCN<sup>ā</sup> is not in equilibrium with (SCN)<sub>2</sub> and SCN<sup>ā</sup>, and that the hydrolysis of (SCN)<sub>2</sub> near neutral pH does not produce OSCN<sup>ā</sup>. Accordingly, we demonstrate that, during the human peroxidase-catalyzed oxidation of SCN<sup>ā</sup>, (SCN)<sub>2</sub> cannot be the precursor of the OSCN<sup>ā</sup> that is produced
Kinetics of Formation of the HostāGuest Complex of a Viologen with Cucurbit[7]uril
Hostāguest complexation between the dicationic viologen 1-tri(ethylene glycol)-1ā²-methyl-<i>m</i>-xylyl-4,4ā²-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 Ā°C by UVāvis spectroscopy. Stopped-flow kinetic experiments were performed to measure the formation and dissociation rate constants of the monoadduct: <i>k</i><sub>1</sub> = (6.01 Ā± 0.03) Ć 10<sup>6</sup> M<sup>ā1Ā </sup>s<sup>ā1</sup> and <i>k</i><sub>ā1</sub> = 52.7 Ā± 0.4 s<sup>ā1</sup>, respectively. Possible mechanisms of complexation are discussed in view of the kinetic results